Antibody isotype responses to experimental infection with bovine herpesvirus 1 in calves with colostrally derived antibody.

Calves, both positive and negative for maternal antibody to bovine herpesvirus 1 (BHV-1), were infected with BHV-1 and their serum antibody isotype responses were measured post-infection. In the case of maternal antibody negative calves there was a classical humoral response to the virus with an early IgM peak followed by IgG1 and IgG2 responses. In maternal antibody positive calves, although infection was established, no active antibody response was detected in serum except for a transient IgM peak in a single calf. By contrast, a delayed-type hypersensitivity test response could be elicited in both groups of calves by intradermal inoculation of nucleocapsid antigen at 4.5 months after the initial infection. The intradermal antigen also stimulated an antibody response. In calves with high levels of maternal antibody at the start of the experiment, and which were not subsequently infected, maternal antibody waned to negative levels by seven months of age. They were skin test negative and did not show a serological response to skin test antigen. It was concluded that isotype specific serology could not be used to distinguish calves with passive immunity to BHV-1 from those with active immunity and putative latent infection. Although the skin test had potential to provide such a distinction, it could compromise future serological monitoring of the animals.