The chinchilla eye - Wiley Online Library

Veterinary Ophthalmology (2010) 13, Supplement 1, 14–25

The chinchilla eye: morphologic observations, echobiometric findings and reference values for selected ophthalmic diagnostic tests Leandro Lima,* Fabaino Montiani-Ferreira,*,† Mariana Tramontin,* Lucianne Leigue dos Santos,* Marcello Machado,‡,§ Roge´rio Ribas Lange* and Heloisa Helena Abil Russ– *Universidade Federal do Parana´ (UFPR), Departamento de Medicina Veterina´ria, Rua dos Funciona´rios, 1540, 80035-050, Curitiba – PR, Brazil; †Michigan State University, Department of Small Animal Clinical Sciences, D-208, Veterinary Medical Center, 48824-1314 East Lansing, MI, USA; ‡Universidade do Contestado (UnC) Campus de Conco´rdia - Rua Victor Sopelsa, 3000, 89700-000, Conco´rdia – SC, Brazil; §FMVZ/USP - Setor de Anatomia Animal, Departmento de Cirurgia, Universidade de Sa˜o Paulo, Av. Prof. Dr. Orlando Marques de Paiva, 87, 05508 270 - Cidade Universita´ria, Sa˜o Paulo- SP – Brazil; and –Faculdade Evange´lica de Medicina de Curitiba, Rua Professora Rosa Saporski, 690; 80810–120, Curitiba – PR, Brazil

Address communications to: Dr. F. Montiani-Ferreira Tel.: 55 41 9969-6983 Fax: 55 41 9969-6983 e-mails: [email protected]; [email protected]

Abstract Purpose To carry out a descriptive investigation into the most relevant morphological features of the chinchilla eye and bony orbit, as well as to perform selected ophthalmic diagnostic tests with the aim of establishing normal anatomic and physiologic references for this species. Method A total of 57 healthy, chinchillas were used to test most of the parameters in this investigation. Besides morphologic observations of the globe and adnexa, selected ocular tests and parameters were investigated, including blink frequency, palpebral fissure length (PFL), Schirmer tear test (STT), esthesiometry, intraocular pressure (IOP), central corneal thickness (CCT), B-mode echobiometric measurements of the globe and culture of the normal conjunctival bacterial microbiota. Morphologic observations were made using six formalin-fixed globes and four macerated skulls. Results and Discussion Normal parameters found for selected ocular diagnostic tests were: blink frequency: 2.6 ± 0.84 blinks per 10 min; STT: 1.07 ± 0.54 mm; esthesiometry: 1.24 ± 0.46 cm; IOP: 17.71 ± 4.17 mmHg; CCT: 0.34 ± 0.03 mm; PFL: 1.44 ± 0.11 cm; anterior chamber depth: 2.01 ± 0.2 mm; axial lens thickness: 5.49 ± 0.43 mm; vitreous chamber depth (internal): 3.69 ± 0.52 mm; axial globe length: 1.14 ± 0.07 cm. The most frequent bacteria isolated from the conjunctiva were Streptococcus sp. (27.45%), Staphylococcus aureus (23.52%) and coagulase-negative Staphylococcus (19.60%). No statistically significant differences between left or right eyes or genders were found for any of the results. Reference data and morphologic observations obtained in this investigation will help veterinary ophthalmologists to recognize unique morphological features and more accurately diagnose ocular diseases in the chinchilla, an animal already being used as a biological model for ophthalmic studies. Key Words: Chinchilla lanigera, esthesiometry, intraocular pressure, morphology, normal microflora, ultrasonic pachymetry, ultrasonography

INTRODUCTION

The term chinchilla is a generic one that refers indistinctly to two species of hystricomorph rodents closely related to

guinea pigs: the short-tailed chinchilla Chinchilla brevicaudata, and the long-tailed or Chinchilla lanigera. Chinchillas occur naturally only in South America (Argentina, Bolivia, Chile and possibly Peru). These animals were once widely 2010 American College of Veterinary Ophthalmologists

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distributed along the central Andes at altitudes of 3000–4000 m.1 The high quality of their fur motivated the harvesting of chinchillas for the fur market. Commercial hunting, beginning in 1828, became a common and widespread activity in northern Chile. The International Union for Conservation of Nature (IUCN) lists the wild short-tailed chinchilla as ‘‘critically endangered.’’ Current populations appear to be limited to the Jujuy province of Argentina and in the Lullaillaco National Park in northern Chile. The long-tailed chinchilla was also considered extinct in the wild, but was ‘re-discovered’ in 1975. Nevertheless, the long-tailed chinchilla is currently classified as vulnerable by the IUCN. Presently, the wild long-tailed chinchilla is known to exist near Illapel in the Chilean Reserva Nacional Las Chinchillas.2 There are enormous numbers bred in captivity outside of South America. These are descendents of single pairs or small populations of long-tailed chinchillas. For instance, most of the chinchillas kept as pets in the USA are descendants of a small group of animals imported into California in the 1920s.3,4 As captive chinchillas found worldwide originated from very small number of animals, the domestic chinchilla gene pool is limited and a genetic component is suspected in many of the commonly encountered diseases, including some commonly seen ocular conditions, especially in the USA.5,6 Adult chinchillas usually weigh between 400 and 800 g with females being slightly larger than males. Relative to other rodent species, the life span of the chinchilla is long, sometimes nearing 20 years. Chinchillas have a short body, large head, delicate limbs, large hairless ears, and a bushy long tail. The wild natural color is a bluish-gray with white fur underneath. Nevertheless, a number of different coat color variations has been developed with selected crosses and are considered mutations.1 Chinchillas are increasingly popular companion animals and are commonly presented to veterinarians including veterinary ophthalmologists. Additionally, chinchillas have recently been used as a laboratory animal model for vision research, particularly the study of semicircular canal function and vestibulo-ocular reflexes.1,7–9 Various attributes make this animal desirable as a research model: ease of handling, small size to permit breeding in small places yet possess a relatively large eye and body for a rodent, trainability for sound avoidance studies, quietness, essentially odor-free urine and feces and lastly, they are relatively inexpensive to acquire and breed.10 An understanding of specific morphological features and physiological parameters of the normal eye of these animals is essential to achieve the best possible ophthalmic diagnosis and further the knowledge base for research purposes. Limited citations about the chinchilla eye morphology and ophthalmic clinical tests are available in the literature. These references include: presence of fine cilia on upper and lower eyelids, rudimentary third eyelid, large cornea, large lens, densely pigmented iris with a vertical slit pupil, and an

anangiotic fundus with variable vascularization of the optic disc. Mean intraocular pressure (IOP) reported is 18.5 ± 5.75 mmHg.10–12 Because of their popularity as pets and their use as research animals, the lack of ocular morphology data and specific reference values for ophthalmic clinical tests for chinchillas, could negatively affect their care. The objective of this study was to make detailed observations about the most important features of chinchilla ocular morphology including gross anatomical descriptions of the eye and orbit and to establish normal parameters for ocular diagnostic tests to serve as reference values for future investigations. MATERIALS AND METHODS

Anatomy of the bony orbit Four skulls of adult chinchillas from a private anatomy collection of one of the investigators (MM) were used for this research and compared with skulls of several other rodent species including guinea pigs, gerbils, hamsters, mice and rats, which also belonged to the same private collection. For the study of the bony orbit, a combination of gentle maceration techniques was used to avoid damage to the delicate bones of the chinchillas’ skull. In this investigation, the skin was removed and the whole head was boiled in water repeatedly to facilitate mechanical removal of the muscles and other tissues. Then fine manual removal of tissues was performed using Adson–Brown forceps, scalpel blades, and iris scissors. After these procedures, the heads were immersed in water for about 3 weeks to remove the remaining debris. For bleaching purposes, the skulls were immersed in 50% hydrogen peroxide for 24 h. Following this, they were washed in distilled water and air dried. Skull anatomy was studied comparatively under 2–4· magnification and digitally photographed. Gross examination of fixed globes Six chinchillas that died from various causes but were free of ocular disease were sent to the UFPR’s Veterinary Teaching Hospital by a local commercial breeder. Immediately after the arrival of the carcasses, shortly after death, bilateral enucleation was performed. The vitreous chamber of the enucleated eye was infused with approximately 0.3 mL of 10% buffered formalin and then immersed in formalin for 1 week. Once adequately fixed, the eyes were sectioned in the sagittal (at the optic axis) and coronal planes for gross anatomy evaluation. Additionally, the optic disc diameter was measured with a Jameson caliper. Ophthalmic procedures on live animals All procedures using live chinchillas were conducted in accordance with UFPR’s Animal Use Committee and with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. Fifty-seven chinchillas of varying ages and color patterns were selected randomly from a commercial breeder collection. Physical examinations, including a

2010 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 13, 14–25

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complete blood count panel, were performed before ocular examinations to exclude animals with indications of systemic disease. Chinchillas with evidence of ocular or systemic diseases were excluded. Procedures and tests necessary to produce this work were split between the investigators. However, to avoid discrepancies related to inter-observer repeatability, the same person always performed the same ocular test on each occasion.

Ophthalmic tests Clinical tests were performed while the chinchillas were manually restrained by an experienced handler, taking care to keep the animal comfortable, supported with one hand under the thorax and a second around the rump. When the head was manually stabilized for taking measurements special attention was given to avoid applying pressure to the neck region, to prevent iatrogenic alterations in IOP. The sequence of procedures performed in this study was: (i) ocular inspection (including blink frequency observation), (ii) Schirmer tear test (STT), (iii) collection of material for bacterial culture analysis, (iv) esthesiometry, (v) tonometry, (vi) central corneal thickness (CCT) measurement with an ultrasonic pachymeter, (vii) B-mode ultrasonography of the globe, (viii) fundoscopy and lastly (ix) palpebral fissure length measurements. Ocular inspection A total of 114 eyes, from 57 healthy chinchillas (25 male and 32 female) were selected and used in this investigation. The anterior ocular structures of all 57 chinchillas were evaluated using a transilluminator and a slit lamp biomicroscope (Hawk Eye; Dioptrix, L’Union, France). Blink frequency was timed and registered at this point. Schirmer tear test Sterile standardized STT strips (Schering Plough Animal Health, Union, NJ, USA) were used to perform the Schirmer type I test (Fig. 1a), which measures the basal plus a portion of the reflex tear secretion in all 57 chinchillas eyes. Microbiological analysis For the microbiological analysis, samples from 36 chinchillas were obtained by carefully touching the conjunctival sac and ocular surface with a cotton swab. No topical anesthetic was used prior to sample collection as this may interfere with the growth of organisms.13 Due to the small size of the eye, the swab touched the cornea, conjunctival sac and eyelid margins from both eyes (Fig. 1b) but never the periocular skin. Aerobic bacterial culture of the microorganisms was performed in BHI broth (brain–heart infusion), and on 5% sheep blood agar and MacConkey plates, which were incubated at 37 C in an aerobic environment for 24–48 h. The same bacterial growth media used in this research was also used elsewhere to establish normal conjunctival microbiota of the opossum, raccoon and ferret in other investiga-

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(b)

(c)

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Figure 1. Photographs of selected ocular tests performed in chinchillas. (a) Schirmer tear test; (b) Swabbing the conjunctiva and eyelid margins; (c) Esthesiometric analysis of central cornea; (e) Corneal pachymetry; and (f) Palpebral fissure length measurement.

tions.14–16 Bacterial colonies were identified by Gram’s stain and standard procedures.

Corneal esthesiometry For the normal corneal sensitivity analysis, all 57 animals were manually restrained, and a Cochet-Bonnet esthesiometer (Luneau Ophtalmologie, Chartres Cedex, France) was used (Fig. 1c). This instrument contains an adjustable nylon filament with a defined diameter, which is applied in different lengths to the center of the cornea. A stimulus produced by the instrument’s nylon monofilament that reaches the corneal touch threshold induces a corneal reflex, consisting of prompt eyelid closure, and discrete retraction of the globe in chinchillas. In this study only the center of the cornea was analyzed for corneal touch threshold, which was repeated five times using the same length of the nylon filament. The length of the nylon filament was then decreased at 5-mm increments until each chinchilla responded with a corneal blink reflex. The corneal touch threshold was then quantified in mm length of the filament necessary to cause a blink reflex. The length of the filament, indicating a correspond-



ing pressure, at which the corneal blink reflex was positive, was deemed the central corneal sensitivity or central corneal touch threshold.

Intraocular pressure Intraocular pressure was measured by applanation tonometry using a Tonopen XL (Mentor, Norwell, MA, USA) (Fig. 1d). Three final IOP readings with variances of 0.05). Fundoscopy Fundoscopy is difficult to perform in chinchillas due to the small size of the globe, heavy choroidal pigmentation and poor response to pharmacological dilation. Choroidal pigmentation of the fundus and consequentially visualization of the distribution of blood vessel varied from animal to animal. The well-demarcated radial choroidal vascular pattern with several prominent vortex vein ampullae can readily be observed at fundoscopy in animals with less choroidal pigmentation, and are conspicuous features of the chinchilla fundus (Fig. 8). The optic disc appears gray and round with a small central depression (physiological cup) and there is no tapetum lucidum. Palpebral fissure length The mean value for palpebral fissure length was 1.43 ± 0.11 cm for males and 1.46 ± 0.10 cm for females. There was no significant difference in palpebral length between males and females (P = 0.46) or between right and left eyes (P = 0.98). DISCUSSION

The presence of a prominent and well-developed zygomatic process of the maxillary bone is what differentiates the zygomatic arch of rodents from that of dogs and cats. Chinchillas and guinea pigs have similar infra-orbital regions in that they look very robust. In smaller rodents, like gerbils, hamsters, mice and rats the zygomatic arch is elongated and proportionally thinner and more delicate. The orbital shape tends to be circular in chinchillas and guinea pigs and ovoid in rats, mice and hamsters. In its general outline, the zygomatic arch is convex in chinchillas, just as it is in rats, mice and guinea pigs, as opposed to the concave shape found in gerbils and hamsters.6 The orbital ligament varies in its insertion point in the zygomatic arch in rodents. It may insert itself caudally at the zygomatic process of the maxillary bone in mice or caudally at the zygomatic bone in chinchillas. Chinchillas, guinea pigs and gerbils have supraorbital margins with a narrow angle, projected laterally, which contributes to making the interorbital breadth much wider in these species than in other rodents.6 Rats, hamsters



Figure 7. Representative B-scan ultrasonogram after optimal positioning was achieved. The four principal landmarks (cornea, anterior lens surface, posterior lens surface and retinal surface) along the globe axis are all perpendicular. Here in this representative example four dashed lines were placed on the B-scan image to capture: (1) Axial globe length (11.6 mm); (2) Anterior chamber depth (2.16 mm); (3) Lens thickness (5.22 mm); (4) Vitreous chamber depth (3.72 mm).

Figure 8. Fundus photograph of the left eye of a chinchilla with lightcolored iris and poorly pigmented choroid. Note the round optic disc with a central depression at the center (arrowhead) and one of the vortex ampullae in the ventral and temporal fundus (arrow).

and mice have smooth and blunt supra-orbital margins that, in addition to proportionally smaller interorbital breadth, also create a shallower orbit.6 Chinchillas and guinea pigs have a well-developed lacrimal bone that participates in the formation of the orbital margin, whereas in rats, mice and hamsters the lacrimal bone does not contribute at all or very little to the orbital margin because it is located at the medial wall of the orbit. An element of great variation among rodent species is the

infra-orbital canal whose size and shape is directly related to the structures that pass through it. The canal is known as the infra-orbital hiatus in species in which the masseter muscle passes through it because of its massive proportion. In these species, the masseter muscle goes through the infra-orbital hiatus after originating at the body of the maxillary bone in an area lateral to the rostrum. Three basic patterns of the morphological features of the masseter muscle’s location contribute to the taxonomic classification of rodents. In sciuromorphs, such as squirrels, kangaroo rats, kangaroo mice and rock pocket mice, no portion of the masseter muscle passes through the small infra-orbital canal.17 In myomorphs such as common rats, gerbils and hamsters, a small portion of the deep masseter muscle passes through the infra-orbital hiatus.17,18 Lastly, in hystricomorphs, such as chinchillas and guinea pigs, a great portion of the deep masseter muscle passes through the infra-orbital hiatus.17,19 The globes of chinchillas possess a large corneal surface. This relative area is only exceeded by some species of bats and the mouse, in which the cornea makes up almost 50% of the globe.11,12 The cat and the rabbit follow closely, with corneas occupying about 30% of the globe area.12 The choroidal vascular anatomy has been studied in humans20 and nonhuman primates,21–23 domestic ungulates (sheep, goat, pig, cow and horse),24,25 dogs,26 cats,27 rabbit,28,29 and rodents30–34 but not yet in the chinchilla. Chinchillas seem to have a pattern for choroidal blood supply similar to that of hamsters and rats.34 In rats and hamsters the principal vein in the choroidal circulatory system has long been known to be the vortex vein.33,34 Just as in rats and hamsters, chinchillas have dense collecting venules with various diameters at the posterior choroid that converge into the vortex veins. The ampullae in these species drain blood into the four individual vortex veins located on the dorsal, ventral, medial (nasal), and lateral (temporal) sides, which pierce the sclera just posterior to the equator of the globe. In chinchillas it was possible to see that the larger veins that converge from all directions toward the vortex vein usually formed two or three ampullae in each quadrant. The larger veins are more demarcated in chinchillas and the ampullae are more conspicuous in this species. However, the exact number of vortex veins in chinchillas needs to be investigated and described in future. We could not observe this feature on formalin-fixed tissues.33,34 We believe that corrosion cast studies must be performed in future for a more accurate characterization of the vascular anatomy in the chinchilla’s eye. The retinal vascular patterns of rodents vary enormously in the various suborders, ranging from anangiotic to holoangiotic.10,35 Using fundoscopy, it was found that the chinchilla has an anangiotic fundus pattern resembling that of the American beaver and capybara10,16,35,36 in contrast to the rat, which has a holangiotic retina with arterioles and venules that radiate from the optic disc35 like the spokes on a bicycle wheel. However, the well-demarcated


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vascular pattern is very characteristic of the chinchilla fundus. The noticeable vortex veins and ampullae serve as very good landmarks for an examiner. These vortex ampullae and veins represent the equator of the eye. They are very important when one wants to describe retinal defects and their relationship to the rest of the chinchilla fundus. Most rodents have pupils that are predominantly circular both when dilated and constricted, with the only exceptions being the marmot, ground squirrel, coipu, degu, chinchilla, and the capybara, all of which have vertically oriented pupils in the constricted state.10,35,37 Nocturnal animals, if they are visually active and aware, have to collect the maximum amount of light possible. A large spherical lens occupying the greater part of the globe positions its posterior surface nearer the retina and the anterior surface near the cornea. This, together with a prominent or protruding cornea ensures the widest possible monocular field of vision. A fully dilated pupil and a shallow anterior chamber give the whole of the lens the opportunity to function as a refracting organ instead of a smaller proportion in other species. It is well known that the slit pupil is more efficient than a round pupil because it permits a maximum degree of closure (more than a round pupil) and also places no obstacle in the way of maximum dilation.12 The chinchilla is nocturnally active and has a suitably sensitive retina, but it also likes to bask in the sun. Thus, a completely closing pupil gives it protection during the day, but at night it allows for maximum light gathering in low light levels. Walls (1942) pointed out that any strongly nocturnal animal which desires to venture out or bask in the sun needs a slit pupil.37 The rudimentary stenopaic (pinhole) aperture observed when chinchilla’s pupils were miotic was also observed by Detwiler (1949).11 The vestigial semilunar fold of conjunctiva (third eyelid) present in the chinchilla is similar to that of the capybara16 and the Canadian beaver.36 The blink frequency of two to four times in 10 min (2.6 ± 0.84), observed in the chinchilla is very low. In comparison with other animals, the dog blinks three to five times per minute,38 the cat one to five times per 5 minutes,39 the horse 5–25 times per minute,40 cattle five times per minute,41 the pig blinks nearly 10 times per minute,42 and the guinea pig blinks only two to five times per 20 min.43 Only the guinea pig has a mean value near that of the chinchilla. In more excited chinchillas, however, the blink frequency increases, as is the case in other animals; for example, the restrained dog blinks 10–20 times per minute.38 Blinking maintains the physiologic thickness of the preocular surface by spreading the tears over the corneal surface. Nonetheless, chinchillas have low STT values, a low eye blink frequency during rest, and lack signs of tear evaporation or corneal dryness. Because of this, questions about quantity and quality of the lipid and mucin layer of the chinchilla tears arise naturally. Maybe this frequency observed in small prey rodents is an evolutionary adaptation selected during the years that

permit more time to ‘keep the eyes open’ and see the predator, ‘without a blink’. These questions, however, need to be better evaluated in future with additional studies about the physical properties of chinchilla’s tears coupled with further morphological studies on their orbital glands. This may help explain the low tear production recorded in this investigation. Normal conjunctival bacterial microflora has been studied in several wild mammals such as the opossum,14 bison,44 deer,45 and elephant.46 In the vast majority of these reports, Gram-positive bacteria were the most common isolates and the present report is no exception. Other reports of ocular microflora in rodents such as the Canadian beaver,16 capybaras,36 and guinea pig,47 also corroborate our findings. Both pathogenic and nonpathogenic bacteria were found in this investigation. Escheichia coli was isolated from the chinchillas in this study. This bacteria was also isolated from normal conjunctival microbiota of dogs48,49 and horses.50 This finding can, alternatively, suggest possible contamination with fecal material and/or maybe this bacteria represents a transient agent of the conjunctiva. Additional studies to try to determine whether or not this bacterium is a normal inhabitant of chinchilla’s ocular microbiota are a possibility. Two uncommonly isolated bacteria were found in our study: Morganella morgani and Proteus vulgaris. The P. vulgaris was observed in healthy dog eyes49 and M. morgani (an enterobacteria) was already isolated from healthy eyes of two different species of nonhuman primates (Cebus apella and Alouatta caraya)51 and even observed in ocular surface of a human patient submitted to cataract surgery.52 It is worth noting that in some cases of bacterial conjunctivitis, a nonpathogenic conjunctival bacterium can overgrow and cause an imbalance of the ocular surface microbiota population, thus becoming pathogenic.53 In conclusion, Gram-positive bacteria were the most predominant bacteria of the normal ocular surface microbiota in healthy chinchillas, and S. aureus and Streptococcus sp. were the predominant common bacterial genera observed. The mean STT value in the chinchilla (1.07 mm) was low in comparison with rabbit (5.3 mm)54 but still higher and somewhat more measurable than the mean value of the guinea pig.43 Previously, Peiffer & Johnson found that chinchillas had an average IOP of 18.5 ± 5.75 mmHg.10 We found a similar mean IOP value of 17.71 ± 4.17 mmHg. However, we used a Tonopen XL applanation tonometer and the other authors used a Mackay–Marg tonometer. This small difference might be explained by either fluctuation of the IOP during the day, differences in physical restraint or, lastly, type of tonometer used. In comparison with the IOP of other rodent species, the chinchilla’s IOP was similar to the Wistar rat (18.4 ± 0.1 mmHg) and to the DBA/21 mice (19.3 ± 0.4 mmHg), even though there are strain differences reported with other three mice types: 10.6 ± 0.6 (BALB/c),



13.3 ± 0.3 (C57-BL/6), and 16.4 ± 0.3 mmHg (CBA).54 The rabbit IOP is reportedly lower with a mean value of 15.21 ± 1.0 mmHg.55 In veterinary medicine the Cochet–Bonnet esthesiometer is a well-established tool to determine the sensitivity of the cornea by evaluating the corneal touch threshold or sensitivity.56,57 In comparison with guinea pig (2.0 ± 0.6 cm) esthesiometry,58 the chinchilla cornea is discretely less sensitive (1.24 ± 0.46 cm). In our study eight corneas showed smallest blink response to the strongest pressure (filament length of 0.5 cm) and two eyes showed no response at all. In these two cases, the corneal sensitivity is so low that could not be evaluated correctly with this instrument. The palpebral fissure length is an important parameter that helps to identify some abnormalities of the upper and lower eyelids. Problems like ankyloblepharon, eyelid agenesis, blepharophimosis, euryblepharon, entropion or ectropion are commonly diagnosed in dogs,59 cats,60 horses,61 and human beings.62 Knowledge of normal palpebral fissure length is an important biometric feature that might help to properly diagnose these conditions and to assist surgeons to restore palpebral form and function with the correct position of upper and lower eyelids. Chinchilla’s central cornea measured using ultrasonic pachymetry (340 ± 30 lm) is thinner than that of several mammalian species that have also been analyzed using ultrasonic pachymetry; for instance, CCT of human beings (548 lm),63 dog64 (562 lm), cat (569, 578, 755 lm),65,66 alpaca (595 lm),67 llama (608 lm),67 miniature horse (785 lm),68 horse (893 lm),69 and capybara (460 ± 30 lm).16 Nevertheless, the chinchilla’s CCT is very similar to that of the ferret (337 ± 20 lm),15 and thicker than the rat (156 ± 30 lm).70 The general ultrasonographic appearance of the chinchilla eye is analogous to other wild and domestic species. The axial globe length of the chinchilla is a bit longer but still close to the guinea pig one (10.17 ± 0.03 mm).71 Conventional laboratory rodents have a smaller axial globe length: 5.15 ± 0.23 mm for the rat and 2.981 ± 0.005 mm for the mouse.72 The lens thickness-to-axial globe length ratio of the chinchilla (1:2), however, is identical with that of the guinea pig (1:2)71 but lower than that of the rat, which is 1:2.6.11,12 Our result was different from the one found previously in the literature11,12 for the chinchilla where a ratio of 1:3 was found. We have previously found that capybaras have a 1:3 ratio,16 which is similar to the ratio of the Saanen goat (1:3.1).73 In man, for instance, the lens-to-eye ratio is 1:12.8 and in the rhesus monkey it is 1:10.12 This anatomical feature shared by chinchillas and guinea pigs (small eye and a proportionally thick lens) might exclude them from being useful biological models for experimental intraocular manipulations such as subretinal injections or posterior segment surgeries. We believe that histological investigations are necessary to better comprehend some of the morphological features

described here in the chinchilla eye. We plan to do so in the near future. Nevertheless, the morphological features and reference data for ocular tests obtained in this investigation will help veterinary ophthalmologists to more accurately diagnose discrete pathological changes of the chinchilla eye. If recognized early on, some of the conditions encountered in the eye of this species can be managed successfully as in other species. ACKNOWLEDGMENTS

The authors thank Dr Gillian Shaw, Johns Hopkins University, Baltimore, MD, USA, as well as Rafaella Drumond, for their invaluable help in this investigation and in the preparation of this manuscript. REFERENCES 1. Keeble E. Rodents: biology and husbandry. In: BSAVA Manual of Rodents and Ferrets (eds Keeble E, Meredith A) British Small animal Veterinary Association, Quedgeley, Gloucester, 2009; 1: 1–17. 2. Jimene´z JE. The extirpation and current status of wild chinchillas Chinchilla lanigera and C. brevicaudata. Biologial Conservation 1996; 77: 1–6. 3. Spotorno AE, Valladares JP, Marin JC et al. Molecular divergence and phylogenetic relationship of chinchillids (Rodentia: Chinchillidae). Journal of Mammalogy 2004; 85: 384–388. 4. Crossley DA, Migue´lez MDM. Skull size and cheek-tooth length in wild-caught and captive-bred chinchillas. Archives of Oral Biology 2001; 46: 919–928. 5. Riggs SM, Mitchell MA. Chinchillas. In: Manual of exotic pet practice (eds Mitchell MA, Tully TN) W.B. Saunders Company, Philadelphia, PA, 2008: 474–491. 6. Montiani-Ferreira F. Rodents: ophthalmology. In: BSAVA Manual of Rodents and Ferrets (eds Keeble E, Meredith A) British Small animal Veterinary Association, Quedgeley, Gloucester, 2009; Vol. 15: 169–180. 7. Hassul M, Daniels PD, Kimm J. Effects of bilateral flocculectomy on the vestibulo-ocular reflex in the chinchilla. Brain Research 1976; 2: 339–343. 8. Daniels PD, Hassul M, Kumm J. Dynamic analysis of the vestibule ocular reflex in the normal and flocculectomized chinchilla. Experimental Neurology 1978; 58: 32–45. 9. Hullar TE, Williams CD. Geometry of the semicircular canals of the chinchilla (Chinchilla lanigera). Hear Research 2006; 213: 17–24. 10. Peiffer RL, Johnson PT. Clinical ocular findings in a colony of chinchillas (Chinchilla lanirera). Laboratory Animals 1980; 14: 331–335. 11. Detwiler SR. The eye of the Chinchilla (C. lanigera). Journal of Morphology 1949; 84: 123–144. 12. Prince HJ. The cornea. In: Comparative Anatomy of the Eye (ed. Charles C) Thomas Publisher, Spreingfield, IL, 1956; 418. 13. Mullin GS, Rubinfeld RS. The antibacterial activity of topical anesthetics. Cornea 1997; 16: 662–665. 14. Pinard CL, Brightman AH, Yeary TJ et al. Normal conjunctival flora in the North American opossum (Didelphis virginiana) and raccoon (Procyon lotor). Journal of Wildlife Disease 2002; 38: 851–855. 15. Montiani-Ferreira F, Mattos BC, Russ HHA. Reference values for selected ophthalmic diagnostic tests of the ferret (Mustela putorius furo). Veterinary Ophthalmology 2006; 9: 209–213.


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16. Montiani-Ferreira F, Truppel J, Tramontin MH et al. The capybara eye: clincial tests, anatomic and biometric features. Veterinary Ophthalmology 2008; 11: 386–394. 17. Wesche P. Rodents: clinical pathology. In: BSAVA Manual of Rodents and Ferrets (eds Keeble E, Meredith A) British Small animal Veterinary Association, Quedgeley, Gloucester, 2009; Vol. 4: 42–51. 18. Elbroch M. Animal skulls: a guide to North American species, Stackpole Books, Mechanicsburg, Pennsylvania, 2006; 727. 19. Popesko P, Rajtova´ V, Hora´k J. A Color Atlas of Anatomy of Small Laboratory Animals: Vol. 2, Rat, Mouse and Hamster. WB Saunders, Philadelphia, PA, 2003; 224. 20. Onda E, Cioffi GA, Bacon DR. Microvasculature of the human optic nerve. American Journal of ophthalmology 1995; 120: 92–102. 21. Snodderly DM, Weinhaus RS. Retinal vasculature of the fovea of the squirrel monkey, Saimiri Sciureus: three-dimensional architecture, visual screening, and relationships to the neuronal layers. Journal of Comparative Neurology 1990; 297: 145–163. 22. Okada S, Ohta Y. Microvascular pattern of the retina in the Japanese monkey (Macaca fuscata fuscata). Scanning Microscopy 1994; 8: 415–427. 23. Funk RH. The vessel architecture of the pars plana in the cynomolgus monkey, rat and rabbit eye. Ophthalmic Research 1993; 25: 337–348. 24. Morrison JC, Defrank MP, Buskirk MV. Comparative microvascular anatomy of mammalian ciliary process. Investigative Ophthalmology and Visual Science 1987; 28: 1325–1340. 25. Simoens P, Van den Broeck W, Lauwers H. Scanning electron microscopic study of the posterior ciliary veins in domestic ungulates. Folia Morphology 2001; 60: 21–26. 26. Brooks DE, Samuelson DA, Gelatt KN et al. Scanning electron microscopy of corrosion casts of the optic nerve microcirculation in dogs. American Journal of Veterinary Research 1989; 50: 908–914. 27. Risco JM, Nopanitaya W. Ocular microcirculation. Scanning electron microscopic study. Investigative Ophthalmology and Visual Science 1980; 19: 5–12. 28. Funk RH, Wagner E, Wild J. Microendoscopic observations of the hemodynamics in the rabbit ciliary processes. Current Eye Research 1992; 11: 543–551. 29. Sugiyama K, Bacon DR, Morrison JC et al. Optic nerve head microvasculature of the rabbit eye. Investigative Ophthalmology and Visual Science 1992; 33: 2251–2261. 30. Sugiyama K, Zhao-Bin Gu, Kawase C et al. Optic nerve and peripapillary choroidal microvasculature of the rat eye. Investigative Ophthalmology and Visual Science 1999; 40: 3084–3090. 31. Bhutto IA, Amemiya T. Corrosion cast demonstration of retinal vasculature of normal Wistar–Kyoto rats. Acta Anatomoy 1995; 153: 290–300. 32. Bhutto IA, Amemiya T. Microvascular architecture of the rat choroid: corrosion cast study. Anatomical Record 2001; 264: 63–71. 33. Ninomiya H, Inomata T. Microvasculature of the hamster eye: scanning electron microscopy of vascular corrosion casts. Veterinary Ophthalmology 2005; 8: 7–12. 34. Ninomiya H, Kuno H. Microvasculature of the rat eye: scanning electron microscopy of vascular corrosion casts. Veterinary Ophthalmology 2001; 4: 55–59. 35. Johnson GL. Contributions to the comparative anatomy of the mammalian eye, chiefly based on ophthalmoscopic examination. Philosophical Transactions of the Royal Society of London: Series B 1901; 194: 1–82.

36. Cullen CL. Normal ocular features, conjunctival microflora and intraocular pressure in the Canadian beaver (Castor canadensis). Veterinary Ophthalmology 2003; 6: 279–284. 37. Walls GL. The Vertebrate Eye. Hafner Publishing Co, New York, 1963; 652. 38. Carrington SD, Bedford PGC, Guillon JP et al. Polarized light biomicroscopic observation on the pre-corneal film, and the normal tear film of the dog. Journal of Small Animal Practice 1987; 28: 605–622. 39. Blakemore C, Cummings RM. Eye-opening in kittens. Visual Research 1975; 15: 1417. 40. Marts BS, Bryan GM, Prieuo DJ. Schirmer tear test measurements and lysozyme concentration of equine tears. Journal of Equine Medical Surgery 1977; 1: 427. 41. Brightman AH, Wachosstock RS, Erskine R. Lysosyme concentrations in the tears of cattle, goats, and sheep. American Journal of Veterinary Research 1989; 50: 2084. 42. Gum GG, Gelatt KN, Ofri O. Physiology of the eye. In: Veterinary Ophthalmology (ed. Gelatt KN) Lippincott Williams & Wilkins, Philadelphia, PA, 1999; 3rd edn, 151–158. 43. Trost K, Skalichy 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. 44. Davidson HJ, Vestweber JG, Brightman AH et al. Ophthalmic examination and conjunctival bacteriologic culture results from a herd of North American bison. Journal of the American Veterinary Medical Association 1999; 215: 1142–1144. 45. Dubay SA, Williams ES, Mills K et al. Bacteria and nematodes in the conjunctiva of mule deer from Wyoming and Utah. Journal of Wildlife Diseases 2000; 36: 783–787. 46. Tuntivanich P, Soontornvipart K, Tuntivanich N et al. Conjunctival microflora in clinically normal Asian elephants in Thailand. Veterinary Research Communications 2002; 26: 251–254. 47. Martin EC, Stiles J, Dacvo SGJ et al. Results of diagnostic ophthalmic testing in healthy guinea pigs. Journal of the American Veterinary Medical Association 2008; 12: 1825–1833. 48. Prado MR, Rocha MFG, Brito EHS et al. Survey of bacterial microorganisms in the conjunctival sac of clinically normal dogs and dogs with ulcerative keratitis in Fortaleza, Ceara, Brazil. Veterinary Ophthalmology 2005; 8: 33–37. 49. Wang L, Pan Q, Zhang QX et al. Investigation of bacteria microorganisms in the conjunctival sac of clinically normal dogs and dogs with ulcerative keratitis in Beijing, China. Veterinary Ophthalmology 2008; 11: 145–149. 50. Andrew SE, Nguyen A, Jones GL et al. Seasonal effects on the aerobic bacterial and fungal conjunctival flora of normal thoroughbred brood mares in Florida. Veterinary Ophthalmology 2003; 6: 45–50. 51. Galera PD, A´vila MO, Ribeiro CR et al. Estudo da microbiota da conjuntiva ocular de macacos-prego (Cebus apella – Linnaeus, 1758) e macacos bugio (Alouatta caraya – Humboldt, 1812), provenientes do reservato´rio de Manso, MT, Brasil. Arquivo do Instituto de Biologia 2002; 69: 33–36. 52. Arantes TE, Cavalcanti RF, Diniz MF et al. Conjunctival bacterial flora na antibiotic resistance pattern in patients undergoing cataract surgery. Arquivo Brasileiro de Oftalmologia 2006; 69: 33–36. 53. Samuelson DA. Ophthalmic anatomy. In: Veterinary Ophthalmology (ed. Gelatt KN) Lippincott Williams & Wilkins, Philadelphia, PA, 1999; 3rd edn, 31–50. 54. Wang WH, Millar JC, Pang IH et al. Noninvasive measurement of rodent intraocular pressure with a rebound tonometer. Investigative Ophthalmology and Visual Science 2005; 46: 4617–4621.


the chinchilla eye 25 55. Zhang L, Li Y, Zhang C et al. Pharmacokinetics and tolerance study of intravitreal injection of dexamethasone-loaded nanoparticles in rabbits. International Journal of Nanomedicine 2009; 4: 175–183. 56. Chan-Ling T. Sensitivity and neural organization of the cat cornea. Investigative Ophthalmology and Visual Science 1989; 30: 1075–1082. 57. Barrett PM, Scagliotti RH, Merideth RE et al. Absolute corneal sensitivity and corneal trigeminal nerve anatomy in normal dogs. Progress in Veterinary and Comparative Ophthalmology 1991; 1: 245–254. 58. Stadtbaumer K, Alkemade F, Nell B. Schirmer tear test and corneal sensitivity in the normal guinea pig. Veterinary Ophthalmology 2004; 7: 426. 59. Bedforg PG. Technique of lateral canthoplasty for the correction of macropalpebral fissure in the dog. Journal of Small Animal Practice 1998; 39: 117–120. 60. Williams DL, Kim JY. Feline entropion: a case series of 50 affected animals (2003–2008). Veterinary Ophthalmology 2009; 12: 221–226. 61. Wilkie DA. Ophthalmic procedures and surgery in the standing horse. Veterinary Clinics of North American Equine Practice 1991; 7: 535–547. 62. Schaefer DP. The graded levator hinge procedure for the correction of upper eyelid retraction. American Ophthalmological Society 2007; 105: 481–512. 63. Susuki S, Oshika T, Oki K et al. Corneal thickness measurements: scanning-slit corneal topography and noncontact specular microscopy versus ultrasonic pachymetry. Journal of Cataract Refractive Surgery 2003; 29: 1313–1318. 64. Gilger BC, Whitley RD, Mclaughlin SA. Canine corneal thickness measured by ultrasonic pachymetry. American Journal of Veterinary Research 1991; 10: 1570–1572.

65. Carrington SD, Woodward EG. Corneal thickness and diameter in the domestic cat. Ophthalmic and Physiological Optics 1986; 6: 385–389. 66. Chang-Ling T, Efron N, Holden BA. Diurnal variation of corneal thickness in the cat. Investigative Ophthalmology and Visual Science 1985; 26: 102–105. 67. Andrew SE, Willis AM, Anderson DE. Density of corneal endothelial cells, corneal thickness, and corneal diameters in normal eyes of llamas and alpacas. American Journal of Veterinary Research 2002; 63: 326–329. 68. Plummer CE, Ramsey DT, Hauptman JG. Assessment of corneal thickness, intraocular pressure, optical corneal diameter, and axial globe dimensions in Miniature horses. American Journal of Veterinary Research 2003; 64: 661–665. 69. Andrew SE, Ramsey DT, Hauptman JG et al. Density of corneal endothelial cells and corneal thichness in eyes of euthanatized horses. American Journal of Veterinary Research 2001; 62: 479–482. 70. Mutti DO, Zadnik K, Murphy CJ. The effect of continuous light on refractive error and the ocular components of the rat. Experimental Eye Research 1998; 67: 631–636. 71. Bantseev V, Oriowo OM, Giblin FJ et al. Effect of hyperbaric oxygen on guinea pig lens optical quality and on the refractive state of the eye. Experimental Eye Research 2004; 78: 925–931. 72. Barathi VA, Boopathi VG, Yap EP et al. Two models of experimental myopia in the mouse. Vision Research 2008; 48: 904–916. 73. Ribeiro AP, Silva ML, Rosa JP et al. Ultrasonographic and echobiometric findings in the eyes of Saanen goats of different ages. Vetinary Opthamology 2009; 12: 313–317.
