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Blindness caused by hypovitaminosis A in feedlot cattle.

Agroup of five yearling steers was examined on arrival at a central Saskatchewan feedlot because of apparent blindness. Three yearlings were blind, with dilated pupils and absent menace reflex; two of these had bilateral exophthalmos. Ophthalmoscopic examination of both eyes of two blind yearlings revealed swollen, heart-shaped optic discs, with indistinct edges (Figure 1). Retinal hemorrhages and tortuous blood vessels were occasionally present. Small, pale shrunken optic discs were seen in one animal. All three steers had white mottling of the nontapetal area, suggesting chorioretinal scarring (Figure 1). A history was obtained from the previous owner of the yearlings. Nine months earlier the calves had been weaned, removed from pasture, and confined in a drylot. Their ration consisted of barley straw and rolled barley, with the occasional inclusion of poor quality hay. There was no supplementation with minerals or vitamins. Weight gains in the last month had apparently been poor, yet reported feed consumption was good. Recent blindness in two steers had been attributed to pinkeye by the owner. Based on this history and the clinical findings, we made a tentative diagnosis of hypovitaminosis A. Blood samples for serum vitamin A were collected. Immediate slaughter for salvage was suggested, but the owner elected to increase the weight of the cattle prior to slaughter. Therefore, all yearlings were treated intramuscularly with two million IU of aqueous vitamin A (Poten ADE, rogar/STB, London, Ontario). The cattle were placed into a small pen, with easy access to water, free-choice green hay, and a ration consisting of 50% barley silage and barley with supplement. The supplement contained vitamin A, fed at a level to provide 87,000 IU per head per day. The yearlings were presented for slaughter two months later, at which time the heads were retrieved from the abattoir for further examination. Concentrations of vitamin A in the serum prior to supplementation with vitamin A ranged from 30 to 50 tg/L. Blood samples were collected two weeks after vitamin A supplementation; serum vitamin A levels were normal, at 410 to 420 tg/L. Gross examination of the heads of the three blind yearlings revealed bilateral exophthalmos and severe bilateral stenosis of the optic foraminae and canals. Thin remnants of the nerve sheaths were all that remained of the 1-1.5 cm segments of the optic nerves which traversed the narrowed optic canals and foraminae. Histological lesions were confined to the retina, optic nerves, and optic tracts, and were present bilaterally in all three animals. The nontapetal fundus showed multifocal retinal degeneration and chorioretinal scarring (Figure 2). These focal lesions varied from loss of the rod outer segments and nuclei of the inner photoreceptor cells to loss of all retinal layers leaving focal chorioretinal scars. Variable atrophy of the inner ganglionic and nerve fiber layers was also visible. The portions of the optic nerves passing through the optic canals were extremely narrowed histologically