VN May 2022

Vetnuus | May 2022 34 Regulars I Ophthalmology Column BACKGROUND Rhinoceroses are peculiar mammals that resemble prehistoric animals, and they are some of the most primitive of the world’s large mammals (1) . They inspired a beautiful and amusing tale by Rudyard Kipling, where he relates how the rhinoceros got his skin (2) . Of the five (5) extant species, four (4) are critically endangered due to poaching, and habitat loss and their distribution limited to Africa and Asia (1) . They are large, robust, odd-toed ungulates part of the order Perissodactyla and family Rhinocerotidae (1) . Three living families are included in the order Perissodactyla: the rhinoceroses, equids and tapirs. All species included in this order bear weight on one (equids) or three (rhinoceroses and tapirs) digits (1) . Rhinoceroses are threatened by severe poaching and habitat loss, with the black rhinoceros appearing on the International Union for Conservation of Nature red list as critically endangered and the white rhinoceros as near threatened (3)(4) . The most numerous of the world’s rhino species by the end of the 20th century was the eir black rhinoceros. During the1960s, anestimated100000animals remained. Their numbers further declined by 98% between 1960 and 1995 due to poaching and habitat loss. Between 1992 and 1997, their population numbers stayed stable and even increased South Africa and Namibia. Black rhinoceros population numbers grew steadily, and by the end of 2018 an estimated 5630 individuals existed, with an increasing current population (3) . By the end of the 19th century, the once widespread white rhinoceros was on the brink of extinction, with a small population of 20-50 animals in KwaZulu-Natal, South Africa, the only remaining animals. By the end of 2017, their numbers had decreased from an estimated 21,316 in 2012 to between 17,212 and 18,915. Five countries account for 99.3% of the remaining population, with South Africa conserving 18,933 individuals (~86.5%) by 2012. The current population trend for white rhinoceros is declining (4) . Traditionally, rhinoceroses have been considered to be nearsighted with relatively poor eyesight (5)(6) . Streak retinoscopy of three white rhinoceroses showed them actually to be mildly farsighted (5) . The retinal ganglion cell (RGC) density in the black rhinoceros suggests that their visual resolution is comparable to that of the rabbit and exceeds the resolution seen in various mammals, including rats, seals and dolphins about half the resolution seen in the domestic cat (6) . RGC density in the visual streak for both the white- and black rhinoceros has a peculiar concentration in the temporal and nasal region (Figure 1) (6)(7) . These studies concluded that the spatial topography of RGC’s allows the rhinoceroses to observe predators and conspecifics from in front and behind themwhile sampling thehorizonduringgrazing in their preferred habitats. The distribution of RGC’s most likely compensate for their reduced head and neck mobility which allows improved observation of their immediate surroundings (6)(7) . OCULAR ABNORMALITIES The available literature describing rhinoceros ocular abnormalities are limited. It may stem from the rarity of the animals and limited baseline ocular data available (8) . Some of the reports describe management of corneal ulceration resulting from suspected trauma in greater one- horned rhinoceroses (9)(10) , one case of enucleation in a black rhinoceros following proptosis (11) and one case of conjunctival habronemiasis in a white rhinoceros female (8) . A report describes an ocular disorder observed in a captive male and female Sumatran rhinoceros. The affected animals developed extensive ocular lesions, which progressed to complete blindness in the female and partial blindness of the right eye, in the male (12) . Eight rhinoceroses that underwent ophthalmological examination by Dr Antony Goodhead of Johannesburg Animal Eye Hospital had incidental ocular pathology discovered. Six of the eight animals examined had bilateral cataracts diagnosed, and one had a unilateral cataract. Embryonic vascular abnormalities / posterior segment pathology was also present in half of the rhinoceroses in this case series. Four animals had a persistent hyaloid artery (PHA), of which three were associated with bilateral cataracts. Retinal detachment was also present in two animals diagnosed with PHA. There are no published records describing cataracts and posterior segment pathology in rhinoceroses. CURRENT RESEARCH PROJECT The project aims to establish population parameters and describe any pathology encountered. Examination of the eye utilising various modalities is the only reliable means of assessing vision and possible pathology. Documenting prevalence of ocular pathology and establishing biometric parameters in rhinoceroses in South Africa JP Burger Figure 1 Topographic map illustrating the distribution of RGC densities in retinal wholemount of a white rhinoceros (6)(7) Lefapha Fo https://www NOTE: All 100 Lefapha la Disaense tša Bongakadiruiwa Follow the link for more information on the UP SHOP: https://www.up.ac.za/media/shared/401/ZP_Files/2020/new-branded-apparel.zp191508.jpg NOTE: All 100 years Veterinary Science products are purchased through the UPSHOP For more info contact Nelson Mosehle sales.upshop@up.ac.za Tel: 012 420 1235/6509 Cell: +27 66 192 3414 >>> 35