Veterinary Ophthalmology (2015) 18, Supplement 1, 132–136
DOI:10.1111/vop.12226
Tear production, intraocular pressure, and conjunctival bacterial flora in a group of captive black-tailed prairie dogs (Cynomys ludovicianus) Jessica M. Meekins, David Eshar and Amy J. Rankin Department of Clinical Sciences, Kansas State University, Manhattan, KS, USA
Address communications to: J. Meekins Tel.: 785-532-5690 Fax: 785-532-4309 e-mail: [email protected]
Abstract Objective To report ocular diagnostic test parameters and normal conjunctival bacterial flora in captive black-tailed prairie dogs. Animals studied Seventeen black-tailed prairie dogs, ranging in age from approximately 4–6 months to 4.5 years. Eleven males came from a zoo collection and 6 females from a wildlife rehabilitation center. Procedures Complete ocular examination was performed under isoflurane anesthesia. Tear and intraocular pressure (IOP) measurements were performed on both (n = 34) eyes of 17 prairie dogs. Phenol red thread test (PRTT) was performed first, and a modified Schirmer tear test I (mSTTI) was performed 10 min later. Indirect rebound tonometry was performed using the TonoVetâ. Attempts to obtain intraocular pressure measurements using an applanation tonometry instrument were unsuccessful. Conjunctival swab samples (n = 17) were taken from both eyes of each prairie dog and pooled. Results The most common ocular abnormality was acquired eyelid margin defects, present in seven eyes of six prairie dogs (35.3%). Mean SD tear production was 13.6 7.8 mm/15 s (range, 3–30) for PRTT and 1.2 0.9 mm/min (range, 0–4) for mSTTI. Mean SD IOP was 7.7 2.2 mmHg (range, 3–11.4). A Staphylococcus xylosus (7/17; 41.2%) organism and a hemolytic Staphylococcus species (5/17; 29.4%) were most commonly isolated from the prairie dog conjunctival sac. Conclusions There was a moderate prevalence of acquired peri-ocular lesions in this group of captive black-tailed prairie dogs. While widely variable, results of tear test and intraocular pressure measurements are reported. Staphylococcus was the most commonly isolated bacterial genus. Key Words: conjunctival, Cynomys ludovicianus, eye, intraocular pressure, prairie dog, tear test
INTRODUCTION
The black-tailed prairie dog (Cynomys ludovicianus) is a member of the order Rodentia and the family Sciuridae (a family shared by squirrels and other small- or mediumsized rodents).1 The black-tailed prairie dog is one of five keystone species, found in the grasslands of North America, stretching from the United States–Canada border to the United States–Mexico border. Black-tailed prairie dogs are the common species found in zoological collections, research facilities, and private homes.1,2 However, reports of ocular disease in captive prairie dogs are lacking.
To the authors’ knowledge, there are describing normal ocular diagnostic test conjunctival bacterial flora in this rodent. our study was to report basic diagnostic and normal conjunctival bacterial flora prairie dogs.
no publications parameters and The purpose of test parameters in black-tailed
MATERIALS AND METHODS
Seventeen black-tailed prairie dogs (Cynomys ludovicianus) from two zoological collections (Kansas, USA) were admitted for an overall health examination. All © 2014 American College of Veterinary Ophthalmologists
ocular diagnostic testing in black-tailed prairie dogs 133
examinations were performed at the Kansas State University Veterinary Health Center (KSU-VHC). The Kansas State University Institutional Animal Care and Use Committee approved this study. The prairie dogs were transported to the KSU-VHC and then placed under general anesthesia for examination (Fig. 1) after arrival at the hospital. Anesthesia was achieved using a chamber induction with 5% isoflurane gas (IsoFlo, Abbott Laboratories, North Chicago, IL, USA) in 2 L/min of oxygen, followed by a small face mask and a Bain nonrebreathing anesthesia circuit. Anesthesia was maintained on 2.5% isoflurane gas in 1.5 L/min of oxygen. Body temperature was monitored using a handheld thermometer inserted in the rectum and maintained using a warm water blanket and heating packs. Heart rate was monitored using a stethoscope, and oxygen saturation was monitored using a pulse oximeter (Nellcor Handheld Pulse Oximeter N20PA; Covidien, Dublin, Ireland). Visual observations were used to monitor the respiratory rate and depth. Following induction of anesthesia, each animal was weighed and a complete physical examination was performed. Complete blood count, serum biochemistry panel, and whole body radiographs were performed, and all but one prairie dog was deemed healthy. A single board-certified ophthalmologist (JMM) performed all ocular examinations, immediately after anesthetic induction and as part of the physical examination. Examinations were conducted in dim lighting conditions and included slit-lamp biomicroscopy (SL-15; Kowa Co, Tokyo, Japan), indirect binocular ophthalmoscopy (Keeler Vantage Plus; Keeler Instruments, Inc., Broomall, PA, USA), and direct ophthalmoscopy (Welch Allyn direct ophthalmoscope, Skaneateles Falls, NY, USA) after attempts at pharmacologic pupil dilation using 1% tropicamide (tropicamide ophthalmic solution; Akorn, Inc., Lake Forest, IL, USA); assessment of vision status was subjectively noted prior to induction of anesthesia by observing each animal’s ability to navigate within the confines of its transportation kennel.
Figure 1. The general anesthesia setup for examination and data collection.
Diagnostic tests included phenol red thread test (PRTT) (Zone-Quick, Showa Yakuhin Kako Co, Tokyo, Japan), modified (mSTTI) Schirmer tear test I (Schirmer tear test, Intervet, Summit, NJ, USA), and intraocular pressure (IOP) evaluation with the TonoVetâ (Tiolat Oy Lumic International, Baltimore, MD) rebound tonometer. Prior to examination and testing, a coin toss determined the sequence of testing for each eye of each prairie dog, with heads representing the right eye (OD; oculus dexter) and tails representing the left eye (OS; oculus sinister). The PRTT was performed first in all eyes, followed by the mSTTI after a 10-min period to allow for reestablishment of the tear film.3 Schirmer tear test strips were modified as previously described 4 by precisely cutting each strip in half on its longitudinal axis through the protective plastic covering (Fig. 2). This modification was performed to accommodate the small palpebral fissure and apparently low liquid tear volume in these animals. Five consecutive IOP measurements were taken for each eye, and the results were averaged to provide a single reading. TonoVetâ measurements were obtained using the D setting (internal calibration for use in dogs), and only readings with a low error or no error bar were accepted. Initial attempts to obtain intraocular pressure measurements using a second (applanation) tonometry instrument (TonoPen-XLâ) were unsuccessful, so this tonometry technique was discontinued after the first 4 animals. Using a microtip culturette swab (BactiSwab; Starplex Scientific, Etobicoke, ON, Canada), a sample was taken from the conjunctival sac of both eyes of each prairie dog, taking care not to contaminate the swab tip by touching the eyelid margins or peri-ocular skin. Eyes of each individual were pooled by using 1 swab per
Figure 2. The modified Schirmer tear test I performed simultaneously in both eyes of an anesthetized prairie dog. The strips were inserted into the ventrolateral conjunctival fornix with the aid of a small ophthalmic forceps.
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 18, 132–136
134 meekins, eshar and rankin
prairie dog (34 eyes; n = 17 samples). Samples were submitted to the Kansas State University Veterinary Diagnostic Laboratory. Statistical analysis was performed with commercially available computer software (WINKS Statistical Software version 6.0.93; TexaSoft, Cedar Hill, TX). Values obtained for mSTTI, PRTT, and IOP were compared between OD and OS by Wilcoxon’s signed rank test because a normal distribution was not established with the small number of prairie dogs examined; OD and OS were considered as independent measures. The effect of gender on measurement of mSTTI was compared by Mann– Whitney U-test. The effects of weight and age on measurement of PRTT, mSTTI, and IOP were compared by Pearson’s Correlation, with OD and OS considered as independent measurements. Results were considered significant at P ≤ 0.05. Figure 4. An example of the most commonly documented ocular abnormality. An acquired eyelid margin defect was present and involving approximately 30% of the ventromedial lower left eyelid.
RESULTS
Prairie dogs ranged in age from approximately 4–6 months to 4.5 years, and body weights ranging from 582 to 1200 g for 11 males and 759–1073 g for six females. The normal prairie dog eye is illustrated in Fig. 3. The most common ocular abnormality was acquired eyelid margin and peri-ocular area defects in 6/17 (35.3%) of examined prairie dogs (Fig. 4). Additionally, 1 prairie dog had an opaque, mucoid ocular discharge from both eyes. The pupil was poorly responsive to pharmacologic dilation in all animals, limiting the examiner’s ability to perform funduscopy. There were no significant differences between OD and OS for PRTT, mSTTI, or IOP. As the eyes were not different, each eye was considered independent for further analysis. There were no significant differences for PRTT, mSTTI, or IOP by gender, and values of PRTT, mSTTI, and IOP did not correlate with age or weight. Tear production and IOP measurements with ranges are reported in Table 1. Tear production was measured by PRTT and mSTTI in all 17 prairie dogs (34 eyes). The mean PRTT was 13.6 7.8 mm/15 s, and the mean mSTTI was 1.2 0.9 mm/min. Intraocular pressure was measured by
indirect tonometry in all 17 prairie dogs (34 eyes). The mean intraocular pressure was 7.7 2.2 mmHg. In total, 16 types of bacteria were isolated with aerobic culture of the conjunctival sac (Table 2). Only 1 prairie dog had no growth on bacterial culture. The remaining 16 had at least one bacterium isolated, and the majority of animals (12/16; 75%) had 2–4 different bacteria isolated. The most common bacteria were a Staphylococcus xylosus (7/17; 41.2%) organism and a hemolytic Staphylococcus species (5/17; 29.4%); Staphylococcus was the major isolated bacterial genus. DISCUSSION
This is the first study to report ocular diagnostic parameters in black-tailed prairie dogs. While widely variable, the results of tear test and IOP measurements provide a baseline range of normal values to which individual animals can be compared. There are no published clinical studies evaluating tear production in ground squirrels, a close relative of the prairie dog. However, the tear production has been measured in other rodent species, including guinea pigs,5 chinchillas,6 and the world’s largest rodent, the Table 1. Mean SD tear and intraocular pressure measurement results in 17 black-tailed prairie dogs (n = 34 eyes)
Figure 3. The normal right eye of a black-tailed prairie dog.
Test
Result
Range
PRTT (mm/15 s) mSTTI (mm/min) IOP (mmHg)
13.6 7.8 1.2 0.9 7.7 2.2
3–30 0–4 3–11.4
PRTT, Phenol red thread test; mSTTI, modified Schirmer tear test I; IOP, intraocular pressure.
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 18, 132–136
ocular diagnostic testing in black-tailed prairie dogs 135 Table 2. Aerobic bacterial conjunctival culture results obtained from a pooled sample of both eyes in 17 black-tailed prairie dogs. Prevalence (%) of each isolated organism is indicated in descending order from most to least common. One of the 17 animals had no growth on bacterial culture, and the majority (13/17) had ≤2 bacteria isolated Organism
No. of prairie dogs
Staphylococcus xylosus #1 Staphylococcus species (hemolytic) Streptococcus species (alpha-hemolytic) Gram + organism Staphylococcus equorum Escherichia coli (nonhemolytic) Enterococcus avium Aerococcus viridans Staphylococcus xylosus #2 Staphylococcus species (nonhemolytic) Staphylococcus arlettae Staphylococcus warneri Staphylococcus epidermidis Neisseria flavescens Corynebacterium stationis Bacillus flexus
7/17 5/17 4/17 3/17 2/17 2/17 2/17 2/17 1/17 1/17 1/17 1/17 1/17 1/17 1/17 1/17
(41.2%) (29.4%) (23.5%) (17.6%) (11.8%)
(5.9%)
capybara.7 The PRTT was evaluated in guinea pigs and chinchillas; the results obtained in chinchillas (14.6 3.5 mm/15 s)6 more closely resembled the results of the prairie dogs, while those of the guinea pigs were higher (21.26 4.19 mm/15 s).5 Schirmer tear test I (STTI) results were similarly low in guinea pigs and chinchillas when compared to prairie dogs; in fact, only Coster et al.5 reported a value for STTI in guinea pigs (3 mm/min), while Muller et al.6 discontinued this tear measurement technique during the chinchilla study due to low and unreliable readings. We faced a similar challenge with low readings using the STTI strip in prairie dogs and attempted a modification4 at the beginning of data collection in an effort to produce more reliable results. Results were consistently low, with a mean of 1.2 mm/min and a range of 0–4. Unlike rodents of a smaller size, the capybara’s tear production as measured by STTI was more robust, with a mean of 18.4 mm wetting per min.7 The STTI and the PRTT are not necessarily directly comparable because they each measure a different aspect of the aqueous portion of tears. The STTI measures 3 components of the tear film: residual, basal, and reflex tear production stimulated by the presence of an irritating test strip in the conjunctival sac.8 In comparison, the PRTT is considered to measure only residual tears present in the lacrimal lake. This residual component of the aqueous tears is a reflection of the balance between lacrimal secretion, tear drainage, and, especially in cases of disease, tear evaporation.8,9 While no technique has been confirmed to eliminate the contribution of basal tear production to the lacrimal lake volume, the rapid nature of the PRTT minimizes its influence.9,10
Tonometry represents the indirect measurement of IOP and has been reported in other rodent species using different commercially available handheld devices.5–7,11 The mean published IOP values in other rodents, with the exception of the chinchilla, were higher when compared to the IOP readings in prairie dogs. Mean IOP in prairie dogs was 7.7 mmHg, which while being lower than that of most other values published in rodents was higher when compared to the mean chinchilla IOP of 2.9 mmHg.6 Initial attempts were made to gather data and compare IOP obtained by two different methods of indirect tonometry; however, it became evident early in the study that the applanation tonometry instrument used (Tonopen-XLâ) did not produce readings at all, even inconsistently or with increased standard error. The use of sedatives and other anesthetic agents have known effects on tear production and IOP measurements. Immobilizing agents such as ketamine and diazepam, as well as other anesthetic and pre-anesthetic agents, cause a transient decrease in the tear production.12,13 The prairie dogs in our study did not receive any premedication or induction drugs, but instead were chamber-induced with isoflurane inhalant anesthetic. A recent study evaluated the impact of duration and type of inhalant anesthetic on the tear production in Beagle dogs.14 It was discovered that the tear production was significantly reduced during anesthesia, but returned to baseline values immediately after recovery and until 10 h after anesthesia in all groups. Duration and type (isoflurane vs. desflurane) of anesthesia were not significant. It is possible that anesthesia influenced the tear production in the prairie dogs, especially with regard to the mSTTI. Similarly, anesthetic and pre-anesthetic agents influence IOP. The combination of ketamine and diazepam caused significant elevation in IOP in normal dogs.15 A study showed that the inhalant agents sevoflurane and desflurane had no clinically significant effects on IOP in dogs, because although IOPs increased significantly from baseline, values remained within normal IOP limits.16 Other clinical studies in larger rodents used anesthetics for purposes of immobilization, examination, and diagnostic testing.7,11 It is impractical and unsafe for animals and investigators to attempt examination and diagnostic testing in awake wild animals. The circumstances in this study, therefore, closely mimic those that would be encountered in a clinical setting, and our results can be interpreted in light of the anesthetic considerations necessary to evaluate these animals. At least one bacterium was cultured from the conjunctival sac of 16/17 prairie dogs; only one animal had no growth of bacteria. A 4-year-old male prairie dog was euthanized shortly after participation in the study due to a general decline in health. Major findings on necropsy showed bilateral ocular and nasal suppurative discharge and an overall poor body condition. This prairie dog presented with opaque, mucoid ocular discharge from both
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 18, 132–136
136 meekins, eshar and rankin
eyes and was one of two prairie dogs with the maximum of 4 bacteria isolated from the conjunctival sac. In comparing the bacterial isolates from this prairie dog and the other normal prairie dogs, Neisseria flavescens was an isolate not recovered from any other animal. Nonhemolytic Escherichia coli and Enterococcus avium were only isolated from one other animal. These could represent bacteria that are not normal commensals of the prairie dog conjunctival sac. The majority of prairie dogs had 2 or fewer bacteria identified from the conjunctival swab samples, and of these, Staphylococcus was the most common bacterial genus. This leads the authors to infer that the predominant bacterial genus of the black-tailed prairie dog conjunctival sac is Staphylococcus. Conjunctival bacterial isolates in other rodents have been described5,7,11 and include some organisms similar to what were found in the prairie dogs. Micrococcus spp. were isolated from Canadian beavers11 and capybara,7 but were not identified in this group of prairie dogs. The bacteria isolated from these animals could represent the normal flora of the captive blacktailed prairie dog conjunctival surface, and this knowledge may facilitate clinical interpretation of ocular surface culture. It is important to note, though, that the bacterial population on the ocular surface is influenced by environmental factors such as the climate and housing conditions. Samples were obtained from captive prairie dogs in the summer and fall in northeastern Kansas. Although these animals were located at two separate facilities, the housing conditions were similar and both facilities were within 35 miles of each other in the Flint Hills of Kansas. The organisms identified do not necessarily reflect what could be present in prairie dogs living in the wild, although the Midwestern prairie is the main natural habitat of this animal. Study limitations include the necessity of general anesthesia to examine and collect diagnostic data, and the use of a single time point for data collection of variables that may be influenced by physiologic variation. The practical considerations in examining wild animals under general anesthesia have already been addressed. Certain quantifiable variables in the study, such as tear production and intraocular pressure, may be additionally influenced by physiologic variation. While we took a series of measurements with low standard error and averaged the readings to achieve a reading for each eye, ideally, IOP should be measured at several time points throughout the day and over several consecutive days to compare and identify physiologic fluctuations. The prevalence of ocular lesions was considered moderate in this group of black-tailed prairie dogs. While widely variable, tear test results and intraocular pressure measurements are reported. Results of aerobic bacterial conjunctival culture in predominantly normal prairie dogs provide information for the diagnosis and management of ocular disease.
ACKNOWLEDGMENTS
The authors thank Dr. James Roush for his assistance with statistical analysis.
REFERENCES 1. Hoogland JL, James DA, Watson L. Nutrition, care, and behavior of captive prairie dogs. The Veterinary Clinics of North America. Exotic Animal Practice 2009; 12: 255–266. 2. Lennox AM. Emergency and critical care procedures in sugar gliders (Petaurus breviceps), African hedgehogs (Atelerix albiventris), and prairie dogs (Cynomys spp). The Veterinary Clinics of North America. Exotic Animal Practice 2007; 10: 533–555. 3. Brown MH, Galland JC, Davidson HJ et al. The phenol red thread tear test in dogs. Veterinary & Comparative Ophthalmology 1996; 6: 274–277. 4. Korbel RLP. Clinical estimation of lacrimal function in various bird species using a modified Schrimer Tear Test. Israel Journal of Veterinary Medicine 1996; 51: 171–175. 5. Coster ME, Stiles J, Krohne SG et al. Results of diagnostic ophthalmic testing in healthy guinea pigs. Journal of the American Veterinary Medical Association 2008; 232: 1825–1833. 6. Muller K, Mauler DA, Eule JC. Reference values for selected ophthalmic diagnostic tests and clinical characteristics of chinchilla eyes (Chinchilla lanigera). Veterinary Ophthalmology 2010; 13(Suppl): 29–34. 7. Montiani-Ferreira F, Truppel J, Tramontin MH et al. The capybara eye: clinical tests, anatomic and biometric features. Veterinary Ophthalmology 2008; 11: 386–394. 8. Saito A, Kotani T. Estimation of lacrimal level and testing methods on normal beagles. Veterinary Ophthalmology 2001; 4: 7–11. 9. Yokoi N, Kinoshita S, Bron AJ et al. Tear meniscus changes during cotton thread and Schirmer testing. Investigative Ophthalmology & Visual Science 2000; 41: 3748–3753. 10. Trost K, Skalicky M, Nell B. Schirmer tear test, phenol red thread tear test, eye blink frequency and corneal sensitivity in the guinea pig. Veterinary Ophthalmology 2007; 10: 143–146. 11. Cullen CL. Normal ocular features, conjunctival microflora and intraocular pressure in the Canadian beaver (Castor canadensis). Veterinary Ophthalmology 2003; 6: 279–284. 12. Herring IP, Pickett JP, Champagne ES et al. Evaluation of aqueous tear production in dogs following general anesthesia. Journal of the American Animal Hospital Association 2000; 36: 427– 430. 13. Dodam JR, Branson KR, Martin DD. Effects of intramuscular sedative and opioid combinations on tear production in dogs. Veterinary Ophthalmology 1998; 1: 57–59. 14. Shepard MK, Accola PJ, Lopez LA et al. Effect of duration and type of anesthetic on tear production in dogs. American Journal of Veterinary Research 2011; 72: 608–612. 15. Hofmeister EH, Mosunic CB, Torres BT et al. Effects of ketamine, diazepam, and their combination on intraocular pressures in clinically normal dogs. American Journal of Veterinary Research 2006; 67: 1136–1139. 16. Almeida DE, Rezende ML, Nunes N et al. Evaluation of intraocular pressure in association with cardiovascular parameters in normocapnic dogs anesthetized with sevoflurane and desflurane. Veterinary Ophthalmology 2004; 7: 265–269.
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 18, 132–136
DOI:10.1111/vop.12226
Tear production, intraocular pressure, and conjunctival bacterial flora in a group of captive black-tailed prairie dogs (Cynomys ludovicianus) Jessica M. Meekins, David Eshar and Amy J. Rankin Department of Clinical Sciences, Kansas State University, Manhattan, KS, USA
Address communications to: J. Meekins Tel.: 785-532-5690 Fax: 785-532-4309 e-mail: [email protected]
Abstract Objective To report ocular diagnostic test parameters and normal conjunctival bacterial flora in captive black-tailed prairie dogs. Animals studied Seventeen black-tailed prairie dogs, ranging in age from approximately 4–6 months to 4.5 years. Eleven males came from a zoo collection and 6 females from a wildlife rehabilitation center. Procedures Complete ocular examination was performed under isoflurane anesthesia. Tear and intraocular pressure (IOP) measurements were performed on both (n = 34) eyes of 17 prairie dogs. Phenol red thread test (PRTT) was performed first, and a modified Schirmer tear test I (mSTTI) was performed 10 min later. Indirect rebound tonometry was performed using the TonoVetâ. Attempts to obtain intraocular pressure measurements using an applanation tonometry instrument were unsuccessful. Conjunctival swab samples (n = 17) were taken from both eyes of each prairie dog and pooled. Results The most common ocular abnormality was acquired eyelid margin defects, present in seven eyes of six prairie dogs (35.3%). Mean SD tear production was 13.6 7.8 mm/15 s (range, 3–30) for PRTT and 1.2 0.9 mm/min (range, 0–4) for mSTTI. Mean SD IOP was 7.7 2.2 mmHg (range, 3–11.4). A Staphylococcus xylosus (7/17; 41.2%) organism and a hemolytic Staphylococcus species (5/17; 29.4%) were most commonly isolated from the prairie dog conjunctival sac. Conclusions There was a moderate prevalence of acquired peri-ocular lesions in this group of captive black-tailed prairie dogs. While widely variable, results of tear test and intraocular pressure measurements are reported. Staphylococcus was the most commonly isolated bacterial genus. Key Words: conjunctival, Cynomys ludovicianus, eye, intraocular pressure, prairie dog, tear test
INTRODUCTION
The black-tailed prairie dog (Cynomys ludovicianus) is a member of the order Rodentia and the family Sciuridae (a family shared by squirrels and other small- or mediumsized rodents).1 The black-tailed prairie dog is one of five keystone species, found in the grasslands of North America, stretching from the United States–Canada border to the United States–Mexico border. Black-tailed prairie dogs are the common species found in zoological collections, research facilities, and private homes.1,2 However, reports of ocular disease in captive prairie dogs are lacking.
To the authors’ knowledge, there are describing normal ocular diagnostic test conjunctival bacterial flora in this rodent. our study was to report basic diagnostic and normal conjunctival bacterial flora prairie dogs.
no publications parameters and The purpose of test parameters in black-tailed
MATERIALS AND METHODS
Seventeen black-tailed prairie dogs (Cynomys ludovicianus) from two zoological collections (Kansas, USA) were admitted for an overall health examination. All © 2014 American College of Veterinary Ophthalmologists
ocular diagnostic testing in black-tailed prairie dogs 133
examinations were performed at the Kansas State University Veterinary Health Center (KSU-VHC). The Kansas State University Institutional Animal Care and Use Committee approved this study. The prairie dogs were transported to the KSU-VHC and then placed under general anesthesia for examination (Fig. 1) after arrival at the hospital. Anesthesia was achieved using a chamber induction with 5% isoflurane gas (IsoFlo, Abbott Laboratories, North Chicago, IL, USA) in 2 L/min of oxygen, followed by a small face mask and a Bain nonrebreathing anesthesia circuit. Anesthesia was maintained on 2.5% isoflurane gas in 1.5 L/min of oxygen. Body temperature was monitored using a handheld thermometer inserted in the rectum and maintained using a warm water blanket and heating packs. Heart rate was monitored using a stethoscope, and oxygen saturation was monitored using a pulse oximeter (Nellcor Handheld Pulse Oximeter N20PA; Covidien, Dublin, Ireland). Visual observations were used to monitor the respiratory rate and depth. Following induction of anesthesia, each animal was weighed and a complete physical examination was performed. Complete blood count, serum biochemistry panel, and whole body radiographs were performed, and all but one prairie dog was deemed healthy. A single board-certified ophthalmologist (JMM) performed all ocular examinations, immediately after anesthetic induction and as part of the physical examination. Examinations were conducted in dim lighting conditions and included slit-lamp biomicroscopy (SL-15; Kowa Co, Tokyo, Japan), indirect binocular ophthalmoscopy (Keeler Vantage Plus; Keeler Instruments, Inc., Broomall, PA, USA), and direct ophthalmoscopy (Welch Allyn direct ophthalmoscope, Skaneateles Falls, NY, USA) after attempts at pharmacologic pupil dilation using 1% tropicamide (tropicamide ophthalmic solution; Akorn, Inc., Lake Forest, IL, USA); assessment of vision status was subjectively noted prior to induction of anesthesia by observing each animal’s ability to navigate within the confines of its transportation kennel.
Figure 1. The general anesthesia setup for examination and data collection.
Diagnostic tests included phenol red thread test (PRTT) (Zone-Quick, Showa Yakuhin Kako Co, Tokyo, Japan), modified (mSTTI) Schirmer tear test I (Schirmer tear test, Intervet, Summit, NJ, USA), and intraocular pressure (IOP) evaluation with the TonoVetâ (Tiolat Oy Lumic International, Baltimore, MD) rebound tonometer. Prior to examination and testing, a coin toss determined the sequence of testing for each eye of each prairie dog, with heads representing the right eye (OD; oculus dexter) and tails representing the left eye (OS; oculus sinister). The PRTT was performed first in all eyes, followed by the mSTTI after a 10-min period to allow for reestablishment of the tear film.3 Schirmer tear test strips were modified as previously described 4 by precisely cutting each strip in half on its longitudinal axis through the protective plastic covering (Fig. 2). This modification was performed to accommodate the small palpebral fissure and apparently low liquid tear volume in these animals. Five consecutive IOP measurements were taken for each eye, and the results were averaged to provide a single reading. TonoVetâ measurements were obtained using the D setting (internal calibration for use in dogs), and only readings with a low error or no error bar were accepted. Initial attempts to obtain intraocular pressure measurements using a second (applanation) tonometry instrument (TonoPen-XLâ) were unsuccessful, so this tonometry technique was discontinued after the first 4 animals. Using a microtip culturette swab (BactiSwab; Starplex Scientific, Etobicoke, ON, Canada), a sample was taken from the conjunctival sac of both eyes of each prairie dog, taking care not to contaminate the swab tip by touching the eyelid margins or peri-ocular skin. Eyes of each individual were pooled by using 1 swab per
Figure 2. The modified Schirmer tear test I performed simultaneously in both eyes of an anesthetized prairie dog. The strips were inserted into the ventrolateral conjunctival fornix with the aid of a small ophthalmic forceps.
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 18, 132–136
134 meekins, eshar and rankin
prairie dog (34 eyes; n = 17 samples). Samples were submitted to the Kansas State University Veterinary Diagnostic Laboratory. Statistical analysis was performed with commercially available computer software (WINKS Statistical Software version 6.0.93; TexaSoft, Cedar Hill, TX). Values obtained for mSTTI, PRTT, and IOP were compared between OD and OS by Wilcoxon’s signed rank test because a normal distribution was not established with the small number of prairie dogs examined; OD and OS were considered as independent measures. The effect of gender on measurement of mSTTI was compared by Mann– Whitney U-test. The effects of weight and age on measurement of PRTT, mSTTI, and IOP were compared by Pearson’s Correlation, with OD and OS considered as independent measurements. Results were considered significant at P ≤ 0.05. Figure 4. An example of the most commonly documented ocular abnormality. An acquired eyelid margin defect was present and involving approximately 30% of the ventromedial lower left eyelid.
RESULTS
Prairie dogs ranged in age from approximately 4–6 months to 4.5 years, and body weights ranging from 582 to 1200 g for 11 males and 759–1073 g for six females. The normal prairie dog eye is illustrated in Fig. 3. The most common ocular abnormality was acquired eyelid margin and peri-ocular area defects in 6/17 (35.3%) of examined prairie dogs (Fig. 4). Additionally, 1 prairie dog had an opaque, mucoid ocular discharge from both eyes. The pupil was poorly responsive to pharmacologic dilation in all animals, limiting the examiner’s ability to perform funduscopy. There were no significant differences between OD and OS for PRTT, mSTTI, or IOP. As the eyes were not different, each eye was considered independent for further analysis. There were no significant differences for PRTT, mSTTI, or IOP by gender, and values of PRTT, mSTTI, and IOP did not correlate with age or weight. Tear production and IOP measurements with ranges are reported in Table 1. Tear production was measured by PRTT and mSTTI in all 17 prairie dogs (34 eyes). The mean PRTT was 13.6 7.8 mm/15 s, and the mean mSTTI was 1.2 0.9 mm/min. Intraocular pressure was measured by
indirect tonometry in all 17 prairie dogs (34 eyes). The mean intraocular pressure was 7.7 2.2 mmHg. In total, 16 types of bacteria were isolated with aerobic culture of the conjunctival sac (Table 2). Only 1 prairie dog had no growth on bacterial culture. The remaining 16 had at least one bacterium isolated, and the majority of animals (12/16; 75%) had 2–4 different bacteria isolated. The most common bacteria were a Staphylococcus xylosus (7/17; 41.2%) organism and a hemolytic Staphylococcus species (5/17; 29.4%); Staphylococcus was the major isolated bacterial genus. DISCUSSION
This is the first study to report ocular diagnostic parameters in black-tailed prairie dogs. While widely variable, the results of tear test and IOP measurements provide a baseline range of normal values to which individual animals can be compared. There are no published clinical studies evaluating tear production in ground squirrels, a close relative of the prairie dog. However, the tear production has been measured in other rodent species, including guinea pigs,5 chinchillas,6 and the world’s largest rodent, the Table 1. Mean SD tear and intraocular pressure measurement results in 17 black-tailed prairie dogs (n = 34 eyes)
Figure 3. The normal right eye of a black-tailed prairie dog.
Test
Result
Range
PRTT (mm/15 s) mSTTI (mm/min) IOP (mmHg)
13.6 7.8 1.2 0.9 7.7 2.2
3–30 0–4 3–11.4
PRTT, Phenol red thread test; mSTTI, modified Schirmer tear test I; IOP, intraocular pressure.
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 18, 132–136
ocular diagnostic testing in black-tailed prairie dogs 135 Table 2. Aerobic bacterial conjunctival culture results obtained from a pooled sample of both eyes in 17 black-tailed prairie dogs. Prevalence (%) of each isolated organism is indicated in descending order from most to least common. One of the 17 animals had no growth on bacterial culture, and the majority (13/17) had ≤2 bacteria isolated Organism
No. of prairie dogs
Staphylococcus xylosus #1 Staphylococcus species (hemolytic) Streptococcus species (alpha-hemolytic) Gram + organism Staphylococcus equorum Escherichia coli (nonhemolytic) Enterococcus avium Aerococcus viridans Staphylococcus xylosus #2 Staphylococcus species (nonhemolytic) Staphylococcus arlettae Staphylococcus warneri Staphylococcus epidermidis Neisseria flavescens Corynebacterium stationis Bacillus flexus
7/17 5/17 4/17 3/17 2/17 2/17 2/17 2/17 1/17 1/17 1/17 1/17 1/17 1/17 1/17 1/17
(41.2%) (29.4%) (23.5%) (17.6%) (11.8%)
(5.9%)
capybara.7 The PRTT was evaluated in guinea pigs and chinchillas; the results obtained in chinchillas (14.6 3.5 mm/15 s)6 more closely resembled the results of the prairie dogs, while those of the guinea pigs were higher (21.26 4.19 mm/15 s).5 Schirmer tear test I (STTI) results were similarly low in guinea pigs and chinchillas when compared to prairie dogs; in fact, only Coster et al.5 reported a value for STTI in guinea pigs (3 mm/min), while Muller et al.6 discontinued this tear measurement technique during the chinchilla study due to low and unreliable readings. We faced a similar challenge with low readings using the STTI strip in prairie dogs and attempted a modification4 at the beginning of data collection in an effort to produce more reliable results. Results were consistently low, with a mean of 1.2 mm/min and a range of 0–4. Unlike rodents of a smaller size, the capybara’s tear production as measured by STTI was more robust, with a mean of 18.4 mm wetting per min.7 The STTI and the PRTT are not necessarily directly comparable because they each measure a different aspect of the aqueous portion of tears. The STTI measures 3 components of the tear film: residual, basal, and reflex tear production stimulated by the presence of an irritating test strip in the conjunctival sac.8 In comparison, the PRTT is considered to measure only residual tears present in the lacrimal lake. This residual component of the aqueous tears is a reflection of the balance between lacrimal secretion, tear drainage, and, especially in cases of disease, tear evaporation.8,9 While no technique has been confirmed to eliminate the contribution of basal tear production to the lacrimal lake volume, the rapid nature of the PRTT minimizes its influence.9,10
Tonometry represents the indirect measurement of IOP and has been reported in other rodent species using different commercially available handheld devices.5–7,11 The mean published IOP values in other rodents, with the exception of the chinchilla, were higher when compared to the IOP readings in prairie dogs. Mean IOP in prairie dogs was 7.7 mmHg, which while being lower than that of most other values published in rodents was higher when compared to the mean chinchilla IOP of 2.9 mmHg.6 Initial attempts were made to gather data and compare IOP obtained by two different methods of indirect tonometry; however, it became evident early in the study that the applanation tonometry instrument used (Tonopen-XLâ) did not produce readings at all, even inconsistently or with increased standard error. The use of sedatives and other anesthetic agents have known effects on tear production and IOP measurements. Immobilizing agents such as ketamine and diazepam, as well as other anesthetic and pre-anesthetic agents, cause a transient decrease in the tear production.12,13 The prairie dogs in our study did not receive any premedication or induction drugs, but instead were chamber-induced with isoflurane inhalant anesthetic. A recent study evaluated the impact of duration and type of inhalant anesthetic on the tear production in Beagle dogs.14 It was discovered that the tear production was significantly reduced during anesthesia, but returned to baseline values immediately after recovery and until 10 h after anesthesia in all groups. Duration and type (isoflurane vs. desflurane) of anesthesia were not significant. It is possible that anesthesia influenced the tear production in the prairie dogs, especially with regard to the mSTTI. Similarly, anesthetic and pre-anesthetic agents influence IOP. The combination of ketamine and diazepam caused significant elevation in IOP in normal dogs.15 A study showed that the inhalant agents sevoflurane and desflurane had no clinically significant effects on IOP in dogs, because although IOPs increased significantly from baseline, values remained within normal IOP limits.16 Other clinical studies in larger rodents used anesthetics for purposes of immobilization, examination, and diagnostic testing.7,11 It is impractical and unsafe for animals and investigators to attempt examination and diagnostic testing in awake wild animals. The circumstances in this study, therefore, closely mimic those that would be encountered in a clinical setting, and our results can be interpreted in light of the anesthetic considerations necessary to evaluate these animals. At least one bacterium was cultured from the conjunctival sac of 16/17 prairie dogs; only one animal had no growth of bacteria. A 4-year-old male prairie dog was euthanized shortly after participation in the study due to a general decline in health. Major findings on necropsy showed bilateral ocular and nasal suppurative discharge and an overall poor body condition. This prairie dog presented with opaque, mucoid ocular discharge from both
© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 18, 132–136
136 meekins, eshar and rankin
eyes and was one of two prairie dogs with the maximum of 4 bacteria isolated from the conjunctival sac. In comparing the bacterial isolates from this prairie dog and the other normal prairie dogs, Neisseria flavescens was an isolate not recovered from any other animal. Nonhemolytic Escherichia coli and Enterococcus avium were only isolated from one other animal. These could represent bacteria that are not normal commensals of the prairie dog conjunctival sac. The majority of prairie dogs had 2 or fewer bacteria identified from the conjunctival swab samples, and of these, Staphylococcus was the most common bacterial genus. This leads the authors to infer that the predominant bacterial genus of the black-tailed prairie dog conjunctival sac is Staphylococcus. Conjunctival bacterial isolates in other rodents have been described5,7,11 and include some organisms similar to what were found in the prairie dogs. Micrococcus spp. were isolated from Canadian beavers11 and capybara,7 but were not identified in this group of prairie dogs. The bacteria isolated from these animals could represent the normal flora of the captive blacktailed prairie dog conjunctival surface, and this knowledge may facilitate clinical interpretation of ocular surface culture. It is important to note, though, that the bacterial population on the ocular surface is influenced by environmental factors such as the climate and housing conditions. Samples were obtained from captive prairie dogs in the summer and fall in northeastern Kansas. Although these animals were located at two separate facilities, the housing conditions were similar and both facilities were within 35 miles of each other in the Flint Hills of Kansas. The organisms identified do not necessarily reflect what could be present in prairie dogs living in the wild, although the Midwestern prairie is the main natural habitat of this animal. Study limitations include the necessity of general anesthesia to examine and collect diagnostic data, and the use of a single time point for data collection of variables that may be influenced by physiologic variation. The practical considerations in examining wild animals under general anesthesia have already been addressed. Certain quantifiable variables in the study, such as tear production and intraocular pressure, may be additionally influenced by physiologic variation. While we took a series of measurements with low standard error and averaged the readings to achieve a reading for each eye, ideally, IOP should be measured at several time points throughout the day and over several consecutive days to compare and identify physiologic fluctuations. The prevalence of ocular lesions was considered moderate in this group of black-tailed prairie dogs. While widely variable, tear test results and intraocular pressure measurements are reported. Results of aerobic bacterial conjunctival culture in predominantly normal prairie dogs provide information for the diagnosis and management of ocular disease.
ACKNOWLEDGMENTS
The authors thank Dr. James Roush for his assistance with statistical analysis.
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© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 18, 132–136
