Ocular abnormalities associated with hypovitaminosis A in Hanwoo calves: a report of two cases

KANG, Seonmi; PARK, Chanho; SEO, Kangmoon

In a previous study, the clinical signs of hypovitaminosis A were observed in male cattle, similar to the calves in this case study [7] . In addition, it is notable that among the herd of 11 cattle, it was the 2 youngest calves that were affected with blindness from vitamin A deficiency. On a diet deficient in vitamin A, the length of time until clinical signs develop is dependent on the extent of the liver stores. Young animals have small stores but require more vitamin A; thus, they are more susceptible to hypovitaminosis A [3] . Because the liver is the normal site for vitamin A storage, measuring the vitamin A concentration in the liver might allow for the detection of hypovitaminosis A [7] . It has also been reported that the clinical signs of hypovitaminosis A develop between 6 and 12 months of being administered a diet deficient in vitamin A, after the depletion of vitamin A stores in the liver [6] . In this study, both calves were suckling or weaning; therefore, it was suggested that the cows had developed early onset hypovitaminosis A [7] . Thus, the other 9 cattle should have been tested to measure their serum vitamin A levels, as cattle can be affected by varying degrees of blindness and early blindness can be reversed by vitamin A therapy [3, 9] . In this study, testing for all cattle was refused by the owner for financial reasons. The measurement of vitamin A content in the feed available to this herd could have been helpful in confirming vitamin A deficiency [5] , but it was unfortunately not tested. However, it has been reported that beef cattle fed with a diet composed of white lees, rice straw and concentrate feed without green forage, similar to the diet in this study, were affected with hypovitaminosis A, and the vitamin A content of the cattle diet was demonstrated to be far below normal values [5] . Blindness resulting from hypovitaminosis A can be classified into one of three types: 1) night blindness related to the deficiency of the visual pigment rhodopsin made from vitamin A, which is reversible; 2) degenerative changes in the retina associated with prolonged vitamin A deficiency; and 3) stenosis of the optic canal, which induces an irreversible loss of vision [9] . However, the earliest signs of night blindness, which can be reversed by administering a vitamin A supplement, generally go unnoticed by cattle owners [3] . In this study, the two affected calves could not regain their vision following dietary supplementation with vitamin A. This suggested that the dietary vitamin supplements alone might not be enough to reverse blindness, or that blindness might be the result of permanent optic nerve necrosis and/or retinal degeneration due to vascular impairment caused by a narrowed optic canal due to altered skeletal growth [3, 6] . TMR is a method of feeding cattle with all the food ingredients blended into a complete ration [8] . TMR consists of minimal forage and a high proportion of grain, thus increasing growth in and profitability of feedlot cattle [8] . However, hypovitaminosis A has been reported to occur in cattle fed on rations with insufficient green forage [6, 7] . Hypovitaminosis A can be treated with an intramuscular injection of aqueous vitamin A or, more economically, a dietary supplement [3, 7] . Treatment with intramuscular injection of vitamin A at 3,000-6,000 iu/kg every 60 days [3] or continuous dietary supplement containing 80 iu/kg (for cattle on a high energy ration) or 100 iu/kg (for a pre