Dietary vitamin A restriction in feedlot calves has been used to increase intramuscular fat deposition, or marbling, resulting in higher quality beef production in terms of palatability. However, it is unclear whether vitamin A restriction which increases the beef carcass value, exerts detrimental effects on the immune response to pathogens or vaccines in cattle. To investigate the effects of vitamin A on antibody (Ab) response, Angus steers (n=40) were randomly assigned to two groups that received either low (LVA group; 1100 IU/kg of dietary dry matter (DM), n=20) or high (HVA group; 3300 IU/kg of dietary DM, n=20) dietary vitamin A. Because approximately 90-112 days are required to decrease vitamin A in serum from the liver stores, all calves were vaccinated intramuscularly with an inactivated bovine coronavirus (BCoV) and rotavirus (BRV) vaccine at post-arrival day (PAD) 112 and boosted at PAD 126 to determine the effect of vitamin A status on vaccine-induced BCoV Ab responses at PAD 140. Twenty calves (50% of total calves) shed BCoV either in feces or nasally at least once shortly after arrival (at PAD 0 and 4). The prevalence of BCoV shedding increased from 21% at PAD 0 to 41% at PAD 4, but with no BCoV shedding detected thereafter (at PAD 35, 112 and 140). The BCoV fecal shedding was negatively associated with pre-existing IgA BCoV Ab titers at PAD 4. In addition, BCoV nasal shedding was negatively associated with pre-existing IgA and IgM Ab titers at PAD 4. Serum IgG1- and IgA-BCoV Ab seroconversion at PAD 35 was positively associated with average BCoV RNA copy numbers detected in fecal samples at PAD 0 and 4. Serum IgG1 BCoV Ab titers and the ratios of IgG1 to IgG2 Abs in the HVA group were significantly higher at PAD 140 than at PAD 112, whereas those in the LVA group did not differ significantly. At PAD 140, serum IgG1 BCoV Ab titers and the ratios of IgG1 to IgG2 Abs were significantly higher in the HVA than in the LVA group. This study suggests that the low vitamin A diet suppresses the Th2 associated Ab (IgG1) responses.
In contrast, there was no vitamin A effect on serum BRV Ab responses to the inactivated BCoV/BRV vaccine. All serum isotype BRV Ab titers were significantly higher at PAD 140 than at PAD 112, regardless of vitamin A dietary status. In addition, no statistically significant differences were observed between the HVA and LVA group at both PADs. In the LVA group, Ab responses to the inactivated BRV and BCoV differed since serum IgG1 antibody responses to the inactivated BCoV vaccine were compromised, whereas those to the inactivated BRV vaccine were significantly increased at PAD 140 compared with PAD 112. This study suggests that vitamin A effects on Ab responses may be vaccine antigen-dependent. Further studies should be performed with various infectious agents or additional vaccines to confirm pathogen-specific vitamin A effects on Ab response.