The eye of the Barbary sheep or aoudad (Ammotragus lervia): reference values for selected ophthalmic diagnostic tests, morphologic and biometric observations

Abstract

The purpose of this study was to describe the normal ocular anatomy and establish reference values for ophthalmic tests in the Barbary sheep or aoudad (Ammotragus lervia). Aoudad eyes are large and laterally positioned in the head with several specialized anatomic features attributed to evolutionary adaptations for grazing. Normal values for commonly used ophthalmic tests were established, Schirmer tear test (STT) - 27.22 ± 3.6 mm/min; Predominant ocular surface bacterial microbiota - Staphylococcus sp.; Corneal esthesiometry- 1.3 ± 0.4 cm; Intraocular pressure by rebound tonometry- 19.47 ± 3.9 mmHg; Corneal thickness- 630.07 ± 20.67 µm, B-mode ultrasonography of the globe-axial eye globe length 29.94 ± 0.96 mm, anterior chamber depth 5.03 ± 0.17 mm, lens thickness 9.4 ± 0.33 mm, vitreous chamber depth 14.1 ± 0.53 mm; Corneal diameter-horizontal corneal diameter 25.05 ± 2.18 mm, vertical corneal diameter 17.95 ± 1.68 mm; Horizontal palpebral fissure length- 34.8 ± 3.12 mm. Knowledge of these normal anatomic variations, biometric findings and normal parameters for ocular diagnostic tests may assist veterinary ophthalmologists in the diagnosis of ocular diseases in this and other similar species.

Keywords: Barbary sheep; Biometry; Ocular parameters; Wild caprid.

Fig. 1

(a) Part of the group of aoudads (Ammotragus lervia) from Curitiba´s Zoo investigated in this study. The picture shows a mixed-aged group but only the adult animals were investigated. (b) A representative example of the general external appearance of the eye of the aoudad. True cilia (longer and thicker at the upper eyelid) are visible. Below the lower eyelid margin are two rows of sparsely distributed longer hairs (asterisk). Note in the anterior uvea the extensive iris collarette and the presence of an upper and a lower (more discrete) corpora nigra (arrows). The pupillary aperture shape was oval with the long axis horizontal. The limbus is relatively large and heavily pigmented.

Fig. 2

Photographs of selected ocular tests performed in aoudads. (a) Schirmer tear test; (b) Esthesiometric analysis of central cornea; (c) Corneal pachymetry; (d) Rebound tonometry; (e) Swabbing the conjunctiva and eyelid margins; and (f) B-mode ocular echobiometry. Besides the globe axial length, the following echobiometric measurements were taken, 1- Anterior chamber depth (axial anterior chamber length); 2- Lens thickness (axial lens length); 3- Vitreous chamber depth (axial vitreous chamber length). Note the superior corpora nigra (arrow).

Fig. 3

Aoudad´s fundoscopic appearance captured using TEFIT (a) and an indirect lens coupled with a slit lamp biomicroscope (Hawk Eye; Dioptrix, L’union, France). Note the extensive tapetum lucidum (a) the holangiotic retinal vascular pattern - (a) and (b). The tapetum has a granular or speckled appearance where stelullae of Winslow are present (b). The optic disc is grayish in color, oval in shape and located just inferior to the inferior border of the tapetum lucidum (a). The major blood vessels of the aoudad’s retina radiate from the center of the optic nerve. (a) Blood vessels arising from the dorsal and ventral quadrants taper toward a region just above the inferior border of the tapetum lucidum. At this region no blood vessels are present and an imaginary line can be traced creating a streak where thin retinal blood vessels are rare or absent (a).