Medical and Veterinary Entomology (2014) 28, 264–272
doi: 10.1111/mve.12046
Multi-scale analysis of the associations among egg, larval and pupal surveys and the presence and abundance of adult female Aedes aegypti (Stegomyia aegypti) in the city of Merida, Mexico P. M A N R I Q U E - S A I D E 1 , P. C O L E M A N 2 , P. J. M C C A L L 3 , ´ Z Q U E Z - P R O K O P E C 4 and C. R. D A V I E S 2 A. L E N H A R T 3 , G. V A 1
Departamento de Zoología, Campus de Ciencias Biol´ogicas y Agropecuarias, Universidad Aut´onoma de Yucat´an, Merida, Mexico, 2 Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, U.K., 3 Vector Group, Liverpool School of Tropical Medicine, Liverpool, U.K. and 4 Department of Environmental Studies, Emory University, Atlanta, GA, U.S.A.
Abstract. Despite decades of research, there is still no agreement on which indices of Aedes aegypti (Stegomyia aegypti ) (Diptera: Culicidae) presence and abundance better quantify entomological risk for dengue. This study reports the results of a multi-scale, cross-sectional entomological survey carried out in 1160 households in the city of Merida, Mexico to establish: (a) the correlation between levels of Ae. aegypti presence and abundance detected with aspirators and ovitraps; (b) which immature and egg indices correlate with the presence and abundance of Ae. aegypti females, and (c) the correlations amongst traditional Aedes indices and their modifications for pupae at the household level and within medium-sized geographic areas used for vector surveillance. Our analyses show that ovitrap positivity was significantly associated with indoor adult Ae. aegypti presence [odds ratio (OR) = 1.50; P = 0.03], that the presence of pupae is associated with adult presence at the household level (OR = 2.27; P = 0.001), that classic Aedes indices are informative only when they account for pupae, and that window screens provide a significant level of protection against peridomestic Ae. aegypti (OR = 0.59; P = 0.02). Results reinforce the potential of using both positive collections in outdoor ovitraps and the presence of pupae as sensitive indicators of indoor adult female presence. Key words. Aedes aegypti , dengue, entomological surveillance, Mexico, productivity
Introduction Counts of female adults of Aedes aegypti (Stegomyia aegypti ) (L.) are considered the reference standard method of conducting entomological surveillance for the risk for dengue because they relate to the risk for virus transmission (Focks, 2003; Morrison et al ., 2008; Scott & Morrison, 2008). However, collecting adult female Aedes is a very labour-intensive activity and is consequently unattainable for the majority of vector
surveillance and control programmes. House-to-house surveys collecting data on the presence and abundance of immature Aedes stages, used to calculate the traditional Aedes indices (e.g. house, container and Breteau indices), often serve as a reference to determine the timing and intensity of control activities, provide an evidence base for optimizing control programmes, and as a surrogate of dengue transmission risk (Scott & Morrison, 2008). To reliably satisfy these requirements, however, the indices must fulfil three fundamental criteria:
Correspondence: Pablo Manrique-Saide, Departamento de Zoología, Campus de Ciencias Biol´ogicas y Agropecuarias, Universidad Aut´onoma de Yucat´an, Merida Yucat´an, Mexico, C.P. 97000. Tel.: + 52 999 942 3205; Fax: + 52 999 942 3200; E-mail: [email protected] 264
© 2014 The Royal Entomological Society
Predicting Ae. aegypti presence and abundance (a) they should correspond with one another; (b) they should be reliable proxies for adult abundance, and (c) they should ultimately be correlated with risk for dengue transmission. Arguably, none of these criteria have been met or confirmed satisfactorily. A number of studies have found that indices do not correspond or correlate with one another (Focks & Chadee, 1997; Focks, 2003; Romero-Vivas & Falconar, 2005), or with adult mosquito infestation (Focks, 2003), and results from field studies have shown null or a very poor correlation with virus transmission (Mendez et al ., 2006; Dibo et al ., 2008; Scott & Morrison, 2008). Given the difficulty of adult collections and questionable correlations between Aedes indices and adult mosquito infestations, the use of Aedes oviposition traps (i.e. ovitraps) and counts of pupae in breeding sites (i.e. pupal surveys) have been recommended as indicators of adult mosquito presence or abundance [Pan American Health Organization (PAHO), 1994; Reiter & Nathan, 2001; Focks, 2003]. Ovitraps have been used in various parts of the world to monitor the presence of dengue vector mosquitoes, particularly at low-density levels, and, because they are inexpensive and easy to use, have been recommended for monitoring the impact of insecticide treatments (Reiter & Nathan, 2001). Counting the absolute number of pupae in each breeding site has been recommended as a method of establishing which containers require treatment in targeted breeding site reduction strategies (Focks & Alexander, 2006). Moreover, pupal counts are considered representative of the local adult mosquito population (Focks & Chadee, 1997; Focks, 2003), and the pupae per person index is under evaluation as an indicator for calculating a minimum threshold of pupal infestation for dengue transmission risk (Focks et al ., 2000; Focks, 2003). Few studies have quantified the relationships among data from larval, pupal, ovitrap and adult surveys under field conditions. In Honduras, Gil-Bellorin (1991) failed to find a direct relationship between larval indices and adult mosquito populations in households. In Colombia, Romero-Vivas and Falconar (2005) failed to find correlations between the immature life-stage indices and adult abundance, but reported that the percentage of positive ovitraps and the percentage of houses positive for fourth-instar Ae. aegypti larvae were more efficient for monitoring adult female presence than direct collections. In Brazil, Dibo et al . (2008) found a very low correlation (kappa coefficient of 0.05) between ovitrap indices and collections of adult Ae. aegypti mosquitoes indoors. Using data from northern Argentina and a stochastic simulation model, Garelli et al . (2009) found moderate correlations (Pearson’s r ∼ 0.4) between Aedes indices and the number of pupae per block. Moreover, no study has simultaneously assessed the correlations among all entomological indices within a well-defined geographic area. Here, we report on cross-sectional entomological surveys carried out in the Mexican city of Merida, in which we explored the relationships among immature-based Ae. aegypti entomological infestation indicators, ovitrap indices and local variations in adult female Aedes density with the aim of contributing to the determination of the most appropriate, accurate and cost-effective surveillance method to predict adult
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mosquito infestations within geographic units operationally used for Ae. aegypti surveillance and control.
Materials and methods Study design As part of a larger study assessing the effectiveness of Ae. aegypti surveillance methods in Mexico (Manrique-Saide et al ., 2013), two entomological surveys were carried out in the city of Merida, Mexico, which has a population of approximately 1 million, at the start and end of the rainy season (22 June to 5 July and 7–13 September) in 2003. Rainy seasons in Merida correspond to the peak period of dengue risk. Both surveys were combined for analysis. The study area and study design have been described previously (Manrique-Saide et al ., 2008). Briefly, a cross-sectional survey was undertaken based on a stratified two-stage cluster sampling design (Bennett et al ., ´ 1991). From the total of 302 AGEBs (Area Geo-Estadística B´asica or Basic Geo-Statistical Areas, defined by the Mexican National Institute of Geography and Statistics) in Merida, a total of 29 clusters, each corresponding to one AGEB, were selected as primary sampling units (Fig. 1). The AGEB is similar to a neighbourhood and is a medium-sized area that is typically used for regular Aedes surveillance. As infestation rates are known to vary among different sectors of Merida (Manrique-Saide et al ., 2013), a separate randomization was made for each, which resulted in the selection of four, 10, seven and eight AGEBs from the northern, southern, eastern and western sectors, respectively (Fig. 1). As AGEBS are delineated by population, they have similar population sizes and thus the same number of households was required to characterize each AGEB. Within each selected AGEB, 40 households were selected by starting at the centre of the AGEB and sampling every third house along a north–south and east–west transect; thus a total of 1160 houses were sampled on one occasion across all 29 AGEBs. Collections of adults (indoor collections), eggs (ovitraps) and larvae/pupae (breeding site surveys) of Ae. aegypti were performed simultaneously at each of the selected houses.
Indoor adult survey Adult mosquito collections were conducted inside houses using modified backpack aspirators (John W. Hock Co., Gainesville, FL, U.S.A.) for 15 min/house between 09.00 hours and 15.00 hours. Collections from all 40 selected houses within each AGEB were made on the same day. Mosquitoes collected were identified to species and sex. Positivity for adults (presence/absence) and the total number of Ae. aegypti females per house was recorded. In addition, two indices were calculated for every AGEB: the Adult House Index (AHI), represented by the (number of houses positive for adult female Ae. aegypti /houses inspected) × 100, and the Adult Density Index (ADI), represented by the mean number of adult female Ae. aegypti collected per house.
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P. Manrique-Saide et al. number of immatures collected, and (f) total number of pupae collected. Based on these household-level entomological variables, six indices (the three classic Aedes indices and their pupae equivalents) were computed at the AGEB level. These included: the Container Index, representing the (number of containers with Ae. aegypti immatures/wet containers inspected) × 100; the Pupae Container Index (PCI), representing the (number of containers with Ae. aegypti pupae/wet containers inspected) × 100; the House Index, representing the (number of houses with Ae. aegypti immatures/houses inspected) × 100; the Pupae House Index (PHI), representing the (number of houses with Ae. aegypti pupae/houses inspected) × 100; the Breteau Index, representing the number of containers positive for Ae. aegypti immatures/houses inspected) × 100, and the Pupae Breteau Index (PBI), representing the (number of containers positive for Ae. aegypti pupae/houses inspected) × 100.
´ Fig. 1. Basic Geo-Statistical Areas [Area Geo-Estadística B´asica (AGEB)] and corresponding neighbourhoods sampled from sectors in Merida (polygons represent AGEBs). North: (1) Chuburna, (2) Jardines de Merida, (3) Francisco de Montejo, (4) Maya. South: (5) 5 Colonias, (6) Castilla Camara, (7) Delio Moreno Canton, (8) Dolores Otero, (9) Mercedes Barrera, (10) Plan de Ayala, (11) San Antonio Xluch, (12) Santa Rosa, (13) Los Cocos, (14) Serapio Rendon. East: (15) Cortes Sarmiento, (16) Brisas, (17) Pacabtun, (18) Poligono 108, (19) San Antonio Kahua, (20) Vergel I, (21) Unidad Habitacional Morelos. West: (22) Bojorquez, (23) Iberica, (24) Sambula, (25) Xoclan, (26) Juan Pablo II, (27) Mulsay, (28) Yucalpeten, (29) Residencial Pensiones.
Ovitrap survey At each of the 40 houses in the 29 AGEBs in which adult mosquitoes were collected, an ovitrap was placed outdoors to monitor oviposition activity. The ovitrap employed was a modified Centers for Disease Control (CDC) ovitrap. A 7-day ovitrapping cycle was initiated on the day the adult survey was carried out. Eggs collected in the ovitraps were counted using a stereomicroscope at ×20 magnification. The positivity of the ovitrap (presence/absence of eggs) and the total number of eggs/ovitrap/house were recorded. Two indices were calculated by AGEB: the Ovitrap House Index (OHI), representing the (number of houses with positive ovitraps/number of houses with ovitraps) × 100, and the Ovitrap Density Index (ODI), representing the number of eggs collected/number of positive ovitraps. Larval and pupal survey As described in Manrique-Saide et al . (2008), mosquito larvae and pupae were collected from all positive containers in surveyed houses and six Aedes entomological indices were recorded: (a) number of containers positive for any immature stages of Ae. aegypti ; (b) number of containers positive for Ae. aegypti pupae; (c) number of houses positive for any immature stages of Ae. aegypti (referred to as ‘positive houses’); (d) number of houses positive for pupae; (e) total
Data management and statistical analyses Adult indoor females vs. ovitrap data. Firstly, using data from each household (n = 1160), analyses were performed to evaluate the effect of the window screens typically found in Merida houses on the presence and abundance of Ae. aegypti females. Logistic regressions were conducted to determine the effect of window screening on the presence of females inside the house and ovitrap positivity. Similarly, negative binomial regression of counts of adult females indoors and ovitrap egg density vs. window screening at the household level were also conducted. Clustering of households at the AGEB level was accounted for in all analyses by adjusting regression coefficient standard errors based on membership of a household in a given AGEB (using robust standard errors). Secondly, using data from each household, logistic regression analyses of the presence of Ae. aegypti females indoors vs. ovitrap positivity at the household level were undertaken. In addition, negative binomial regression analyses of counts of the number of adult females indoors vs. ovitrap egg density was also undertaken, in both cases controlling for window screening and clustering by AGEB.
Adult indoor females vs. larvae and pupae (immatures) or pupae-only data at the household level. Logistic regression analyses of the presence of Ae. aegypti females vs. immature (larvae + pupae) or pupae data collected at household level were undertaken. Negative binomial regression analyses were also performed on the counts of Ae. aegypti adult females indoors vs. data for immatures collected at household level, in all cases controlling for window screening, survey and clustering at AGEB.
Ovitrap data vs. immatures and pupae-only data at the household level. Logistic regression analyses were applied to the presence of ovitrap positivity vs. immature and pupae data collected at the household level, controlling for window screening, survey and clustering at AGEB; negative binomial
© 2014 The Royal Entomological Society, Medical and Veterinary Entomology, 28, 264–272
Predicting Ae. aegypti presence and abundance regression analyses were performed on the number of eggs per ovitrap vs. the number of immatures and pupae collected at household level, controlling for window screening, survey and clustering by AGEB.
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abundant (86%) of all Culex spp. collected (0.5 adults/house, 0.3 females/house).
Ovitrap collections Correlations among traditional Aedes indices at AGEB level. Non-parametric Pearson’s correlations between all of the ‘traditional’ Aedes indices (Container Index, PCI, House Index, PHI, Breteau Index and PBI) were performed to describe how they correlate to one another.
Adult and ovitrap indices vs. immature indices at AGEB level. Logistic regression analyses of the proportion of houses with Ae. aegypti females indoors and the proportion of houses with positive ovitraps vs. immature indices at AGEB level were performed controlling for window screening, survey and clustering by AGEB. Negative binomial regression analyses were undertaken of total counts of females indoors and the total number of eggs collected at AGEB level vs. each of the Aedes indices, controlling for window screening, survey and clustering by AGEB (the number of ovitraps recovered in each AGEB was included as a variable in the analyses of the number of eggs). All analyses were carried out using stata Version 8.1 (StataCorp LP, College Station, TX, U.S.A.).
Of the 1160 ovitraps deployed, 74.9% (n = 869) were recovered 1 week later. Missing ovitraps had fallen over, been removed by householders or disappeared without explanation. The OHI was 55.9% (houses with positive ovitraps/houses with ovitraps), with a mean number of 26.2 eggs per positive ovitrap (range: 10–50 eggs per positive ovitrap). The mean number of eggs collected per AGEB was 441.6; those with the highest egg yields (n = 892) had over 10 times as many eggs as those with the lowest egg yields (n = 73). Window screening had no significant effect on the likelihood of ovitraps being positive (OR = 1.06, 95% CI 0.740–1.521; P = 0.75) or the number of eggs per ovitrap (IRR = 1.06, 95% CI 0.728–1.556; P = 0.75), which was not unexpected as the ovitraps were located outdoors. After adjusting for window screening, a positive ovitrap at a house was significantly associated with the presence of indoor adult female Ae. aegypti (OR = 1.50, 95% CI 1.04–2.18; P = 0.03), but the number of eggs collected per ovitrap was not (IRR = 1.01, 95% CI 0.998–1.017; P = 0.10).
Comparison of indoor adult females with presence and abundance of all immatures Results Collections of indoor adult female Ae. aegypti In total, 530 adult Ae. aegypti , 46.4% of which were females, were collected from the 1160 sampled houses. All of the adult sample was collected from 227 houses (19.6%). Aedes aegypti females (n = 246) were present in 145 houses (AHI: 12.5%), at a mean (ADI) geometric mean of 1.4 females/house [95% confidence interval (CI) 1.3–1.5]. Most of the houses positive for females recorded a single female (77.6%, 111 Ae. aegypti females in 111 houses); the maximum number of females per house was nine (in one house only). However, 44.8% (13 of 29) of all surveyed AGEBs had houses in which more than two mosquitoes were found. The presence of window screening in a house significantly decreased both the odds of having adult mosquitoes inside the house (13.6% of unscreened houses vs. 8.5% of screened houses were positive for female Ae. aegypti ) and the number of females found indoors (means of 0.24 and 0.13 in unscreened and screened houses, respectively) [odds ratio (OR) = 0.59, 95% CI 0.378–0.933; P = 0.02; incidence rate ratio (IRR) = 0.52, 95% CI 0.330–0.824; P = 0.005]. In addition, 715 other adult mosquitoes were found inside houses during the study; these included specimens of Culex coronator (Dyar and Knab), Culex interrogator (Dyar and Knab), Culex nigripalpus (Theobald), Culex quinquefasciatus (Say), Culex thriambus (Dyar) and Culex salinarius (Coquillet) (Diptera: Culicidae). Culex quinquefasciatus was the most
The means of the six indices derived from entomological data on larval and pupal infestations at the AGEB level were: Container Index: 14.8% (95% CI 12.9–16.8); PCI: 7.5% (95% CI 6.2–8.7); House Index: 34.2% (95% CI 29.9–38.6); PHI: 20.1% (95% CI 17.4–22.8); Breteau Index: 59.7% (95% CI 48.8–70.7), and PBI: 29.1% (95% CI 23.8–34.4). After adjusting for window screening, the associations between the number of indoor adult females and these indices were highly significant, with the exception of the number of pupae collected per house (Table 1). From all six Aedes immature indices measured at the AGEB level, House Index and PHI showed the most highly significant associations with both the proportion of houses per AGEB found to be positive for indoor Ae. aegypti females and the total number of females collected in each AGEB (Table 2). The only other significant association was between PBI and the proportion of houses with Ae. aegypti females per AGEB (Table 2).
Comparison of ovitrap data with presence and abundance of immatures The presence/absence and number of eggs were significantly associated with house positivity for immatures and the number of containers positive for pupae per house (Table 3). From the six Aedes immature indices measured at the AGEB level, all pupal indices (PCI, PHI, PBI) were significantly associated
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Table 1. Association between the presence and abundance of indoor Aedes aegypti females and Aedes immature infestation indicators at household level, after adjusting for window screening and clustering at AGEB level. Presence of female Ae. aegypti indoors Immature-positive containers per house, n Pupa-positive containers per house Immatures collected per house, n Pupae collected per house, n Positive for immatures, n Positive for pupae, n Abundance of female Ae. aegypti indoors Immature-positive containers per house, n Pupa-positive containers per house Immatures collected per house, n Pupae collected per house, n Positive for immatures, n Positive for pupae, n
OR
P -value
Table 2. Associations between the proportion of houses positive for adult female Aedes aegypti [Adult House Index (AHI)] and the number of Ae. aegypti females collected indoors [Adult Density Index (ADI)] and Aedes immature indices measured at AGEB level (adjusting for window screening).
95% CI of OR
AHI
OR
P -value
95% CI of OR
Container Index PCI House Index PHI Breteau Index PBI
1.009 1.014 1.021 1.046 1.006 1.015
(0.645) (0.596) 0.035 0.007 (0.052) 0.026
0.971–1.049 0.962–1.058 1.002–1.041 1.012–1.082 1.000–1.013 1.002–1.028
ADI
IRR
P -value
95% CI of IRR
Container Index PCI House Index PHI Breteau Index PBI
1.016 1.024 1.028 1.054 1.006 1.016
(0.496) (0.448) 0.025 0.015 (0.085) (0.069)
0.971–1.062 0.963–1.089 1.003–1053 1.010–1.010 0.999–1.014 0.999–1.034
1.167
0.009
1.040–1.311
1.374
0.007
1.091–1.729
1.003
0.005
1.001–1.005
1.023
(0.131)
0.993–1.053
1.927 2.274
0.002 0.001
1.281–2.897 1.420–3.641
IRR
P -value
95% CI of IRR
1.266
< 0.001
1.111–1.443
1.634
< 0.001
1.251–2.135
1.004
0.003
1.001–1.007
1.032
< 0.001
1.015–1.050
2.363 2.547
< 0.001 < 0.001
1.521–3.674 1.648–3.937
P -values in brackets are not significant (P > 0.05). ´ AGEB, Basic Geo-Statistical Area (Area Geo-Estadística B´asica); OR, odds ratio; 95% CI, 95% confidence interval; IRR, incidence rate ratio.
with the number of houses with positive ovitraps and with the number of Ae. aegypti eggs collected in ovitraps (Table 4). The Container Index was significantly associated only with the number of Ae. aegypti eggs collected at the AGEB level (Table 4).
Correlations between Aedes indices at AGEB level All pairwise correlations between the different Aedes indices and their modifications for pupae at the AGEB level are summarized in Table 5. Each Aedes immature-based index was highly correlated with its corresponding pupal index. Only the relationship between the PCI and the House Index was not significant. The PCI and Breteau Index were marginally significantly correlated (P = 0.05). The strongest correlation occurred between the House Index and Breteau Index.
Discussion This is the first comprehensive study of direct adult collections and proxy measures of Ae. aegypti adult infestation in Mexico. Our statistical analyses, clustered at an operational scale used for Ae. aegypti surveillance throughout Mexico, show that ovitrap collections are a sensitive method for
P -values in brackets are not significant (P > 0.05). ´ AGEB, Basic Geo-Statistical Area (Area Geo-Estadística B´asica); OR, odds ratio; 95% CI, 95% confidence interval; IRR, incidence rate ratio; PCI, Pupae Container Index; PHI, Pupae House Index; PBI, Pupae Breteau Index. Container Index: number of positive containers/containers examined × 100; House Index: number of houses infested by Ae. aegypti immatures/houses inspected × 100; Breteau Index: number of positive containers/houses inspected × 100; PCI, PHI, PBI: indices calculated for pupae only.
detecting Ae. aegypti infestations, that classic Aedes indices are informative only when they are corrected to account for pupae, and that the presence of pupae correlates with adult presence at the household level. These findings also suggest that window screens provide a significant level of protection against peridomestic Ae. aegypti . Garcia-Rejon et al . (2008) reported no correlation between outdoor abundance of larvae or pupae and indoor abundance of adult females collected in the homes of dengue patients in Merida. However, their study was structured to detect the presence of dengue virus-infected mosquitoes in the home environment, and thus their entomological surveys were conducted only in households that had reported recent cases of dengue, rather than a sample representative of the community. By contrast, our study systematically sampled the city of Merida, specifically investigating the relationships between Ae. aegypti infestations at different life stages. It is notoriously challenging to collect adult Ae. aegypti (Reiter & Nathan, 2001; Morrison et al ., 2004b). Detecting the presence of gravid Ae. aegypti using ovitraps has been considered very sensitive, even if there is often no clear correlation between the number of eggs collected in ovitraps and adult abundance (Reiter & Nathan, 2001; Focks, 2003), as was corroborated by the present study. A strong association was observed between the presence of a positive ovitrap collection and the odds of having at least one female Ae. aegypti indoors. This association between outdoor ovitrap and indoor adult positivity suggests that ovitrap collections may represent
© 2014 The Royal Entomological Society, Medical and Veterinary Entomology, 28, 264–272
Predicting Ae. aegypti presence and abundance Table 3. Associations between a positive ovitrap at household level and number of Aedes aegypti eggs collected and Aedes immature infestation indicators at household level, after adjusting for window screening and clustering at AGEB level. Positive ovitrap
OR
P -value
95% CI of OR
Positive containers per house, n Pupa-positive containers per house, n Immatures collected per house, n Pupae collected per house, n Positive for immatures, n Positive for pupae, n
1.077 1.280
(0.139) 0.032
0.976–1.188 1.022–1.602
1.000 1.011 1.407 1.496
(0.672) (0.123) 0.032 0.026
Ae. aegypti eggs collected per ovitrap
IRR
Positive containers per house, n Pupa-positive containers per house, n Immatures collected per house, n Pupae collected per house, n Positive for immatures, n Positive for pupae, n
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Table 4. Associations between the number of houses with positive ovitraps [Ovitrap House Index (OHI)] and the number of Aedes aegypti eggs [Ovitrap Density Index (ODI)] collected in ovitraps at AGEB level (adjusting for window screening and number of ovitraps recovered) and Aedes immature indices. OHI
OR
P -value
95% CI of OR
0.999–1.002 0.997–1.026 1.030–1.924 1.050–2.130
Container Index PCI House Index PHI Breteau Index PBI
1.025 1.081 1.004 1.023 1.001 1.013
(0.098) < 0.001 (0.528) 0.038 (0.555) 0.004
0.955–1.056 1.034–1.125 0.991–1.017 1.001–1.046 0.997–1.006 1.004–1.023
P -value
95% CI of IRR
ODI
IRR
P -value
95% CI of IRR
1.097 1.234
(0.062) 0.035
0.995–1.208 1.015–1.500
1.000 1.002 1.391 1.428
(0.816) (0.729) 0.014 0.031
0.998–1.002 0.990–1.015 1.068–1.811 1.034–1.971
Container Index PCI House Index PHI Breteau Index PBI
1.030 1.063 1.012 1.029 1.004 1.015
0.011 < 0.001 (0.091) 0.004 (0.093) < 0.001
1.001–1.054 1.036–1.090 0.998–1.026 1.009–1.050 0.999–1.009 1.007–1.023
P -values in brackets are not significant (P > 0.05). ´ AGEB, Basic Geo-Statistical Area (Area Geo-Estadística B´asica); OR, odds ratio; 95% CI, 95% confidence interval; IRR, incidence rate ratio.
a practical method of monitoring the presence of indoor Aedes females. Current surveillance programmes in Mexico, Brazil and Vietnam actively rely on ovitraps as a proxy measure of Ae. aegypti infestation levels (Secretaría de Salud, 2007; de Melo et al ., 2012; Wu et al ., 2013). Presence or absence surveys based on oviposition traps have been considered more sensitive than the direct collection of adults at particular sites for monitoring the spatial and temporal distributions of adult Ae. aegypti , particularly in areas with low levels of infestation (Furlow & Young, 1970; Ritchie, 1984; PAHO, 1994; Reiter & Gubler, 1997; Focks, 2003). As observed in this study, ovitraps detected on average 4.5 times as many houses positive for Ae. aegypti females as adult indoor collections. As the adult Aedes female is the most epidemiologically important stage for dengue surveillance and control (Morrison et al ., 2008), there remains substantial debate on the applicability of traditional Aedes larvae- or pupae-based indices as indicators of Aedes adult infestations and as accurate measures of dengue entomological risk (Focks, 2003). Important biological (i.e. physiological status) and epidemiological (i.e. location and probability of contact with the host) factors can differentially influence direct indoor and indirect outdoor measures of adult Aedes presence. Therefore, we examined the correlations with larval and pupal measures for both types of data separately. A current criticism of the traditional Aedes indices is that they are poor proxies for adult vector abundance (Focks, 2003). Two key reasons for questioning the strength of associations between the presence of immature stages and that of adults (at least for larvae) are that high mortalities during the larval stage preclude the assumption that larval densities are indicative of adult densities, and that flying adults can be spatially dissociated from their development sites (Morrison
P -values in brackets are not significant (P > 0.05). ´ AGEB, Basic Geo-Statistical Area (Area Geo-Estadística B´asica); OR, odds ratio; 95% CI, 95% confidence interval; IRR, incidence rate ratio; PCI, Pupae Container Index; PHI, Pupae House Index; PBI, Pupae Breteau Index. Container Index: number of positive containers/containers examined × 100; House Index: number of houses infested by Ae. aegypti immatures/houses inspected × 100; Breteau Index: number of positive containers/houses inspected × 100; PCI, PHI, PBI: indices calculated for pupae only.
et al ., 2004b). To overcome the first of these issues, Focks and Chadee (1997) described a method of counting pupae to estimate the number of adult vectors because pupae can be easily identified and counted and pupal numbers are thought to correlate highly with the local population of adult mosquitoes. Results from the present study were contradictory to these predictions because the numbers of both immatures and pupae collected per house correlated with the counts of indoor Ae. aegypti females. The comparison of adult indices and immature indices at the AGEB level in Merida concurs, in part, with criticism of the utility of traditional indices for predicting adult female Ae. aegypti presence and/or abundance; indeed the Container Index and Breteau Index did not show strong correlations with the presence of adult vectors indoors at AGEB level. However, the House Index and PHI were both strongly associated with the presence and number of adult Ae. aegypti collected indoors at AGEB level. Based on the results of several field studies (Chan et al ., 1971; Bang et al ., 1981; Strickman & Kittayapong, 2002; Romero-Vivas & Falconar, 2005), it can be argued that larvaebased indicators that include a measure of larval productivity in medium-sized areas are better associated with adult abundance and occurrence than indices that do not account for such factor. Tun-Lin et al . (1996) reported high correlations between larval and adult densities calculated from outdoor surveys. However, similar associations of total immature counts and adults were not seen at the AGEB level in this study. Although the number of immatures per house was associated with the presence
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Table 5. Pearson’s correlation coefficients and coefficients of determination (R 2 indicated in parenthesis) between Aedes aegypti immature infestation indices recorded in Merida during the rainy season in 2003.
PCI House Index PHI Breteau Index PBI
Container Index
PCI
House Index
PHI
Breteau Index
0.79 0.50 0.42 0.55 0.61
– 0.1(0.09)† 0.43 (0.21) 0.34* 0.64 (0.45)
– – 0.82 (0.70) 0.88 (0.72) 0.75 (0.48)
– – – 0.80 (0.57) 0.88 (0.73)
– – – – 0.89 (0.71)
(0.59) (0.24) (0.18) (0.36) (0.39)
P < 0.05 unless otherwise indicated; *P = 0.05; †P > 0.05. PCI, Pupae Container Index; PHI, Pupae House Index; PBI, Pupae Breteau Index. Container Index: number of positive containers/wet containers examined × 100; House Index: number of houses infested with Ae. aegypti immatures/houses inspected × 100; Breteau Index: number of positive containers/houses inspected × 100; PCI, PHI, PBI: indices calculated for pupae only.
and abundance of Ae. aegypti females inside the same house, neither the total number of immatures nor the total number of pupae collected in an AGEB were associated with the presence and total abundance of Aedes adult females collected indoors at AGEB level. The lack of agreement of indices across spatial scales may relate to the very focal and heterogeneous nature of Ae. aegypti infestations (Getis et al ., 2003). Romero-Vivas and Falconar (2005) reported a negative correlation between larval density (for fourth-instar larvae exclusively) and ODI (from ovitraps placed outdoors or indoors), attributing their findings to density-dependent mortality occurring during the first two larval stages. In the present study, no significant correlation was observed between total immatures or pupae and the egg density recorded in the same house. However, a significant positive correlation was detected between the number of positive containers per house and the presence and abundance of Aedes females indoors, and also between the Container Index and the total number of eggs at AGEB level. Focks (2003), in an influential review of Aedes surveillance methods, questioned the utility of the traditional Aedes indices (container, house and Breteau indices), suggesting they were of limited use partly because of the lack of correlation between the different measures. In the past, different authors have reported variable results on the correlations among the traditional Aedes indices (Tinker, 1978; Tun-Lin et al ., 1996; Focks & Chadee, 1997; Mahadev et al ., 2004; Romero-Vivas & Falconar, 2005). It has been argued that correlations between indices are generally limited (30–50%), even when they are positive and significant (Focks, 2003). The results of the present study show positive correlations among the three traditional indices at AGEB level, but also that the House and Breteau Indices were strongly associated. Highly significant correlations between the Breteau and House Indices have been reported by Tinker (1978) in Jamaica and El Salvador, by Chan (1985) in Singapore, and by Sanchez et al . (2006) in Cuba, among others. They particularly observed that correlations occurred when infestations were low (House Index: < 5%) and weakened as the level of infestation increased (Tinker, 1978; Chan, 1985). At low infestation levels (as in Singapore in the mid-1980s), infestations are often characterized by a single positive container per house (Chan, 1985); as infestations increase, more houses are expected to have multiple foci.
However, it has been observed that at all levels of infestation, houses are most commonly infested by a single focus (Tinker, 1978). This was also observed in the present study: whereas overall findings showed houses to have an average of 5.4 (95% CI 4.7–6.1) Ae. aegypti -positive containers on their patios, the mode was one positive container per house, and the House and Breteau Indices were found to be highly correlated. The complete count of every larva and pupa in every container in a community is too practically difficult to be considered as a routine surveillance measure. In this study, indices based on pupal presence (i.e. the numbers of containers and houses positive for pupae) and ovitrap positivity showed significant associations with all of the adult Aedes female measurements at house level. Furthermore, the number of houses positive for pupae at AGEB level (PHI) was significantly associated with all of the adult Aedes female measurements at the same resolution, and the PCI showed the strongest association with ovitrap indices at AGEB level. Although the data presented herein were collected over 10 years ago, we consider the results remain relevant. Currently there are no methods that can be considered to represent a reference standard for the surveillance of Ae. aegypti (Scott & Morrison, 2008), but the present study has contributed to the debate on which may be more correlated with female presence and abundance. Additionally, Mexico is currently exploring the use of a network of ovitraps for Ae. aegypti surveillance ´ (Hern´andez-Avila et al ., 2013), and the results from this study can be used to inform policies regarding which method may correlate better with female adult abundance. Open windows and doors are important points of entry into a house and such apertures can be screened to prevent the entry of flying insects (Schofield et al ., 1990). Recently, long-lasting insecticidal nets deployed as window nets or indoor curtains in houses have been field-tested with good results in different settings worldwide as part of an integrated environmental management approach to complement and enhance current dengue vector control (Kroeger et al ., 2006; Tun-Lin et al ., 2009; Vanlerberghe et al ., 2011; Rizzo et al ., 2012). Our study validates the importance of window screening in reducing the abundance of Ae. aegypti indoors and contributes to the literature on the importance of evaluating methods of making houses ‘Aedes-proof’ in order to support the reduction of human exposure to Ae. aegypti indoors.
© 2014 The Royal Entomological Society, Medical and Veterinary Entomology, 28, 264–272
Predicting Ae. aegypti presence and abundance The ovitrap has been designated as a tool for Ae. aegypti surveillance, particularly to provide a reliable indirect measure of the presence/absence of adult females and of relative changes in adult female populations (PAHO, 1994; Reiter & Gubler, 1997). Recent innovations of the Dengue Surveillance and Control Programme in Mexico have included the use of ovitraps located outdoors in high-risk and dengue-endemic localities, including Merida. The information arising from this network of ovitraps is currently being employed to evaluate local effects on control methods (i.e. reductions after ultralow-volume spraying), but has also started to be used as an indicator of the relative risk for dengue transmission based on vector abundance (egg counts from ovitrapping). The association between ovitrap (outdoors) positivity and indoor adult positivity observed in this study suggests that ovitrapping may represent a practical method of monitoring the prevalence of Aedes females indoors in that the monitoring or targeting of specific areas selected according to the prevalence of positive catches in ovitraps may generate results similar to those of the monitoring or targeting of adult female prevalence. This relatively low labour-intensive sampling method may be more cost-effective than traditional immature surveys (which are more labour-intensive and require more personnel to cover an entire city). Recent studies in Brazil and Taiwan have demonstrated moderate but significant correlations between ovitrap indices and dengue virus transmission (Dibo et al ., 2008; de Melo et al ., 2012; Wu et al ., 2013), indicating the need for more studies to quantitatively explore the potential of ovitraps to serve as entomological proxies of dengue virus transmission activity.
Acknowledgements Regrettably, Professor Clive R. Davies passed away prior to the completion of this manuscript. We wish to dedicate this study as a small memorial to his significant contribution to tropical medicine research. We would like to express our sincere gratitude to Consejo Nacional de Ciencia y Tecnologia (CONACYT Mexico), Fondo Sectorial de Investigaci´on en Salud y Seguridad Social (SSA/IMSS/ISSSTE-CONACYT) (SALUD-2011-1-161551) and the Programa de Impulso y Orientacion a la Investigacion (PRIORI-UADY) for their financial support of this project. We also wish to express our sincere gratitude to the communities in Merida for their permission, participation and cooperation throughout the study. References Bang, Y.H. & Pant, C.P. (1972) A field trial of Abate larvicide for the control of Aedes aegypti in Bangkok, Thailand. Bulletin of the World Health Organization, 46, 416–425. Bang, Y.H., Bown, D.N. & Onwubiko, O. (1981) Prevalence of larvae of potential yellow fever vectors in domestic water containers in south-east Nigeria. Bulletin of the World Health Organization, 59, 107–114. Bennett, S., Woods, T., Liyanage, W.M. & Smith, D.L. (1991) A simplified general method for cluster-sample surveys of health in developing countries. World Health Statistics Quarterly, 44, 98–106.
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Schoeler, G.B., Schleich, S.S., Manweiler, S.A. & Sifuentes, V.L. (2004) Evaluation of surveillance devices for monitoring Aedes aegypti in an urban area of northeastern Peru. Journal of the American Mosquito Control Association, 20, 6–11. Schofield, C.J., Briceno-Leon, R., Kolstrup, N., Webb, D.J.T. & White, G.B. (1990) The role of house design in limiting vector-borne disease. Appropriate Technology in Vector Control (ed. by C.E. Curtis), pp. 187–212. CRC Press, Boca Raton, FL. Scott, T.W. & Morrison, A.C. (2002) Ecological aspects for application of genetically modified mosquitoes. Proceedings of the Frontis Workshop on Ecological Challenges Concerning the Use of Genetically Modified Mosquitoes for Disease Control (ed. by W. Takken & T.W. Scott). http://library.wur.nl/frontis/malaria [Accessed 12 December 2013]. Scott, T.W. & Morrison, A.C. (2008) Longitudinal field studies will guide a paradigm shift in dengue prevention. Vector-borne Diseases: Understanding the Environmental, Human Health, and Ecological Connections Workshop Summary (Forum on Microbial Threats), pp. 132–149. National Academies Press, Washington, DC. Smith, D.L., Dushoff, J. & McKenzie, F.E. (2004) The risk of a mosquito-borne infection in a heterogeneous environment. PLoS Biology, 2, e368. Secretaría de Salud (2007) Programa de Acci´on Específico 2007–2012: Dengue. Ministry of Health, Mexico City, DF. Strickman, D. & Kittayapong, P. (2002) Dengue and its vectors in Thailand: introduction to the study and seasonal distribution of Aedes larvae. American Journal of Tropical Medicine and Hygiene, 67, 247–259. Tinker, M.E. (1978) Relationships of the house index and Breteau index for Aedes aegypti . PAHO/WHO Newsletter on Dengue, Yellow Fever and Aedes aegypti in the Americas, 7, 11–13. Tonn, R.J., Sheppard, P.M., Macdonald, W.W. & Bang, Y.H. (1969) Replicate surveys of larval habitats of Aedes aegypti in relation to dengue haemorrhagic fever in Bangkok, Thailand. Bulletin of the World Health Organization, 40, 819–829. Tun-Lin, W., Kay, B.H., Barnes, A. & Forsyth, S. (1996) Critical examination of Aedes aegypti indices: correlations with abundance. American Journal of Tropical Medicine and Hygiene, 54, 543–547. Tun-Lin, W., Lenhart, A., Nam, V.S. et al. (2009) Reducing costs and operational constraints of dengue vector control by targeting productive breeding places: a multi-country non-inferiority cluster randomized trial. Tropical Medicine and International Health, 14, 1143–1153. Vanlerberghe, V., Villegas, E., Oviedo, M., Baly, A., Lenhart, A., McCall, P.J. & Van der Stuyft, P. (2011) Evaluation of the effectiveness of insecticide-treated materials for household level dengue vector control. PLoS Neglected Tropical Diseases, 5, e9. World Health Organization (1997) Dengue Haemorrhagic Fever: Diagnosis, Treatment, Prevention and Control , 2nd edn. WHO, Geneva. World Health Organization. (1999) Dengue/Dengue Haemorrhagic Fever Prevention and Control Programme in Thailand . (WHO Project: ICP CTD 001, SEA/Haem. Fev./68, SEA/VBC/68.) WHO, Geneva. Wu, H.H., Wang, C.Y., Teng, H.J. et al. (2013) A dengue vector surveillance by human population-stratified ovitrap survey for Aedes (Diptera: Culicidae) adult and egg collections in high dengue-risk areas of Taiwan. Journal of Medical Entomology, 50, 261–269. Accepted 9 October 2013 First published online 6 May 2014
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doi: 10.1111/mve.12046
Multi-scale analysis of the associations among egg, larval and pupal surveys and the presence and abundance of adult female Aedes aegypti (Stegomyia aegypti) in the city of Merida, Mexico P. M A N R I Q U E - S A I D E 1 , P. C O L E M A N 2 , P. J. M C C A L L 3 , ´ Z Q U E Z - P R O K O P E C 4 and C. R. D A V I E S 2 A. L E N H A R T 3 , G. V A 1
Departamento de Zoología, Campus de Ciencias Biol´ogicas y Agropecuarias, Universidad Aut´onoma de Yucat´an, Merida, Mexico, 2 Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, U.K., 3 Vector Group, Liverpool School of Tropical Medicine, Liverpool, U.K. and 4 Department of Environmental Studies, Emory University, Atlanta, GA, U.S.A.
Abstract. Despite decades of research, there is still no agreement on which indices of Aedes aegypti (Stegomyia aegypti ) (Diptera: Culicidae) presence and abundance better quantify entomological risk for dengue. This study reports the results of a multi-scale, cross-sectional entomological survey carried out in 1160 households in the city of Merida, Mexico to establish: (a) the correlation between levels of Ae. aegypti presence and abundance detected with aspirators and ovitraps; (b) which immature and egg indices correlate with the presence and abundance of Ae. aegypti females, and (c) the correlations amongst traditional Aedes indices and their modifications for pupae at the household level and within medium-sized geographic areas used for vector surveillance. Our analyses show that ovitrap positivity was significantly associated with indoor adult Ae. aegypti presence [odds ratio (OR) = 1.50; P = 0.03], that the presence of pupae is associated with adult presence at the household level (OR = 2.27; P = 0.001), that classic Aedes indices are informative only when they account for pupae, and that window screens provide a significant level of protection against peridomestic Ae. aegypti (OR = 0.59; P = 0.02). Results reinforce the potential of using both positive collections in outdoor ovitraps and the presence of pupae as sensitive indicators of indoor adult female presence. Key words. Aedes aegypti , dengue, entomological surveillance, Mexico, productivity
Introduction Counts of female adults of Aedes aegypti (Stegomyia aegypti ) (L.) are considered the reference standard method of conducting entomological surveillance for the risk for dengue because they relate to the risk for virus transmission (Focks, 2003; Morrison et al ., 2008; Scott & Morrison, 2008). However, collecting adult female Aedes is a very labour-intensive activity and is consequently unattainable for the majority of vector
surveillance and control programmes. House-to-house surveys collecting data on the presence and abundance of immature Aedes stages, used to calculate the traditional Aedes indices (e.g. house, container and Breteau indices), often serve as a reference to determine the timing and intensity of control activities, provide an evidence base for optimizing control programmes, and as a surrogate of dengue transmission risk (Scott & Morrison, 2008). To reliably satisfy these requirements, however, the indices must fulfil three fundamental criteria:
Correspondence: Pablo Manrique-Saide, Departamento de Zoología, Campus de Ciencias Biol´ogicas y Agropecuarias, Universidad Aut´onoma de Yucat´an, Merida Yucat´an, Mexico, C.P. 97000. Tel.: + 52 999 942 3205; Fax: + 52 999 942 3200; E-mail: [email protected] 264
© 2014 The Royal Entomological Society
Predicting Ae. aegypti presence and abundance (a) they should correspond with one another; (b) they should be reliable proxies for adult abundance, and (c) they should ultimately be correlated with risk for dengue transmission. Arguably, none of these criteria have been met or confirmed satisfactorily. A number of studies have found that indices do not correspond or correlate with one another (Focks & Chadee, 1997; Focks, 2003; Romero-Vivas & Falconar, 2005), or with adult mosquito infestation (Focks, 2003), and results from field studies have shown null or a very poor correlation with virus transmission (Mendez et al ., 2006; Dibo et al ., 2008; Scott & Morrison, 2008). Given the difficulty of adult collections and questionable correlations between Aedes indices and adult mosquito infestations, the use of Aedes oviposition traps (i.e. ovitraps) and counts of pupae in breeding sites (i.e. pupal surveys) have been recommended as indicators of adult mosquito presence or abundance [Pan American Health Organization (PAHO), 1994; Reiter & Nathan, 2001; Focks, 2003]. Ovitraps have been used in various parts of the world to monitor the presence of dengue vector mosquitoes, particularly at low-density levels, and, because they are inexpensive and easy to use, have been recommended for monitoring the impact of insecticide treatments (Reiter & Nathan, 2001). Counting the absolute number of pupae in each breeding site has been recommended as a method of establishing which containers require treatment in targeted breeding site reduction strategies (Focks & Alexander, 2006). Moreover, pupal counts are considered representative of the local adult mosquito population (Focks & Chadee, 1997; Focks, 2003), and the pupae per person index is under evaluation as an indicator for calculating a minimum threshold of pupal infestation for dengue transmission risk (Focks et al ., 2000; Focks, 2003). Few studies have quantified the relationships among data from larval, pupal, ovitrap and adult surveys under field conditions. In Honduras, Gil-Bellorin (1991) failed to find a direct relationship between larval indices and adult mosquito populations in households. In Colombia, Romero-Vivas and Falconar (2005) failed to find correlations between the immature life-stage indices and adult abundance, but reported that the percentage of positive ovitraps and the percentage of houses positive for fourth-instar Ae. aegypti larvae were more efficient for monitoring adult female presence than direct collections. In Brazil, Dibo et al . (2008) found a very low correlation (kappa coefficient of 0.05) between ovitrap indices and collections of adult Ae. aegypti mosquitoes indoors. Using data from northern Argentina and a stochastic simulation model, Garelli et al . (2009) found moderate correlations (Pearson’s r ∼ 0.4) between Aedes indices and the number of pupae per block. Moreover, no study has simultaneously assessed the correlations among all entomological indices within a well-defined geographic area. Here, we report on cross-sectional entomological surveys carried out in the Mexican city of Merida, in which we explored the relationships among immature-based Ae. aegypti entomological infestation indicators, ovitrap indices and local variations in adult female Aedes density with the aim of contributing to the determination of the most appropriate, accurate and cost-effective surveillance method to predict adult
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mosquito infestations within geographic units operationally used for Ae. aegypti surveillance and control.
Materials and methods Study design As part of a larger study assessing the effectiveness of Ae. aegypti surveillance methods in Mexico (Manrique-Saide et al ., 2013), two entomological surveys were carried out in the city of Merida, Mexico, which has a population of approximately 1 million, at the start and end of the rainy season (22 June to 5 July and 7–13 September) in 2003. Rainy seasons in Merida correspond to the peak period of dengue risk. Both surveys were combined for analysis. The study area and study design have been described previously (Manrique-Saide et al ., 2008). Briefly, a cross-sectional survey was undertaken based on a stratified two-stage cluster sampling design (Bennett et al ., ´ 1991). From the total of 302 AGEBs (Area Geo-Estadística B´asica or Basic Geo-Statistical Areas, defined by the Mexican National Institute of Geography and Statistics) in Merida, a total of 29 clusters, each corresponding to one AGEB, were selected as primary sampling units (Fig. 1). The AGEB is similar to a neighbourhood and is a medium-sized area that is typically used for regular Aedes surveillance. As infestation rates are known to vary among different sectors of Merida (Manrique-Saide et al ., 2013), a separate randomization was made for each, which resulted in the selection of four, 10, seven and eight AGEBs from the northern, southern, eastern and western sectors, respectively (Fig. 1). As AGEBS are delineated by population, they have similar population sizes and thus the same number of households was required to characterize each AGEB. Within each selected AGEB, 40 households were selected by starting at the centre of the AGEB and sampling every third house along a north–south and east–west transect; thus a total of 1160 houses were sampled on one occasion across all 29 AGEBs. Collections of adults (indoor collections), eggs (ovitraps) and larvae/pupae (breeding site surveys) of Ae. aegypti were performed simultaneously at each of the selected houses.
Indoor adult survey Adult mosquito collections were conducted inside houses using modified backpack aspirators (John W. Hock Co., Gainesville, FL, U.S.A.) for 15 min/house between 09.00 hours and 15.00 hours. Collections from all 40 selected houses within each AGEB were made on the same day. Mosquitoes collected were identified to species and sex. Positivity for adults (presence/absence) and the total number of Ae. aegypti females per house was recorded. In addition, two indices were calculated for every AGEB: the Adult House Index (AHI), represented by the (number of houses positive for adult female Ae. aegypti /houses inspected) × 100, and the Adult Density Index (ADI), represented by the mean number of adult female Ae. aegypti collected per house.
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P. Manrique-Saide et al. number of immatures collected, and (f) total number of pupae collected. Based on these household-level entomological variables, six indices (the three classic Aedes indices and their pupae equivalents) were computed at the AGEB level. These included: the Container Index, representing the (number of containers with Ae. aegypti immatures/wet containers inspected) × 100; the Pupae Container Index (PCI), representing the (number of containers with Ae. aegypti pupae/wet containers inspected) × 100; the House Index, representing the (number of houses with Ae. aegypti immatures/houses inspected) × 100; the Pupae House Index (PHI), representing the (number of houses with Ae. aegypti pupae/houses inspected) × 100; the Breteau Index, representing the number of containers positive for Ae. aegypti immatures/houses inspected) × 100, and the Pupae Breteau Index (PBI), representing the (number of containers positive for Ae. aegypti pupae/houses inspected) × 100.
´ Fig. 1. Basic Geo-Statistical Areas [Area Geo-Estadística B´asica (AGEB)] and corresponding neighbourhoods sampled from sectors in Merida (polygons represent AGEBs). North: (1) Chuburna, (2) Jardines de Merida, (3) Francisco de Montejo, (4) Maya. South: (5) 5 Colonias, (6) Castilla Camara, (7) Delio Moreno Canton, (8) Dolores Otero, (9) Mercedes Barrera, (10) Plan de Ayala, (11) San Antonio Xluch, (12) Santa Rosa, (13) Los Cocos, (14) Serapio Rendon. East: (15) Cortes Sarmiento, (16) Brisas, (17) Pacabtun, (18) Poligono 108, (19) San Antonio Kahua, (20) Vergel I, (21) Unidad Habitacional Morelos. West: (22) Bojorquez, (23) Iberica, (24) Sambula, (25) Xoclan, (26) Juan Pablo II, (27) Mulsay, (28) Yucalpeten, (29) Residencial Pensiones.
Ovitrap survey At each of the 40 houses in the 29 AGEBs in which adult mosquitoes were collected, an ovitrap was placed outdoors to monitor oviposition activity. The ovitrap employed was a modified Centers for Disease Control (CDC) ovitrap. A 7-day ovitrapping cycle was initiated on the day the adult survey was carried out. Eggs collected in the ovitraps were counted using a stereomicroscope at ×20 magnification. The positivity of the ovitrap (presence/absence of eggs) and the total number of eggs/ovitrap/house were recorded. Two indices were calculated by AGEB: the Ovitrap House Index (OHI), representing the (number of houses with positive ovitraps/number of houses with ovitraps) × 100, and the Ovitrap Density Index (ODI), representing the number of eggs collected/number of positive ovitraps. Larval and pupal survey As described in Manrique-Saide et al . (2008), mosquito larvae and pupae were collected from all positive containers in surveyed houses and six Aedes entomological indices were recorded: (a) number of containers positive for any immature stages of Ae. aegypti ; (b) number of containers positive for Ae. aegypti pupae; (c) number of houses positive for any immature stages of Ae. aegypti (referred to as ‘positive houses’); (d) number of houses positive for pupae; (e) total
Data management and statistical analyses Adult indoor females vs. ovitrap data. Firstly, using data from each household (n = 1160), analyses were performed to evaluate the effect of the window screens typically found in Merida houses on the presence and abundance of Ae. aegypti females. Logistic regressions were conducted to determine the effect of window screening on the presence of females inside the house and ovitrap positivity. Similarly, negative binomial regression of counts of adult females indoors and ovitrap egg density vs. window screening at the household level were also conducted. Clustering of households at the AGEB level was accounted for in all analyses by adjusting regression coefficient standard errors based on membership of a household in a given AGEB (using robust standard errors). Secondly, using data from each household, logistic regression analyses of the presence of Ae. aegypti females indoors vs. ovitrap positivity at the household level were undertaken. In addition, negative binomial regression analyses of counts of the number of adult females indoors vs. ovitrap egg density was also undertaken, in both cases controlling for window screening and clustering by AGEB.
Adult indoor females vs. larvae and pupae (immatures) or pupae-only data at the household level. Logistic regression analyses of the presence of Ae. aegypti females vs. immature (larvae + pupae) or pupae data collected at household level were undertaken. Negative binomial regression analyses were also performed on the counts of Ae. aegypti adult females indoors vs. data for immatures collected at household level, in all cases controlling for window screening, survey and clustering at AGEB.
Ovitrap data vs. immatures and pupae-only data at the household level. Logistic regression analyses were applied to the presence of ovitrap positivity vs. immature and pupae data collected at the household level, controlling for window screening, survey and clustering at AGEB; negative binomial
© 2014 The Royal Entomological Society, Medical and Veterinary Entomology, 28, 264–272
Predicting Ae. aegypti presence and abundance regression analyses were performed on the number of eggs per ovitrap vs. the number of immatures and pupae collected at household level, controlling for window screening, survey and clustering by AGEB.
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abundant (86%) of all Culex spp. collected (0.5 adults/house, 0.3 females/house).
Ovitrap collections Correlations among traditional Aedes indices at AGEB level. Non-parametric Pearson’s correlations between all of the ‘traditional’ Aedes indices (Container Index, PCI, House Index, PHI, Breteau Index and PBI) were performed to describe how they correlate to one another.
Adult and ovitrap indices vs. immature indices at AGEB level. Logistic regression analyses of the proportion of houses with Ae. aegypti females indoors and the proportion of houses with positive ovitraps vs. immature indices at AGEB level were performed controlling for window screening, survey and clustering by AGEB. Negative binomial regression analyses were undertaken of total counts of females indoors and the total number of eggs collected at AGEB level vs. each of the Aedes indices, controlling for window screening, survey and clustering by AGEB (the number of ovitraps recovered in each AGEB was included as a variable in the analyses of the number of eggs). All analyses were carried out using stata Version 8.1 (StataCorp LP, College Station, TX, U.S.A.).
Of the 1160 ovitraps deployed, 74.9% (n = 869) were recovered 1 week later. Missing ovitraps had fallen over, been removed by householders or disappeared without explanation. The OHI was 55.9% (houses with positive ovitraps/houses with ovitraps), with a mean number of 26.2 eggs per positive ovitrap (range: 10–50 eggs per positive ovitrap). The mean number of eggs collected per AGEB was 441.6; those with the highest egg yields (n = 892) had over 10 times as many eggs as those with the lowest egg yields (n = 73). Window screening had no significant effect on the likelihood of ovitraps being positive (OR = 1.06, 95% CI 0.740–1.521; P = 0.75) or the number of eggs per ovitrap (IRR = 1.06, 95% CI 0.728–1.556; P = 0.75), which was not unexpected as the ovitraps were located outdoors. After adjusting for window screening, a positive ovitrap at a house was significantly associated with the presence of indoor adult female Ae. aegypti (OR = 1.50, 95% CI 1.04–2.18; P = 0.03), but the number of eggs collected per ovitrap was not (IRR = 1.01, 95% CI 0.998–1.017; P = 0.10).
Comparison of indoor adult females with presence and abundance of all immatures Results Collections of indoor adult female Ae. aegypti In total, 530 adult Ae. aegypti , 46.4% of which were females, were collected from the 1160 sampled houses. All of the adult sample was collected from 227 houses (19.6%). Aedes aegypti females (n = 246) were present in 145 houses (AHI: 12.5%), at a mean (ADI) geometric mean of 1.4 females/house [95% confidence interval (CI) 1.3–1.5]. Most of the houses positive for females recorded a single female (77.6%, 111 Ae. aegypti females in 111 houses); the maximum number of females per house was nine (in one house only). However, 44.8% (13 of 29) of all surveyed AGEBs had houses in which more than two mosquitoes were found. The presence of window screening in a house significantly decreased both the odds of having adult mosquitoes inside the house (13.6% of unscreened houses vs. 8.5% of screened houses were positive for female Ae. aegypti ) and the number of females found indoors (means of 0.24 and 0.13 in unscreened and screened houses, respectively) [odds ratio (OR) = 0.59, 95% CI 0.378–0.933; P = 0.02; incidence rate ratio (IRR) = 0.52, 95% CI 0.330–0.824; P = 0.005]. In addition, 715 other adult mosquitoes were found inside houses during the study; these included specimens of Culex coronator (Dyar and Knab), Culex interrogator (Dyar and Knab), Culex nigripalpus (Theobald), Culex quinquefasciatus (Say), Culex thriambus (Dyar) and Culex salinarius (Coquillet) (Diptera: Culicidae). Culex quinquefasciatus was the most
The means of the six indices derived from entomological data on larval and pupal infestations at the AGEB level were: Container Index: 14.8% (95% CI 12.9–16.8); PCI: 7.5% (95% CI 6.2–8.7); House Index: 34.2% (95% CI 29.9–38.6); PHI: 20.1% (95% CI 17.4–22.8); Breteau Index: 59.7% (95% CI 48.8–70.7), and PBI: 29.1% (95% CI 23.8–34.4). After adjusting for window screening, the associations between the number of indoor adult females and these indices were highly significant, with the exception of the number of pupae collected per house (Table 1). From all six Aedes immature indices measured at the AGEB level, House Index and PHI showed the most highly significant associations with both the proportion of houses per AGEB found to be positive for indoor Ae. aegypti females and the total number of females collected in each AGEB (Table 2). The only other significant association was between PBI and the proportion of houses with Ae. aegypti females per AGEB (Table 2).
Comparison of ovitrap data with presence and abundance of immatures The presence/absence and number of eggs were significantly associated with house positivity for immatures and the number of containers positive for pupae per house (Table 3). From the six Aedes immature indices measured at the AGEB level, all pupal indices (PCI, PHI, PBI) were significantly associated
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Table 1. Association between the presence and abundance of indoor Aedes aegypti females and Aedes immature infestation indicators at household level, after adjusting for window screening and clustering at AGEB level. Presence of female Ae. aegypti indoors Immature-positive containers per house, n Pupa-positive containers per house Immatures collected per house, n Pupae collected per house, n Positive for immatures, n Positive for pupae, n Abundance of female Ae. aegypti indoors Immature-positive containers per house, n Pupa-positive containers per house Immatures collected per house, n Pupae collected per house, n Positive for immatures, n Positive for pupae, n
OR
P -value
Table 2. Associations between the proportion of houses positive for adult female Aedes aegypti [Adult House Index (AHI)] and the number of Ae. aegypti females collected indoors [Adult Density Index (ADI)] and Aedes immature indices measured at AGEB level (adjusting for window screening).
95% CI of OR
AHI
OR
P -value
95% CI of OR
Container Index PCI House Index PHI Breteau Index PBI
1.009 1.014 1.021 1.046 1.006 1.015
(0.645) (0.596) 0.035 0.007 (0.052) 0.026
0.971–1.049 0.962–1.058 1.002–1.041 1.012–1.082 1.000–1.013 1.002–1.028
ADI
IRR
P -value
95% CI of IRR
Container Index PCI House Index PHI Breteau Index PBI
1.016 1.024 1.028 1.054 1.006 1.016
(0.496) (0.448) 0.025 0.015 (0.085) (0.069)
0.971–1.062 0.963–1.089 1.003–1053 1.010–1.010 0.999–1.014 0.999–1.034
1.167
0.009
1.040–1.311
1.374
0.007
1.091–1.729
1.003
0.005
1.001–1.005
1.023
(0.131)
0.993–1.053
1.927 2.274
0.002 0.001
1.281–2.897 1.420–3.641
IRR
P -value
95% CI of IRR
1.266
< 0.001
1.111–1.443
1.634
< 0.001
1.251–2.135
1.004
0.003
1.001–1.007
1.032
< 0.001
1.015–1.050
2.363 2.547
< 0.001 < 0.001
1.521–3.674 1.648–3.937
P -values in brackets are not significant (P > 0.05). ´ AGEB, Basic Geo-Statistical Area (Area Geo-Estadística B´asica); OR, odds ratio; 95% CI, 95% confidence interval; IRR, incidence rate ratio.
with the number of houses with positive ovitraps and with the number of Ae. aegypti eggs collected in ovitraps (Table 4). The Container Index was significantly associated only with the number of Ae. aegypti eggs collected at the AGEB level (Table 4).
Correlations between Aedes indices at AGEB level All pairwise correlations between the different Aedes indices and their modifications for pupae at the AGEB level are summarized in Table 5. Each Aedes immature-based index was highly correlated with its corresponding pupal index. Only the relationship between the PCI and the House Index was not significant. The PCI and Breteau Index were marginally significantly correlated (P = 0.05). The strongest correlation occurred between the House Index and Breteau Index.
Discussion This is the first comprehensive study of direct adult collections and proxy measures of Ae. aegypti adult infestation in Mexico. Our statistical analyses, clustered at an operational scale used for Ae. aegypti surveillance throughout Mexico, show that ovitrap collections are a sensitive method for
P -values in brackets are not significant (P > 0.05). ´ AGEB, Basic Geo-Statistical Area (Area Geo-Estadística B´asica); OR, odds ratio; 95% CI, 95% confidence interval; IRR, incidence rate ratio; PCI, Pupae Container Index; PHI, Pupae House Index; PBI, Pupae Breteau Index. Container Index: number of positive containers/containers examined × 100; House Index: number of houses infested by Ae. aegypti immatures/houses inspected × 100; Breteau Index: number of positive containers/houses inspected × 100; PCI, PHI, PBI: indices calculated for pupae only.
detecting Ae. aegypti infestations, that classic Aedes indices are informative only when they are corrected to account for pupae, and that the presence of pupae correlates with adult presence at the household level. These findings also suggest that window screens provide a significant level of protection against peridomestic Ae. aegypti . Garcia-Rejon et al . (2008) reported no correlation between outdoor abundance of larvae or pupae and indoor abundance of adult females collected in the homes of dengue patients in Merida. However, their study was structured to detect the presence of dengue virus-infected mosquitoes in the home environment, and thus their entomological surveys were conducted only in households that had reported recent cases of dengue, rather than a sample representative of the community. By contrast, our study systematically sampled the city of Merida, specifically investigating the relationships between Ae. aegypti infestations at different life stages. It is notoriously challenging to collect adult Ae. aegypti (Reiter & Nathan, 2001; Morrison et al ., 2004b). Detecting the presence of gravid Ae. aegypti using ovitraps has been considered very sensitive, even if there is often no clear correlation between the number of eggs collected in ovitraps and adult abundance (Reiter & Nathan, 2001; Focks, 2003), as was corroborated by the present study. A strong association was observed between the presence of a positive ovitrap collection and the odds of having at least one female Ae. aegypti indoors. This association between outdoor ovitrap and indoor adult positivity suggests that ovitrap collections may represent
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Predicting Ae. aegypti presence and abundance Table 3. Associations between a positive ovitrap at household level and number of Aedes aegypti eggs collected and Aedes immature infestation indicators at household level, after adjusting for window screening and clustering at AGEB level. Positive ovitrap
OR
P -value
95% CI of OR
Positive containers per house, n Pupa-positive containers per house, n Immatures collected per house, n Pupae collected per house, n Positive for immatures, n Positive for pupae, n
1.077 1.280
(0.139) 0.032
0.976–1.188 1.022–1.602
1.000 1.011 1.407 1.496
(0.672) (0.123) 0.032 0.026
Ae. aegypti eggs collected per ovitrap
IRR
Positive containers per house, n Pupa-positive containers per house, n Immatures collected per house, n Pupae collected per house, n Positive for immatures, n Positive for pupae, n
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Table 4. Associations between the number of houses with positive ovitraps [Ovitrap House Index (OHI)] and the number of Aedes aegypti eggs [Ovitrap Density Index (ODI)] collected in ovitraps at AGEB level (adjusting for window screening and number of ovitraps recovered) and Aedes immature indices. OHI
OR
P -value
95% CI of OR
0.999–1.002 0.997–1.026 1.030–1.924 1.050–2.130
Container Index PCI House Index PHI Breteau Index PBI
1.025 1.081 1.004 1.023 1.001 1.013
(0.098) < 0.001 (0.528) 0.038 (0.555) 0.004
0.955–1.056 1.034–1.125 0.991–1.017 1.001–1.046 0.997–1.006 1.004–1.023
P -value
95% CI of IRR
ODI
IRR
P -value
95% CI of IRR
1.097 1.234
(0.062) 0.035
0.995–1.208 1.015–1.500
1.000 1.002 1.391 1.428
(0.816) (0.729) 0.014 0.031
0.998–1.002 0.990–1.015 1.068–1.811 1.034–1.971
Container Index PCI House Index PHI Breteau Index PBI
1.030 1.063 1.012 1.029 1.004 1.015
0.011 < 0.001 (0.091) 0.004 (0.093) < 0.001
1.001–1.054 1.036–1.090 0.998–1.026 1.009–1.050 0.999–1.009 1.007–1.023
P -values in brackets are not significant (P > 0.05). ´ AGEB, Basic Geo-Statistical Area (Area Geo-Estadística B´asica); OR, odds ratio; 95% CI, 95% confidence interval; IRR, incidence rate ratio.
a practical method of monitoring the presence of indoor Aedes females. Current surveillance programmes in Mexico, Brazil and Vietnam actively rely on ovitraps as a proxy measure of Ae. aegypti infestation levels (Secretaría de Salud, 2007; de Melo et al ., 2012; Wu et al ., 2013). Presence or absence surveys based on oviposition traps have been considered more sensitive than the direct collection of adults at particular sites for monitoring the spatial and temporal distributions of adult Ae. aegypti , particularly in areas with low levels of infestation (Furlow & Young, 1970; Ritchie, 1984; PAHO, 1994; Reiter & Gubler, 1997; Focks, 2003). As observed in this study, ovitraps detected on average 4.5 times as many houses positive for Ae. aegypti females as adult indoor collections. As the adult Aedes female is the most epidemiologically important stage for dengue surveillance and control (Morrison et al ., 2008), there remains substantial debate on the applicability of traditional Aedes larvae- or pupae-based indices as indicators of Aedes adult infestations and as accurate measures of dengue entomological risk (Focks, 2003). Important biological (i.e. physiological status) and epidemiological (i.e. location and probability of contact with the host) factors can differentially influence direct indoor and indirect outdoor measures of adult Aedes presence. Therefore, we examined the correlations with larval and pupal measures for both types of data separately. A current criticism of the traditional Aedes indices is that they are poor proxies for adult vector abundance (Focks, 2003). Two key reasons for questioning the strength of associations between the presence of immature stages and that of adults (at least for larvae) are that high mortalities during the larval stage preclude the assumption that larval densities are indicative of adult densities, and that flying adults can be spatially dissociated from their development sites (Morrison
P -values in brackets are not significant (P > 0.05). ´ AGEB, Basic Geo-Statistical Area (Area Geo-Estadística B´asica); OR, odds ratio; 95% CI, 95% confidence interval; IRR, incidence rate ratio; PCI, Pupae Container Index; PHI, Pupae House Index; PBI, Pupae Breteau Index. Container Index: number of positive containers/containers examined × 100; House Index: number of houses infested by Ae. aegypti immatures/houses inspected × 100; Breteau Index: number of positive containers/houses inspected × 100; PCI, PHI, PBI: indices calculated for pupae only.
et al ., 2004b). To overcome the first of these issues, Focks and Chadee (1997) described a method of counting pupae to estimate the number of adult vectors because pupae can be easily identified and counted and pupal numbers are thought to correlate highly with the local population of adult mosquitoes. Results from the present study were contradictory to these predictions because the numbers of both immatures and pupae collected per house correlated with the counts of indoor Ae. aegypti females. The comparison of adult indices and immature indices at the AGEB level in Merida concurs, in part, with criticism of the utility of traditional indices for predicting adult female Ae. aegypti presence and/or abundance; indeed the Container Index and Breteau Index did not show strong correlations with the presence of adult vectors indoors at AGEB level. However, the House Index and PHI were both strongly associated with the presence and number of adult Ae. aegypti collected indoors at AGEB level. Based on the results of several field studies (Chan et al ., 1971; Bang et al ., 1981; Strickman & Kittayapong, 2002; Romero-Vivas & Falconar, 2005), it can be argued that larvaebased indicators that include a measure of larval productivity in medium-sized areas are better associated with adult abundance and occurrence than indices that do not account for such factor. Tun-Lin et al . (1996) reported high correlations between larval and adult densities calculated from outdoor surveys. However, similar associations of total immature counts and adults were not seen at the AGEB level in this study. Although the number of immatures per house was associated with the presence
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Table 5. Pearson’s correlation coefficients and coefficients of determination (R 2 indicated in parenthesis) between Aedes aegypti immature infestation indices recorded in Merida during the rainy season in 2003.
PCI House Index PHI Breteau Index PBI
Container Index
PCI
House Index
PHI
Breteau Index
0.79 0.50 0.42 0.55 0.61
– 0.1(0.09)† 0.43 (0.21) 0.34* 0.64 (0.45)
– – 0.82 (0.70) 0.88 (0.72) 0.75 (0.48)
– – – 0.80 (0.57) 0.88 (0.73)
– – – – 0.89 (0.71)
(0.59) (0.24) (0.18) (0.36) (0.39)
P < 0.05 unless otherwise indicated; *P = 0.05; †P > 0.05. PCI, Pupae Container Index; PHI, Pupae House Index; PBI, Pupae Breteau Index. Container Index: number of positive containers/wet containers examined × 100; House Index: number of houses infested with Ae. aegypti immatures/houses inspected × 100; Breteau Index: number of positive containers/houses inspected × 100; PCI, PHI, PBI: indices calculated for pupae only.
and abundance of Ae. aegypti females inside the same house, neither the total number of immatures nor the total number of pupae collected in an AGEB were associated with the presence and total abundance of Aedes adult females collected indoors at AGEB level. The lack of agreement of indices across spatial scales may relate to the very focal and heterogeneous nature of Ae. aegypti infestations (Getis et al ., 2003). Romero-Vivas and Falconar (2005) reported a negative correlation between larval density (for fourth-instar larvae exclusively) and ODI (from ovitraps placed outdoors or indoors), attributing their findings to density-dependent mortality occurring during the first two larval stages. In the present study, no significant correlation was observed between total immatures or pupae and the egg density recorded in the same house. However, a significant positive correlation was detected between the number of positive containers per house and the presence and abundance of Aedes females indoors, and also between the Container Index and the total number of eggs at AGEB level. Focks (2003), in an influential review of Aedes surveillance methods, questioned the utility of the traditional Aedes indices (container, house and Breteau indices), suggesting they were of limited use partly because of the lack of correlation between the different measures. In the past, different authors have reported variable results on the correlations among the traditional Aedes indices (Tinker, 1978; Tun-Lin et al ., 1996; Focks & Chadee, 1997; Mahadev et al ., 2004; Romero-Vivas & Falconar, 2005). It has been argued that correlations between indices are generally limited (30–50%), even when they are positive and significant (Focks, 2003). The results of the present study show positive correlations among the three traditional indices at AGEB level, but also that the House and Breteau Indices were strongly associated. Highly significant correlations between the Breteau and House Indices have been reported by Tinker (1978) in Jamaica and El Salvador, by Chan (1985) in Singapore, and by Sanchez et al . (2006) in Cuba, among others. They particularly observed that correlations occurred when infestations were low (House Index: < 5%) and weakened as the level of infestation increased (Tinker, 1978; Chan, 1985). At low infestation levels (as in Singapore in the mid-1980s), infestations are often characterized by a single positive container per house (Chan, 1985); as infestations increase, more houses are expected to have multiple foci.
However, it has been observed that at all levels of infestation, houses are most commonly infested by a single focus (Tinker, 1978). This was also observed in the present study: whereas overall findings showed houses to have an average of 5.4 (95% CI 4.7–6.1) Ae. aegypti -positive containers on their patios, the mode was one positive container per house, and the House and Breteau Indices were found to be highly correlated. The complete count of every larva and pupa in every container in a community is too practically difficult to be considered as a routine surveillance measure. In this study, indices based on pupal presence (i.e. the numbers of containers and houses positive for pupae) and ovitrap positivity showed significant associations with all of the adult Aedes female measurements at house level. Furthermore, the number of houses positive for pupae at AGEB level (PHI) was significantly associated with all of the adult Aedes female measurements at the same resolution, and the PCI showed the strongest association with ovitrap indices at AGEB level. Although the data presented herein were collected over 10 years ago, we consider the results remain relevant. Currently there are no methods that can be considered to represent a reference standard for the surveillance of Ae. aegypti (Scott & Morrison, 2008), but the present study has contributed to the debate on which may be more correlated with female presence and abundance. Additionally, Mexico is currently exploring the use of a network of ovitraps for Ae. aegypti surveillance ´ (Hern´andez-Avila et al ., 2013), and the results from this study can be used to inform policies regarding which method may correlate better with female adult abundance. Open windows and doors are important points of entry into a house and such apertures can be screened to prevent the entry of flying insects (Schofield et al ., 1990). Recently, long-lasting insecticidal nets deployed as window nets or indoor curtains in houses have been field-tested with good results in different settings worldwide as part of an integrated environmental management approach to complement and enhance current dengue vector control (Kroeger et al ., 2006; Tun-Lin et al ., 2009; Vanlerberghe et al ., 2011; Rizzo et al ., 2012). Our study validates the importance of window screening in reducing the abundance of Ae. aegypti indoors and contributes to the literature on the importance of evaluating methods of making houses ‘Aedes-proof’ in order to support the reduction of human exposure to Ae. aegypti indoors.
© 2014 The Royal Entomological Society, Medical and Veterinary Entomology, 28, 264–272
Predicting Ae. aegypti presence and abundance The ovitrap has been designated as a tool for Ae. aegypti surveillance, particularly to provide a reliable indirect measure of the presence/absence of adult females and of relative changes in adult female populations (PAHO, 1994; Reiter & Gubler, 1997). Recent innovations of the Dengue Surveillance and Control Programme in Mexico have included the use of ovitraps located outdoors in high-risk and dengue-endemic localities, including Merida. The information arising from this network of ovitraps is currently being employed to evaluate local effects on control methods (i.e. reductions after ultralow-volume spraying), but has also started to be used as an indicator of the relative risk for dengue transmission based on vector abundance (egg counts from ovitrapping). The association between ovitrap (outdoors) positivity and indoor adult positivity observed in this study suggests that ovitrapping may represent a practical method of monitoring the prevalence of Aedes females indoors in that the monitoring or targeting of specific areas selected according to the prevalence of positive catches in ovitraps may generate results similar to those of the monitoring or targeting of adult female prevalence. This relatively low labour-intensive sampling method may be more cost-effective than traditional immature surveys (which are more labour-intensive and require more personnel to cover an entire city). Recent studies in Brazil and Taiwan have demonstrated moderate but significant correlations between ovitrap indices and dengue virus transmission (Dibo et al ., 2008; de Melo et al ., 2012; Wu et al ., 2013), indicating the need for more studies to quantitatively explore the potential of ovitraps to serve as entomological proxies of dengue virus transmission activity.
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