Vetnuus | May 2024 41 Email: assistant@sava.co.za ADVERTISE IN VETNEWS MAGAZINE leading to granulomatous inflammatory lesions. Penetration of CNS by axonal transport, a prominent feature of viral infections, is also noted in listeriosis. Electric shocks and lightning strikes can produce direct ocular sequelae and also cause injury to the CNS (may manifest as blindness, papilledema or cranial nerve palsies). Lead poisoning is diagnosed in cattle, and, to a lesser extent, sheep and horses. In acute lead poisoning of cattle, clinical signs include staggering, muscle tremors (mainly of the head and neck), clamping of the jaws, and frothing at the mouth, in addition to ocular signs such as snapping of the eyelids, rolling of the eyes, and blindness. Hypovitaminosis A Vitamin A deficiency occurs most commonly due to a lack of vitamin A in the diet, or inhibition of absorption from the digestive tract. It is commonly seen in cattle and sheep in feedlots, and also those being fed high-concentrate diets in the absence of green pasture, as is common during droughts. Cattle and sheep cannot generate their own Vitamin A, they need to eat green plants containing carotenoid precursors such as β-carotene – found in the orange-yellow pigments in the green leaves of plants. Although there are many signs of Vitamin A deficiency, those related to the visual system occur through two mechanisms. In young animals, it causes abnormal thickening of growing bones, including the bones around the optic canal. This bone remodelling leads to constriction and compressions of the optic nerve. Clinical signs include retrograde degeneration of the retina, especially in the peripapillary region. Reduced cerebrospinal fluid (CSF) absorption, leading to increased CSF pressure, ataxia, tetraparesis, and seizures. As the deficiency progresses, papilledema worsens, the optic disc enlarges and becomes pink and pale, and details of the central optic disc are obscured. In the later stages or if treatment is not given, irreversible optic neuropathy occurs, and the optic disc becomes grey, flat, and shrunken. Blindness is accompanied by fixed, dilated pupils, and on ophthalmoscopy, papilledema will be present. Blindness may result from retinal degeneration at all ages or constriction and ischemic necrosis of the optic nerve at the optic foramen in growing animals. The second main mechanism of action of Vit A deficiency in the visual system is that Vit A is used in the synthesis of the visual photopigment in rods. Hypovitaminosis A will also impair rod function, leading to night blindness. In chronic deficiencies, progressive loss of vision and complete retinal degeneration will occur. The ubiquitous protozoan T. gondii is an abortifacient in sheep and goats and occasionally in pigs, although infection of ruminants with the protozoan is often asymptomatic. Ocular signs are rare, but T. gondii may infiltrate the retina and uvea, causing retinitis and chorioretinitis as a result of primary posterior segment infection. Anterior uveitis may also be present. From the above one can see that a production animal vet must consider a wide variety of possible causes of visual impairment in small stock and be able to act quickly for the benefit of the herd/flock situation. v Regulars I Ophthalmology Column
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