Vetnuus | May 2024 39 Although infectious keratoconjunctivitis is an important disease in ruminants, we have had ample exposure to the topic through recent SAVA webinars and online discussions, so this article is going to be geared to causes of ocular disease that can cause vision loss in sheep and goats. This is a relatively common problem, and it is not uncommon for farmers or production animal veterinarians to call regarding visual problems in sheep and goats. We are going to discuss several differential diagnoses that should be on the differential list for any veterinarian dealing with small ruminants. Obvious pathologies like keratitis, corneal oedema, and cataracts will not be covered. The importance of these is that multiple animals in a flock, or even an entire flock, may be affected. Some of the significant problems encountered include: Polioencephalomalacia (PEM), also called “cerebrocortical necrosis”, results in necrosis of the cortical grey matter. PEM has a worldwide distribution, in cattle, sheep and goats. There are several causes of PEM including altered thiamine (vitamin B1) status caused by such factors as excessive ruminal consumption of thiamine by bacterial thiaminases, and ingestion of thiaminase-containing plants like bracken fern could further exacerbate the problem. PEM usually occurs in a feedlot-type environment or pasture situation where stock is fed a low fibre, high concentrate ration or a sudden unusual amount of carbohydrates. Ruminal concentrations of thiamine are decreased in these conditions. Lambs between 2 and 4 months of age, and 6-month-old calves are most commonly affected. The initial ocular sign in sheep is trochlear nerve paralysis, which causes dorsomedial strabismus. Further signs include head pressing, aimless wandering or motionless standing, and cortical blindness. These initial signs progress to recumbency, opisthotonus, hyperesthesia, tonic-clonic convulsions, and nystagmus. Blindness is often the first sign to appear and the last to resolve. It may take up to 1 week after the resolution of the other signs for vision to return. Although the blindness is central, papilledema and decreased pupillary light responses [PLRs] may occur, and bilateral dorsomedial strabismus may be present. Bracken fern (Pteris aguilinum) also known as bright blindness in sheep, can cause subconjunctival haemorrhages, dilated pupils, decreased PLRs, tapetal hyperreflectivity, attenuation of the retinal blood vessels, outer retinal degeneration, and blindness. The optic disc is normal. Pteris aquilinum has been associated with progressive retinal degeneration (PRD) of the outer layers in sheep. Affected animals have variously been called “bright blind,” “moonlight blind,” “clear blind,” or simply “glass-eyed”; this terminology has arisen based on the abnormal shine of the affected eyes in semidarkness as a result of retinal atrophy. Ptaquiloside, which is a norsesquiterpine glucoside of the illudane type, is a bracken carcinogen and the principal causative agent of PRD. It is most common in 3-4 year old sheep. It is seldom seen in sheep younger than 2 years. Clinically, the sheep may be permanently blind. Ingestion of Sarcostemma spp. [Climbing milkweed] causes neurologic disease among sheep in Africa, and they may also be associated with visual impairment. Helichrysum argyrosphaerum plant toxicity in sheep in Africa may result in paresis, paralysis and blindness. Bilateral optic neuropathy follows the compression of the nerve in the optic canal and the resulting pathology includes gliosis and extensive oedema of the myelin in the intraorbital and intracranial portions of the optic nerves. This progresses to degeneration and loss of photoreceptor outer segments predominantly in the nontapetal retina. The brain shows bilaterally symmetrical status spongiosis of the white matter of the brain. Antiparasitic drugs, such as rafoxanide. Rafoxanide and Closantel are salicylanilides used as anthelmintic. It is most commonly used to treat adult liver flukes of the species Fasciola hepatica and Fasciola gigantica, roundworms and nasal bots. Closantel mainly interrupts Regulars I Ophthalmology Column Visual Impairment in Sheep Dr Antony Goodhead, Specialist Veterinary Ophthalmologist, Johannesburg and Cape Animal Eye Hospitals, www.animaleyehospital.co.za >>>40
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