Vitamin A is an important immune system nutrient. Vitamin A has a significant role in providing immunologic protections against viral, bacterial and protozoan infections. The function of virtually all immune cells (neutrophils, macrophages, NK cells, T and B lymphocytes etc.) are affected by vitamin A. Vitamin A is associated with the maintenance of the protective mucous membranes of the respiratory and digestive tracts.
It is thought that a deficiency in vitamin A causes damage to these membranes which allows bacteria and viruses an opportunity for invasion. Overall vitamin A deficiency rather than vitamin A excess through
supplementation has the most profound and significant effects on the immune system. It has been suggested that in cases of BRD a vitamin A injection to calves known to be deficient or marginal in vitamin A status would benefit the animal and its immune system by rapidly increasing body stores of vitamin A.
It also functions in visual purple, a compound in the eye needed for sight when an animal adapts from light to dark. Vitamin A is also essential for proper kidney function and normal development of bones, teeth and nerve tissue. Vitamin A deficiency results in tissue changes associated with vision, bone development,
skin and the digestive tract. One of the first easily detected signs of vitamin A deficiency in cattle is night blindness. An easy way to check for this condition is to place an obstacle in pathway of cattle and notice if they stumble over it at twilight. Other early signs are loss of appetite, rough hair coat, dull eyes, slowed gains and reduced feed efficiency. Diarrhea and pneumonia may be the first indicators, especially in young animals. Later developments include excessive watering of the eyes, staggering gait, lameness or stiffness in the knees and hock joints and swelling of the legs and brisket (and sometimes in the abdominal region). Feedlot cattle with advanced vitamin A deficiency often pant excessively at high temperatures and go
into convulsions when excited. Signs of vitamin A deficiency in breeding herds include lowered
fertility and calving percentage