Production of bovine herpesvirus type 1-seronegative latent carriers by administration of a live-attenuated vaccine in passively immunized calves

The consequences of the vaccination of neonatal carves with the widely used live-attenuated temperature-sensitive (ts) bovine herpesvirus type 1 (BHV- 1) were investigated. The ts strain established acute and latent infections in all vaccinated calves either with or without passive immunity. Four of seven calves vaccinated under passive immunity became clearly BHV-1 seroneg ative by different serological tests, as did uninfected control calves afte r the disappearance of maternal antibodies, and they remained so for long p eriods. A cell-mediated immune response was detected by a BHV-1 gamma inter feron assay, but this test failed to detect the seronegative latent carrier s (SNLCs). While they are not detected, SNLCs represent a threat for BHV-1- free herds or countries. This study demonstrates that SNLCs can be easily o btained by inoculation with a live-attenuated BHV-1 under passive immunity and that latent carrier animals without any antibody do exist. Consequently , this situation could represent a good model to experimentally produce SNL Cs.