Vitamin A of grazing animals is totally dependent on exogenous supply. According to previous studies, the minimum requirement of vitamin A in cow is about 30 IU/kg each day, and the demand should be increased by 50% during lactation and pregnancy . The long-term lack of dietary vitamin A or carotene in cow can easily result in hypovitaminosis A in newborn calves . The rapid growth and development of calves need enough vitamin A, through liver storage and extra implements, mainly dependent on breast milk. However, in our current study, beef cattle were fed with rice straw and distillers’ grains for a long time, without green feed, carrots and other vitamin A-rich feed, which resulted in vitamin A deficiency in pregnant and lactation cows. Under normal circumstances, the content of vitamin A in the cattle feed should not be less than 400,000 IU/kg in order to ensure normal growth and development. However, the vitamin A content was 1360 IU/kg in feed of this farm. In the serums of affected calves, the average content of vitamin A was 130 IU/L at the late stage in this farm, far below the normal value , confirming this disease was related with vitamin A deficiency in calves.
The blindness disorders are regarded as the typical signs in affected calves of vitamin A deficiency [21, 22]. The mechanisms of blindness due to vitamin A deficiency have been explored for many years. In terms of blindness or associated signs, fundus examination plays an important role in the quick diagnosis of hypovitaminosis A. At the early stage of hypovitaminosis A, papilledema is the first sign of changes in the optic disc and is reversible under the experimental conditions. The most representative characteristic, however, is pigmentation with various sizes and shapes on the tapetum nigrum of retina. In this study, no clear papilledema was found, which might be due to the delay of taking photos of fundus. However, the irregular pigmentation was found clearly with varied sizes at local areas in the tapetum nigrum and tapetum lucidum, especially in tapetum nigrum (Figure 1).
Vitamin A plays a significant role in immune system function of animals. In humans, it is well accepted that `vitamin A deficiency impairs innate immunity by impeding normal regeneration of mucosal barriers damaged by infection, and by diminishing the function of immune cells [23, 24]. In the case of diarrheal diseases, vitamin A could promote regeneration of damaged mucosal epithelium and enhance the phagocytic activity of neutrophils and macrophages. It has also been shown that vitamin A can reduce the incidence and duration of diarrhea in children [25, 26]. In this study, based on the clinical findings, his-pathological changes in intestines and vitamin A determinations, we could conclude that hypovitaminosis A impeded mucosal barriers of intestines and lowered the immunity of cattle which made the cattle more susceptible to E. coli, and the intractable diarrhea of cattle was likely attributed to both hypovitaminosis A and pathogenic E. coli infections, which was in accordance to previous studies [27, 28]. Therefore, the corresponding treatment should include vitamin A supplement and antibacterial drugs utilization according the antibiotic susceptibility test. It is intriguing that Rumen distention was present in affected calves, whether it is due to hypovitaminosis A or E. coli infections remains further studies. Further research on E. coli isolates should be performed to identify genetic material encoding for specific virulence factors of E. coli and to demonstrate the effect of vitamin A deficiency on expression of innate immunity-related genes and development of immune cells in gastrointestinal tract of cattle .