Effect of levamisole in immune responses to bovine herpesvirus-1.

The effect of levamisole on bovine immune responses to infectious bovine rhinotracheitis virus was assessed under laboratory and commercial feedlot situations. In all instances, levamisole appeared to have a beneficial effect on antibody responses of the cattle after vaccination. In the smaller scale pilot trials, levamisole appeared to be more efficacious when given 7 days after vaccination, presumably when a large amount of viral antigens was present as a result of viral replication. In the larger feedlot trial, however, response to administration of levamisole at the time of vaccination appeared to be slightly better than if given 7 days later. In all instances that animals had an antibody response before they were challenge-exposed with virulent virus, rectal temperature responses remained below 40 C, indicating that a threshold level of immunity may be acquired after the vaccination and that elevation of this threshold level does not necessarily alter the clinical disease. However, the amount of virus replication and shedding after challenge exposure seemed to be correlated with the level of immunity. These results are discussed in relationship to the role of immunity levels to spread of virus within a feedlot.