Annals of Tropical Medicine & Parasitology
ISSN: 0003-4983 (Print) 1364-8594 (Online) Journal homepage: http://www.tandfonline.com/loi/ypgh19
Schistosome infection rates in field snails: Schistosoma mansoni in Biomphalaria pfeifferi from Kenya R. F. Sturrock, S. J. Karamsadkar & J. Ouma To cite this article: R. F. Sturrock, S. J. Karamsadkar & J. Ouma (1979) Schistosome infection rates in field snails: Schistosoma mansoni in Biomphalaria pfeifferi from Kenya, Annals of Tropical Medicine & Parasitology, 73:4, 369-375, DOI: 10.1080/00034983.1979.11687272 To link to this article: http://dx.doi.org/10.1080/00034983.1979.11687272
Published online: 11 Mar 2016.
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Date: 23 August 2017, At: 09:12
Annals of Tropical Medicine and Parasitology, Vol. 73, No.4 (1979)
Schistosome infection rates in field snails: Schistosoma mansoni in Biomphalaria pfeijferi from Kenya BY R. F. STURROCK, S. J. KARAMSADKAR
Wellcome Trust Research Laboratories, P.O. Box 43640, Nairobi, Kenya
AND]. OUMA Downloaded by [Australian Catholic University] at 09:12 23 August 2017
Division of Vector Borne Diseases, Ministry of Health, Nairobi, Kenya Received 3 July 1978 The measurement of infection rates in field populations of snails has been used for many years in epidemiological studies of schistosomiasis (Pesigan et al., 1958; Webbe, 1962a, b; Chu and Dawood, 1970b; Sturrock, 1973a). The data obtained can provide valuable information in planning molluscicidal and chemotherapeutic control of transmission (Dazo et al., 1966; Webbe, 1965, Jordan and Webbe, 1969; Chu and Dawood, 1970a; Sturrock, 1973b). The data can be compared with the predictions from mathematical models such as those of Hairston (1965), Macdonald (1965), Goffman and Warren (1970) and Hirsch and Nasell (1975), although Fine and Lehman (1977) found that predicted rates are generally much higher than those observed in the field. This anomaly may be explained in part because observed rates usually record mature, 'patent' infections and ignore immature 'prepatent' infections which are not yet shedding cercariae. There are virtually no published data to show the relationship, if any, between 'patent' and 'prepatent' infection rates. Field snail infection rates are generally measured in one of two ways: crushing or shedding (Webbe, 1965). The first involves the low power microscopical examination of crushed snails (Pesigan et al., 1958; Chernin and Dunaven, 1962; Chu and Dawood, 1970a). This reveals cercariae from 'patent' infections which can then, with some difficulty, be used to infect laboratory animals so as to assist in the identification of the particular schistosome species. Theoretically, high power microscopy should reveal immature stages of the parasite (mother and daughter sporocysts and, later on, germ balls of developing cercariae in the by then diffuse and hardly recognizable daughter sporocysts). Upatham (1970) detected daughter sporocysts 11-19 days after he had exposed laboratory-bred snails to Schistosoma mansoni, and he saw developing cercariae from day 19 onwards. He also distinguished daughter sporocysts in laboratory-bred snails exposed 12 days previously in contaminated field sites (Upatham, 1976). However, it is uncertain whether daughter sporocysts and, more especially, the less distinctive mother sporocysts and germ balls would be easily distinguished in naturally infected field snail crushes. The second method involves searching for shed cercariae by naked eye or low power microscope from individual snails illuminated in glass tubes containing a small volume of water; this technique has been widely used (Pesigan et al., 1958; Chu and Dawood, 1970a), especially in longitudinal field studies in which the snails are returned to the field to avoid artificial depletion of small snail populations (Web be, 1962a, b; Sturrock, 1973a). Mass shedding in field snails has also been used to detect infected snail colonies but gives no information on the actual infection rate (Butler et al., 1971). Examination of the snails is quick and simple, and any cercariae can easily be used to infect experimental animals. 0003--4983/79/040369+07 SOI.00/0
© 1979 Liverpool School of Tropical Medicine
370
SCHISTOSOME INFECTION RATES IN KENYAN FIELD SNAILS
However, the advantage of keeping the snails alive is plainly offset by the failure to detect 'prepatent' infections. It is also conceivable that the stress suffered by the snails during collection and transportation to the laboratory might prevent some infected snails from shedding cercariae if the examination period is relatively short. This paper reports the results of examining several field collections of Biomphalaria pfeifferi for both 'patent' and 'prepatent' S. mansoni infections in order to determine the relationship, if any, between the two.
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MATERIALS AND METHODS
Field Snail Collections Collections I and II were made in the Nairobi area on 29 September 1976 and 16 March 1977 respectively. Collections III to VI were made in the Lower Nduu Area ofMachakos District on 19 and 23 October 1976 and on 16 February and 24 August 1977, respectively. Only B. pfeifferi were examined. The Nairobi site was a slow-flowing stream running through the Kibera suburb to the Nairobi dam. Around the site primitive houses, without sanitation or water supplies, accommodate a human population which is too unstable for the prevalence of S. mansoni to be estimated with accuracy. Infected snails have been recovered from the site since 1974 (Sturrock et al., 1976) during dry periods from January to March and from August to December, when the snail numbers are usually highest. Other snails species present were Lymnaea natalensis and Bulinus tropicus. Snails were collected in the Machakos district from slow-flowing or stagnant pools in tributaries of the Kalala River. The high human prevalence of S. mansoni in this area has been documented by Siongok et al. (1976). Other snail species collected included B.forskalii, B. (Physopsis) africanus, L. natalensis and Gyraulus sp. The snails were collected in metal scoops. The Nairobi snails were placed in damp vegetation in metal trays and carried by car to the main Nairobi laboratory within 30 minutes of collection. The Machakos snails were treated similarly except that they were held overnight at a field laboratory before being transported the 100 km to Nairobi. Examination for S. mansoni Infection On arrival in the Nairobi laboratory, all the snails from each collection were examined by Method A. They were set up individually in 25 X 50 mm glass specimen tubes each containing about 10 ml of filtered water from the Nairobi site, and were exposed immediately and again the following morning to artificial light for 2 h, the optimal time for detecting S. mansoni cercariae (Webbe, 1962a, b). The tubes were examined with a low power binocular microscope and any snails seen to be shedding S. mansoni cercariae were removed. About 50-60 of the remaining snails (or half in the case of the smaller collections I and III) were selected at random and crushed (Method B) as described by Chernin and Dunaven (1962). The remaining non-shedding snails were examined by Method C. They were kept in aerated five-litre glass aquaria at 25°C, at densities of less than ten snails per litre and fed on lettuce. They were examined individually for cercaria! shedding as in method A at weekly intervals for the following five weeks. RESULTS The results of the three tests for infection with S. mansoni on the six collections of field snails are summarized in the Table.
189 195
24 (30·0) 30 (34·5)
81 87 9 (4-6)
3 (1·5)
54 0 135 0 135 0 105 2 (1·5) 105 2 (1·5) 97 2 (1·5) 88 2 (1·5)
4 (9·8) 0 2 (5·0) 4 (10·0) 8 (20·0) 12 (30·0) 12 (30·0)
6 (3·1)
41 40 40 37 37 37 35
195
6 (6·9)
87
Ill
100
66
33 33 33 32 29 29 29
100
34 (34-0)
0
0 0 0 0 0 0 0
34 (34·0)
T+ve (%)
(19.10.76)
Machakos
IV
121
117
62 55 55 53 49 45 42
121
4 (3·3)
0
0 0 0 0 0 0 0
4 (3-3)
T +ve (%)
(23.10.76)
Machakos
v
117
107
52 55 45 41 41 37 37
117
24 (20·5)
14 (12·7)
0 0 6 (10•9) 6 (10·9) 6 (10·9) 6 (10·9) 7 (12·7)
10 (8·5)
T +ve (%)
(16.2.77)
Machakos
VI
0 0 0 5 5 5 19
(1·8) (1·8) (1·8) (7·2)
5 (1·6)
318 28 (8·8)
313 23 (7·2)
49 264 254 218 216 216 216
318
T+ve (%)
(24.8.77)
Machakos
• Total number of snails examined. (InC, diminishing numbers due to snail mortality during holding period.) t Number of snails infected with S. m~nsoni expressed as a percentage in brackets. (InC, the cumulative total of infected snails is shown and expressed as a percentage of the initial number of snails on day 0.)
(a) PATENT INFECTIONS Immediate shedding (Method A) (b) PREPATENT INFECTION (i) Immediate crushing (Method B) 0 (ii) Repeated I shedding on 2 weeks 3 (Method C) 4 5 (c) EsTIMATED (i) Extra +ve snails in A from C (ii) Total +ve snails (Methods A & C)
T*+ve (%)t
T+ve (%)
(16.3.77)
(22.9.76)
--
II Nairobi
Nairobi
I
Showing the S. mansoni infection rate among six .field collections q[Biomphalaria pfeifferi examined in three ways: immediate cercaria[ shedding, immediate crushing and repeated cercaria[ shedding
TABLE
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Cll
.... _,
r-
:...
...,t::l
0 Q ?::
~
c
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372
SCHISTOSOME INFECTION RATES IN KENYAN FIELD SNAILS
Method A (shedding) gave an overall 'patent' infection rate of 6·9% (65/938). The rates for the individual collections ranged from 3·1% to 34%. Method B (crushing) was used on 33% (291/873) of the remaining and apparently uninfected snails. None yielded matureS. mansoni cercariae, showing that Method A had detected all the patent infections. Secondary sporocysts from 'prepatent' infections were found in 1·4% (4/291) of these snails-all from collection I. Assuming that the average prepatent period for S. mansoni in snails at 25°C is 27 days (Foster, 1964), that young secondary sporocysts are clearly detectable between days II and 19 after infection (Upatham, 1970) and that there is an equal chance of any snail having a prepatent infection at any given stage of development, then the observed infection rate might be multiplied two or three times in order to estimate the true 'prepatent' infection rate. The estimate would probably still be too low because the variable amounts of tissue from field snails of different sizes, variable pigmentation of these tissues, other extraneous organisms and, in some cases, dirt, all hindered the examination of the crushed field snails, even by very experienced microscopists. Method C (serial shedding) was used on 67% (582/873) of the apparently uninfected snails. During the five-week holding period 6·9% (40/582) of these snails began to shed S. mansoni cercariae, denoting the presence of 'prepatent' infections at the time of collection. The cumulative totals of snails starting to shed cercariae were 8, I 7, 21, 25 and 40 in weeks l-5 respectively. Although these figures indicate a fairly steady rate of maturation of infections with time, examination of the individual collection data showed that this was not the case in three of the collections (II, V and VI), in which the majority of the infections matured at weeks 2, I and 5 respectively. Thus, one cannot assume that all the apparently uninfected snails had an equal chance of an infection at any stage of maturation within the prepatent period. As 23·2% (135/582) of these snails died within the five-week holding period, and there was no reason to assume that this mortality was due to S. mansoni infection, the true 'prepatent' infection rate could well have been about 8-9%. Among the different collections, the observed rates ranged from zero to 30%, and immature infections were detected in three collections (II, V and VI) from which none were detected by Method B. Using the data from Method C, the number was estimated proportionately of 'prepatent' infections present in each collection at the time of sampling. Adding these 'prepatent' infections to the 'patent' infections detected by Method A permitted the calculation of the 'total' infection rate at the time of collection (see Table, section (c)]. There was no clear correlation between the 'total' infection rate and either the 'patent' or 'prepatent' rates: in collections I and VI the 'total' equalled the 'patent' rate; in II and V it comprised a mixture of the two; while in III and IV 'prepatent' infections predominated. DISCUSSION The simplest and most widely used method for the determination of snail infection rates, especially for longitudinal transmission studies, is Method A, which gave a reliable estimate of the 'patent' infection rate, and was not, apparently, affected by the stress suffered by the snails before examination began in the case of the Machakos collections. Unfortunately, Method A does not detect 'prepatent' infections and, as our data amply demonstrate, the 'patent' and 'prepatent' infection rates are not necessarily correlated. Of the two methods used to detect 'prepatent' infections, serial shedding (Method C) was considerably more effective than crushing (Method B), despite quite substantial snail mortality during the holding period. Serial shedding was also simpler and did not require
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STURROCK ET AL.
373
skilled microscopists. Unfortunately, it is not suitable for large scale, longitudinal field studies. Failure to return the snails to the field has already been mentioned as a disadvantage in studying small field populations: for large field snail populations, inordinately large snail holding facilities would be required. In practice, the repeated use of Method A on field populations at two- or three-weekly intervals is in some ways equivalent to Method C, although not exactly. Successive rises in the 'patent' infection rate reflect the maturation of 'prepatent' infections. Moreover, mortality in the laboratory during the holding period is replaced by at least an estimate of the natural mortality which occurs in the field. An interesting result was that the maturation of 'prepatent' infections occurred in pulses in three collections, two of which were from Machakos. This implies that infection of the snails and, hence, faecal contamination of their habitats, occurs irregularly rather than as a steady, continuous process, even in a very heavily endemic area. Whether such contamination is random or, as seems more probable in view of the focal nature of schistosomiasis transmission (Bradley et al., 1967; Bradley, 1972), non-random, cannot be ascertained from our limited number of sites which, in any case, had been deliberately selected because they were known to contain infected snails. Discontinuities in the contamination of snail populations could have important implications for mathematical modellers (Fine and Lehman, 1977), particularly if taken in conjunction with the observation that not all snail colonies within an endemic area are involved in transmission (Sturrock, 1973a). Mathematical models of schistosome transmission should differentiate between those snail colonies which are involved in transmission and those which are not. Within colonies active in transmission 'prepatent' infections will not be detected, unless special techniques are used to identify them, especially in snails less than about four weeks old, which may form a significant proportion of the total population (Dazo et al., 1966; Jordan and Web be, 1969; Sturrock, 1973a). Failure to incorporate these features may well lead to the prediction of infection rates higher than the 'patent' rates normally observed in the field; a discrepancy which has been noted by Fine and Lehman (1977). Their statement that 'reported infection rates of snails in endemic areas are typically lower than would be predicted on theoretical grounds ... ' is undoubtedly true, if average 'patent' infection rates for large areas are being considered, for S. mansoni, S. haematobium and S. japonicum in their various snail hosts in different countries (Pesigan et al., 1958; Webbe, 1962a; Chu and Dawood, 1970b; Sturrock, 1973a). If, however, data from individual snail colonies for a specific time are considered, most of these authors record 'patent' rates of at least 30% and Webbe and Jordan (1966) report values of 50-80°/o. These rates, of course, concern the snails actually involved in active transmission at that time. For a full description of transmission one would need to know the total cercaria} output from these snails and this, in turn, is governed by many variables, among them the age and intensity of the infections, the nutritional status of the snails and climatic conditions. However, judging by our present findings, 'total' infection rates could well be approaching 100% in some, at least, of these colonies because 'prepatent' infections are excluded. One factor, which might prevent 'patent' infection rates ever achieving the highest predicted theoretical levels, is differential mortality-which produces an increased death rate among infected snails (Hairston, 1965; Sturrock and Sturrock, 1970, 1971 ; Sturrock and Webbe, 1971; Cohen, 1973; Sturrock et al., 1975). Macdonald (1965) deliberately ignored differential mortality, on the grounds that its effects would be quantitative and would not affect the essential dynamics of the transmission he was trying to describe. Hairston ( 1971) implied that Macdonald used unrealistically high 'patent' infection rates in snails when predicting the efficacy of certain control measures with his model. The examples already cited show that very high 'patent' infection rates can, and do, occur in nature at the individual site or snail population level rather than at the area
374
SCHISTOSOME INFECTION RATES IN KENYAN FIELD SNAILS
level. At the site level, the high rates assumed by Macdonald are probably close to reality, although it is not clear whether they would have a limiting role on transmission in the area as a whole.
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SUMMARY Six collections of Biomphalaria pfeifferi were made in the Nairobi and Machakos areas of Kenya. Individual cercaria! shedding (Method A) showed that 6·9% (65/938) of the snails had mature 'patent' Schistosoma mansoni infections (range 3·1-34% in the six collections). Crushing (Method B) about one-third of the remaining snails detected no additional mature infections but yielded 1·4% (four snails from one collection) with secondary sporocysts from immature 'prepatent' infections. Weekly shedding observations for five weeks (Method C) on the remainder of the snails showed that 6·9% (40 snails from four collections-range 1·5-30%) shed cercariae maturing from infections immature at the time of collection. As Method B detected no cercariae, Method A apparently detected all 'patent' infections reliably, despite the stress suffered by the snails before examination. Method C was simpler and more efficient than Method B in the detection of infections which were immature at the time of collection. Judging from the rate at which infections matured in Method C, field snails in heavily endemic areas are subjected to pulses of infection rather than to a continuous flow of miracidia. The 'patent' infection rate was not correlated to the 'prepatent' infection rate. Some of the implications of these findings to mathematical models of schistosome transmission are discussed. The work was supported by the Wellcome Trust and formed part of a collaborative project with the World Health Organization and the National Public Health Laboratories of the Government of Kenya. This paper is published by kind permission of the Director of Medical Services, Ministry of Health, Kenya. ACKNOWLEDGEMENTS.
REFERENCES BRADLEY, D. J. (1972). Regulation of parasite populations. A general theory of the epidemiology and control of parasitic infections. Transactions of the Royal Society of Tropical Medicine and Hygiene, 66, 697-708. BRADLEY, D. J., STURROCK, R. F. & WILLIAMS, P. N. (1967). Circumstantial epidemiology of Schistosoma haematobium in Lango District, Uganda. East African Medical Journal, 44, 194-204. BuTLER, J. M., RUiz-TIBEN, E. & FERGUSON, F. F. (1971). Evaluation of two methods for the detection of Schistosoma mansoni cercariae shed by Biomphalaria glabrata. American Journal of Tropical Medicine and Hygiene, 20, 157-159. CHERNIN, E. & DuNAVEN, C. A. (1962). The influence of host-parasite dispersion upon the capacity of Schistosoma mansoni miracidia to infect Australorbis glabratus. American Journal rif Tropical Medicine and Hygiene, 11, 455-471. CHu, K. Y. & DAWOOD, I. K. (1970a). Cercaria) production from Biomphalaria alexandrina infected with Schistosoma mansoni. Bulletin of the World Health Organization, 4:2, 569-574. CHU, K. Y. & DAWOOD, I. K. (1970b). Cercarial transmission seasons of Schistosoma mansoni in the Nile Delta. Bulletin of the World Health Organization, 4:2, 575-580. CoHEN,j. E. (1973). Selected host mortality in a catalytic model applied to schistosomiasis. American Naturalist, 107, 198-212. DAzo, B. C., HAIRSTON, N. S. & DAWOOD, I. K. (1966). The ecology of Bulinus truncatus and Biomphalaria alexandrina and its implications for the control of bilharzia in the Egypt-49 project. Bulletin of the World Health Organization, 35, 339-356. FINE, P. E. M. & LEHMAN, J. S. (1977). Mathematical models of schistosomiasis: report of a workshop. American Journal rif Tropical Medicine and Hygiene, 26, 500-504. FosTER, R. (1964). The effect of temperature on the development of Schistosoma mansoni Sambon 1907 in the intermediate host. Journal rif Tropical Medicine and Hygiene, 67, 289-292.
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GoFFMAN, W. & WARREN, K. S. (1970). An application of the Kermack-McKendrick theory to the epidemiology of schistosomiasis. American Journal of Tropical Medicine and Hygiene, 19, 278-283. HAIRSTON, N. S. (1965). On the mathematical analysis of schistosome populations. Bulletin rif the World Health Organization, 33, 4~2. HAIRSTON, N. S. (1971). Book review of Human Schistosomiasis by P. Jordan and G. Webbe. American Journal of Tropical Medicine and Hygiene, 20, 164-165. HIRSCH, W. M. & NASELL, I. (1975). The transmission and control of schistosome infections. In Mathematical Anarysis rif Decision Problems in Ecology, ed. Chemes, A. & Lynn, W. New York and Heidelberg: SpringerVerlag. jORDAN, P. & WEBBE, G. (1969). Human Schistosomiasis. Springfield, Illinois: Charles C. Thomas. MACDONALD, G. (1965). The dynamics of helminth infections with special reference to schistosomes. Transactions rif the Royal Society rif Tropical Medicine and Hygiene, 59, 489-506. PESIGAN, T. P., HAIRSTON, N. s., JAUREGUI, J. J., GARCIA, E. G., SANTOS, A. T., SANTOS, B. c. & BESA, A. A. (1958). Studies on Schistosomajaponicum infection in the Philippines. 2. The molluscan host. Bulletin of the World Health Organization, 18, 481-578. SIONGOK, T. K. A., MAHMOUD, A. A. F., 0UMA, J. H., WARREN, K. s., MULLER, A. s., HANDA, A. K. & HousER, H. B. (1976). Morbidity in schistosomiasis mansoni in relation to intensity of infection: study of a community in Machakos, Kenya. AmJrican Journal of Tropical Medicine and Hygiene, 25, 273-284. STURROCK, B. M. & STuRROCK, R. F. (1970). Laboratory studies on the host-parasite relationship of Schistosoma m!111Soni and Biomphalaria glabrata from St. Lucia, West Indies. Annals of Tropical Medicine and Parasitology, 64, 357-363. STURROCK, R. F. (1973a). Field studies on the transmission of Schistosoma mansoni and on the bionomics of its intermediate host, Biomphalaria glabrata, on St Lucia, West Indies. International Journal for Parasitology, 3, 175-194. STURROCK, R. F. (1973b). Field studies on the population dynamics of Biomphalaria glabrata, intermediate host of Schistosoma mansoni on the West Indian island of St Lucia. International Journal for Parasitology, 3, 165-174. STURROCK, R. F., BuTTERWORTH, A. E. & HousA, V. (1976). Schistosoma mansoni in the baboon (Papio anubis): parasitological responses of Kenyan baboons to different exposures of a local parasite. Parasitology, 73, 239-252. STURROCK, R. F., CoHEN, J. E. & WEBBE, G. (1975). Catalytic curve analysis of schistosomiasis in snails. Annals of Tropical Medicine and Parasitology, 69, 133-134. STURROCK, R. F. & STURROCK, B. M. ( 1971). Shell abnormalities in Biomphalaria glabrata infected with Schistosoma mansoni and their significance in field transmission studies. Journal of Helminthology, 45, 201-212. STURROCK, R. F. & WEBBE, G. ( 1971). The application of catalytic models to schistosomiasis in snails. Journal of Helminthology, 45, 189-200. UPATHAM, E. S. ( 1970). Bionomics of miracidia rif Schistosoma mansoni. Ph.D. thesis. University of Michigan, pp. 41-42. UPATHAM, E. S. (1976). Field studies on the bionomics of the free-living stages of St Lucian Schistosoma mansoni. International Journal for Parasitology, 6, 239-245. WEBBE, G. (1962a). The transmission of Schistosoma haematobium in an area of Lake Province, Tanganyika. Bulletin of the World Health Organization, 27, 59-85. WEBBE, G. (1962b). Population studies on intermediate hosts in relation to transmission of Bilharzia in East Africa. In Bilharziasis: Ciba Foundation Symposium, ed. Wolstenholme, G. E. & O'Connor, M., pp. 7-22. London: Churchill. WEBBE, G. (1965). Transmission of bilharziasis 2. Production of cercariae. Bulletin of the World Health Organization, 33, 155-162. WEBBE, G. & joRDAN, P. ( 1966). Recent advances in knowledge of schistosomiasis in E:tst Africa. Transactions rifthe Royal Society rif Tropical Medicine and Hygiene, 60, 279-312.
ISSN: 0003-4983 (Print) 1364-8594 (Online) Journal homepage: http://www.tandfonline.com/loi/ypgh19
Schistosome infection rates in field snails: Schistosoma mansoni in Biomphalaria pfeifferi from Kenya R. F. Sturrock, S. J. Karamsadkar & J. Ouma To cite this article: R. F. Sturrock, S. J. Karamsadkar & J. Ouma (1979) Schistosome infection rates in field snails: Schistosoma mansoni in Biomphalaria pfeifferi from Kenya, Annals of Tropical Medicine & Parasitology, 73:4, 369-375, DOI: 10.1080/00034983.1979.11687272 To link to this article: http://dx.doi.org/10.1080/00034983.1979.11687272
Published online: 11 Mar 2016.
Submit your article to this journal
View related articles
Citing articles: 11 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ypgh19 Download by: [Australian Catholic University]
Date: 23 August 2017, At: 09:12
Annals of Tropical Medicine and Parasitology, Vol. 73, No.4 (1979)
Schistosome infection rates in field snails: Schistosoma mansoni in Biomphalaria pfeijferi from Kenya BY R. F. STURROCK, S. J. KARAMSADKAR
Wellcome Trust Research Laboratories, P.O. Box 43640, Nairobi, Kenya
AND]. OUMA Downloaded by [Australian Catholic University] at 09:12 23 August 2017
Division of Vector Borne Diseases, Ministry of Health, Nairobi, Kenya Received 3 July 1978 The measurement of infection rates in field populations of snails has been used for many years in epidemiological studies of schistosomiasis (Pesigan et al., 1958; Webbe, 1962a, b; Chu and Dawood, 1970b; Sturrock, 1973a). The data obtained can provide valuable information in planning molluscicidal and chemotherapeutic control of transmission (Dazo et al., 1966; Webbe, 1965, Jordan and Webbe, 1969; Chu and Dawood, 1970a; Sturrock, 1973b). The data can be compared with the predictions from mathematical models such as those of Hairston (1965), Macdonald (1965), Goffman and Warren (1970) and Hirsch and Nasell (1975), although Fine and Lehman (1977) found that predicted rates are generally much higher than those observed in the field. This anomaly may be explained in part because observed rates usually record mature, 'patent' infections and ignore immature 'prepatent' infections which are not yet shedding cercariae. There are virtually no published data to show the relationship, if any, between 'patent' and 'prepatent' infection rates. Field snail infection rates are generally measured in one of two ways: crushing or shedding (Webbe, 1965). The first involves the low power microscopical examination of crushed snails (Pesigan et al., 1958; Chernin and Dunaven, 1962; Chu and Dawood, 1970a). This reveals cercariae from 'patent' infections which can then, with some difficulty, be used to infect laboratory animals so as to assist in the identification of the particular schistosome species. Theoretically, high power microscopy should reveal immature stages of the parasite (mother and daughter sporocysts and, later on, germ balls of developing cercariae in the by then diffuse and hardly recognizable daughter sporocysts). Upatham (1970) detected daughter sporocysts 11-19 days after he had exposed laboratory-bred snails to Schistosoma mansoni, and he saw developing cercariae from day 19 onwards. He also distinguished daughter sporocysts in laboratory-bred snails exposed 12 days previously in contaminated field sites (Upatham, 1976). However, it is uncertain whether daughter sporocysts and, more especially, the less distinctive mother sporocysts and germ balls would be easily distinguished in naturally infected field snail crushes. The second method involves searching for shed cercariae by naked eye or low power microscope from individual snails illuminated in glass tubes containing a small volume of water; this technique has been widely used (Pesigan et al., 1958; Chu and Dawood, 1970a), especially in longitudinal field studies in which the snails are returned to the field to avoid artificial depletion of small snail populations (Web be, 1962a, b; Sturrock, 1973a). Mass shedding in field snails has also been used to detect infected snail colonies but gives no information on the actual infection rate (Butler et al., 1971). Examination of the snails is quick and simple, and any cercariae can easily be used to infect experimental animals. 0003--4983/79/040369+07 SOI.00/0
© 1979 Liverpool School of Tropical Medicine
370
SCHISTOSOME INFECTION RATES IN KENYAN FIELD SNAILS
However, the advantage of keeping the snails alive is plainly offset by the failure to detect 'prepatent' infections. It is also conceivable that the stress suffered by the snails during collection and transportation to the laboratory might prevent some infected snails from shedding cercariae if the examination period is relatively short. This paper reports the results of examining several field collections of Biomphalaria pfeifferi for both 'patent' and 'prepatent' S. mansoni infections in order to determine the relationship, if any, between the two.
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MATERIALS AND METHODS
Field Snail Collections Collections I and II were made in the Nairobi area on 29 September 1976 and 16 March 1977 respectively. Collections III to VI were made in the Lower Nduu Area ofMachakos District on 19 and 23 October 1976 and on 16 February and 24 August 1977, respectively. Only B. pfeifferi were examined. The Nairobi site was a slow-flowing stream running through the Kibera suburb to the Nairobi dam. Around the site primitive houses, without sanitation or water supplies, accommodate a human population which is too unstable for the prevalence of S. mansoni to be estimated with accuracy. Infected snails have been recovered from the site since 1974 (Sturrock et al., 1976) during dry periods from January to March and from August to December, when the snail numbers are usually highest. Other snails species present were Lymnaea natalensis and Bulinus tropicus. Snails were collected in the Machakos district from slow-flowing or stagnant pools in tributaries of the Kalala River. The high human prevalence of S. mansoni in this area has been documented by Siongok et al. (1976). Other snail species collected included B.forskalii, B. (Physopsis) africanus, L. natalensis and Gyraulus sp. The snails were collected in metal scoops. The Nairobi snails were placed in damp vegetation in metal trays and carried by car to the main Nairobi laboratory within 30 minutes of collection. The Machakos snails were treated similarly except that they were held overnight at a field laboratory before being transported the 100 km to Nairobi. Examination for S. mansoni Infection On arrival in the Nairobi laboratory, all the snails from each collection were examined by Method A. They were set up individually in 25 X 50 mm glass specimen tubes each containing about 10 ml of filtered water from the Nairobi site, and were exposed immediately and again the following morning to artificial light for 2 h, the optimal time for detecting S. mansoni cercariae (Webbe, 1962a, b). The tubes were examined with a low power binocular microscope and any snails seen to be shedding S. mansoni cercariae were removed. About 50-60 of the remaining snails (or half in the case of the smaller collections I and III) were selected at random and crushed (Method B) as described by Chernin and Dunaven (1962). The remaining non-shedding snails were examined by Method C. They were kept in aerated five-litre glass aquaria at 25°C, at densities of less than ten snails per litre and fed on lettuce. They were examined individually for cercaria! shedding as in method A at weekly intervals for the following five weeks. RESULTS The results of the three tests for infection with S. mansoni on the six collections of field snails are summarized in the Table.
189 195
24 (30·0) 30 (34·5)
81 87 9 (4-6)
3 (1·5)
54 0 135 0 135 0 105 2 (1·5) 105 2 (1·5) 97 2 (1·5) 88 2 (1·5)
4 (9·8) 0 2 (5·0) 4 (10·0) 8 (20·0) 12 (30·0) 12 (30·0)
6 (3·1)
41 40 40 37 37 37 35
195
6 (6·9)
87
Ill
100
66
33 33 33 32 29 29 29
100
34 (34-0)
0
0 0 0 0 0 0 0
34 (34·0)
T+ve (%)
(19.10.76)
Machakos
IV
121
117
62 55 55 53 49 45 42
121
4 (3·3)
0
0 0 0 0 0 0 0
4 (3-3)
T +ve (%)
(23.10.76)
Machakos
v
117
107
52 55 45 41 41 37 37
117
24 (20·5)
14 (12·7)
0 0 6 (10•9) 6 (10·9) 6 (10·9) 6 (10·9) 7 (12·7)
10 (8·5)
T +ve (%)
(16.2.77)
Machakos
VI
0 0 0 5 5 5 19
(1·8) (1·8) (1·8) (7·2)
5 (1·6)
318 28 (8·8)
313 23 (7·2)
49 264 254 218 216 216 216
318
T+ve (%)
(24.8.77)
Machakos
• Total number of snails examined. (InC, diminishing numbers due to snail mortality during holding period.) t Number of snails infected with S. m~nsoni expressed as a percentage in brackets. (InC, the cumulative total of infected snails is shown and expressed as a percentage of the initial number of snails on day 0.)
(a) PATENT INFECTIONS Immediate shedding (Method A) (b) PREPATENT INFECTION (i) Immediate crushing (Method B) 0 (ii) Repeated I shedding on 2 weeks 3 (Method C) 4 5 (c) EsTIMATED (i) Extra +ve snails in A from C (ii) Total +ve snails (Methods A & C)
T*+ve (%)t
T+ve (%)
(16.3.77)
(22.9.76)
--
II Nairobi
Nairobi
I
Showing the S. mansoni infection rate among six .field collections q[Biomphalaria pfeifferi examined in three ways: immediate cercaria[ shedding, immediate crushing and repeated cercaria[ shedding
TABLE
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372
SCHISTOSOME INFECTION RATES IN KENYAN FIELD SNAILS
Method A (shedding) gave an overall 'patent' infection rate of 6·9% (65/938). The rates for the individual collections ranged from 3·1% to 34%. Method B (crushing) was used on 33% (291/873) of the remaining and apparently uninfected snails. None yielded matureS. mansoni cercariae, showing that Method A had detected all the patent infections. Secondary sporocysts from 'prepatent' infections were found in 1·4% (4/291) of these snails-all from collection I. Assuming that the average prepatent period for S. mansoni in snails at 25°C is 27 days (Foster, 1964), that young secondary sporocysts are clearly detectable between days II and 19 after infection (Upatham, 1970) and that there is an equal chance of any snail having a prepatent infection at any given stage of development, then the observed infection rate might be multiplied two or three times in order to estimate the true 'prepatent' infection rate. The estimate would probably still be too low because the variable amounts of tissue from field snails of different sizes, variable pigmentation of these tissues, other extraneous organisms and, in some cases, dirt, all hindered the examination of the crushed field snails, even by very experienced microscopists. Method C (serial shedding) was used on 67% (582/873) of the apparently uninfected snails. During the five-week holding period 6·9% (40/582) of these snails began to shed S. mansoni cercariae, denoting the presence of 'prepatent' infections at the time of collection. The cumulative totals of snails starting to shed cercariae were 8, I 7, 21, 25 and 40 in weeks l-5 respectively. Although these figures indicate a fairly steady rate of maturation of infections with time, examination of the individual collection data showed that this was not the case in three of the collections (II, V and VI), in which the majority of the infections matured at weeks 2, I and 5 respectively. Thus, one cannot assume that all the apparently uninfected snails had an equal chance of an infection at any stage of maturation within the prepatent period. As 23·2% (135/582) of these snails died within the five-week holding period, and there was no reason to assume that this mortality was due to S. mansoni infection, the true 'prepatent' infection rate could well have been about 8-9%. Among the different collections, the observed rates ranged from zero to 30%, and immature infections were detected in three collections (II, V and VI) from which none were detected by Method B. Using the data from Method C, the number was estimated proportionately of 'prepatent' infections present in each collection at the time of sampling. Adding these 'prepatent' infections to the 'patent' infections detected by Method A permitted the calculation of the 'total' infection rate at the time of collection (see Table, section (c)]. There was no clear correlation between the 'total' infection rate and either the 'patent' or 'prepatent' rates: in collections I and VI the 'total' equalled the 'patent' rate; in II and V it comprised a mixture of the two; while in III and IV 'prepatent' infections predominated. DISCUSSION The simplest and most widely used method for the determination of snail infection rates, especially for longitudinal transmission studies, is Method A, which gave a reliable estimate of the 'patent' infection rate, and was not, apparently, affected by the stress suffered by the snails before examination began in the case of the Machakos collections. Unfortunately, Method A does not detect 'prepatent' infections and, as our data amply demonstrate, the 'patent' and 'prepatent' infection rates are not necessarily correlated. Of the two methods used to detect 'prepatent' infections, serial shedding (Method C) was considerably more effective than crushing (Method B), despite quite substantial snail mortality during the holding period. Serial shedding was also simpler and did not require
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373
skilled microscopists. Unfortunately, it is not suitable for large scale, longitudinal field studies. Failure to return the snails to the field has already been mentioned as a disadvantage in studying small field populations: for large field snail populations, inordinately large snail holding facilities would be required. In practice, the repeated use of Method A on field populations at two- or three-weekly intervals is in some ways equivalent to Method C, although not exactly. Successive rises in the 'patent' infection rate reflect the maturation of 'prepatent' infections. Moreover, mortality in the laboratory during the holding period is replaced by at least an estimate of the natural mortality which occurs in the field. An interesting result was that the maturation of 'prepatent' infections occurred in pulses in three collections, two of which were from Machakos. This implies that infection of the snails and, hence, faecal contamination of their habitats, occurs irregularly rather than as a steady, continuous process, even in a very heavily endemic area. Whether such contamination is random or, as seems more probable in view of the focal nature of schistosomiasis transmission (Bradley et al., 1967; Bradley, 1972), non-random, cannot be ascertained from our limited number of sites which, in any case, had been deliberately selected because they were known to contain infected snails. Discontinuities in the contamination of snail populations could have important implications for mathematical modellers (Fine and Lehman, 1977), particularly if taken in conjunction with the observation that not all snail colonies within an endemic area are involved in transmission (Sturrock, 1973a). Mathematical models of schistosome transmission should differentiate between those snail colonies which are involved in transmission and those which are not. Within colonies active in transmission 'prepatent' infections will not be detected, unless special techniques are used to identify them, especially in snails less than about four weeks old, which may form a significant proportion of the total population (Dazo et al., 1966; Jordan and Web be, 1969; Sturrock, 1973a). Failure to incorporate these features may well lead to the prediction of infection rates higher than the 'patent' rates normally observed in the field; a discrepancy which has been noted by Fine and Lehman (1977). Their statement that 'reported infection rates of snails in endemic areas are typically lower than would be predicted on theoretical grounds ... ' is undoubtedly true, if average 'patent' infection rates for large areas are being considered, for S. mansoni, S. haematobium and S. japonicum in their various snail hosts in different countries (Pesigan et al., 1958; Webbe, 1962a; Chu and Dawood, 1970b; Sturrock, 1973a). If, however, data from individual snail colonies for a specific time are considered, most of these authors record 'patent' rates of at least 30% and Webbe and Jordan (1966) report values of 50-80°/o. These rates, of course, concern the snails actually involved in active transmission at that time. For a full description of transmission one would need to know the total cercaria} output from these snails and this, in turn, is governed by many variables, among them the age and intensity of the infections, the nutritional status of the snails and climatic conditions. However, judging by our present findings, 'total' infection rates could well be approaching 100% in some, at least, of these colonies because 'prepatent' infections are excluded. One factor, which might prevent 'patent' infection rates ever achieving the highest predicted theoretical levels, is differential mortality-which produces an increased death rate among infected snails (Hairston, 1965; Sturrock and Sturrock, 1970, 1971 ; Sturrock and Webbe, 1971; Cohen, 1973; Sturrock et al., 1975). Macdonald (1965) deliberately ignored differential mortality, on the grounds that its effects would be quantitative and would not affect the essential dynamics of the transmission he was trying to describe. Hairston ( 1971) implied that Macdonald used unrealistically high 'patent' infection rates in snails when predicting the efficacy of certain control measures with his model. The examples already cited show that very high 'patent' infection rates can, and do, occur in nature at the individual site or snail population level rather than at the area
374
SCHISTOSOME INFECTION RATES IN KENYAN FIELD SNAILS
level. At the site level, the high rates assumed by Macdonald are probably close to reality, although it is not clear whether they would have a limiting role on transmission in the area as a whole.
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SUMMARY Six collections of Biomphalaria pfeifferi were made in the Nairobi and Machakos areas of Kenya. Individual cercaria! shedding (Method A) showed that 6·9% (65/938) of the snails had mature 'patent' Schistosoma mansoni infections (range 3·1-34% in the six collections). Crushing (Method B) about one-third of the remaining snails detected no additional mature infections but yielded 1·4% (four snails from one collection) with secondary sporocysts from immature 'prepatent' infections. Weekly shedding observations for five weeks (Method C) on the remainder of the snails showed that 6·9% (40 snails from four collections-range 1·5-30%) shed cercariae maturing from infections immature at the time of collection. As Method B detected no cercariae, Method A apparently detected all 'patent' infections reliably, despite the stress suffered by the snails before examination. Method C was simpler and more efficient than Method B in the detection of infections which were immature at the time of collection. Judging from the rate at which infections matured in Method C, field snails in heavily endemic areas are subjected to pulses of infection rather than to a continuous flow of miracidia. The 'patent' infection rate was not correlated to the 'prepatent' infection rate. Some of the implications of these findings to mathematical models of schistosome transmission are discussed. The work was supported by the Wellcome Trust and formed part of a collaborative project with the World Health Organization and the National Public Health Laboratories of the Government of Kenya. This paper is published by kind permission of the Director of Medical Services, Ministry of Health, Kenya. ACKNOWLEDGEMENTS.
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GoFFMAN, W. & WARREN, K. S. (1970). An application of the Kermack-McKendrick theory to the epidemiology of schistosomiasis. American Journal of Tropical Medicine and Hygiene, 19, 278-283. HAIRSTON, N. S. (1965). On the mathematical analysis of schistosome populations. Bulletin rif the World Health Organization, 33, 4~2. HAIRSTON, N. S. (1971). Book review of Human Schistosomiasis by P. Jordan and G. Webbe. American Journal of Tropical Medicine and Hygiene, 20, 164-165. HIRSCH, W. M. & NASELL, I. (1975). The transmission and control of schistosome infections. In Mathematical Anarysis rif Decision Problems in Ecology, ed. Chemes, A. & Lynn, W. New York and Heidelberg: SpringerVerlag. jORDAN, P. & WEBBE, G. (1969). Human Schistosomiasis. Springfield, Illinois: Charles C. Thomas. MACDONALD, G. (1965). The dynamics of helminth infections with special reference to schistosomes. Transactions rif the Royal Society rif Tropical Medicine and Hygiene, 59, 489-506. PESIGAN, T. P., HAIRSTON, N. s., JAUREGUI, J. J., GARCIA, E. G., SANTOS, A. T., SANTOS, B. c. & BESA, A. A. (1958). Studies on Schistosomajaponicum infection in the Philippines. 2. The molluscan host. Bulletin of the World Health Organization, 18, 481-578. SIONGOK, T. K. A., MAHMOUD, A. A. F., 0UMA, J. H., WARREN, K. s., MULLER, A. s., HANDA, A. K. & HousER, H. B. (1976). Morbidity in schistosomiasis mansoni in relation to intensity of infection: study of a community in Machakos, Kenya. AmJrican Journal of Tropical Medicine and Hygiene, 25, 273-284. STURROCK, B. M. & STuRROCK, R. F. (1970). Laboratory studies on the host-parasite relationship of Schistosoma m!111Soni and Biomphalaria glabrata from St. Lucia, West Indies. Annals of Tropical Medicine and Parasitology, 64, 357-363. STURROCK, R. F. (1973a). Field studies on the transmission of Schistosoma mansoni and on the bionomics of its intermediate host, Biomphalaria glabrata, on St Lucia, West Indies. International Journal for Parasitology, 3, 175-194. STURROCK, R. F. (1973b). Field studies on the population dynamics of Biomphalaria glabrata, intermediate host of Schistosoma mansoni on the West Indian island of St Lucia. International Journal for Parasitology, 3, 165-174. STURROCK, R. F., BuTTERWORTH, A. E. & HousA, V. (1976). Schistosoma mansoni in the baboon (Papio anubis): parasitological responses of Kenyan baboons to different exposures of a local parasite. Parasitology, 73, 239-252. STURROCK, R. F., CoHEN, J. E. & WEBBE, G. (1975). Catalytic curve analysis of schistosomiasis in snails. Annals of Tropical Medicine and Parasitology, 69, 133-134. STURROCK, R. F. & STURROCK, B. M. ( 1971). Shell abnormalities in Biomphalaria glabrata infected with Schistosoma mansoni and their significance in field transmission studies. Journal of Helminthology, 45, 201-212. STURROCK, R. F. & WEBBE, G. ( 1971). The application of catalytic models to schistosomiasis in snails. Journal of Helminthology, 45, 189-200. UPATHAM, E. S. ( 1970). Bionomics of miracidia rif Schistosoma mansoni. Ph.D. thesis. University of Michigan, pp. 41-42. UPATHAM, E. S. (1976). Field studies on the bionomics of the free-living stages of St Lucian Schistosoma mansoni. International Journal for Parasitology, 6, 239-245. WEBBE, G. (1962a). The transmission of Schistosoma haematobium in an area of Lake Province, Tanganyika. Bulletin of the World Health Organization, 27, 59-85. WEBBE, G. (1962b). Population studies on intermediate hosts in relation to transmission of Bilharzia in East Africa. In Bilharziasis: Ciba Foundation Symposium, ed. Wolstenholme, G. E. & O'Connor, M., pp. 7-22. London: Churchill. WEBBE, G. (1965). Transmission of bilharziasis 2. Production of cercariae. Bulletin of the World Health Organization, 33, 155-162. WEBBE, G. & joRDAN, P. ( 1966). Recent advances in knowledge of schistosomiasis in E:tst Africa. Transactions rifthe Royal Society rif Tropical Medicine and Hygiene, 60, 279-312.
