Identification of a mutant bovine herpesvirus-1 (BHV-1) in post-arrival outbreaks of IBR in feedlot calves and protection with conventional vaccination

Outbreaks of infectious bovine rhinotracheitis (IBR) have recently been obs erved in vaccinated feedlot calves in Alberta a few months post-arrival. To investigate the cause of these outbreaks, lung and tracheal tissues were c ollected from calves that died of IBR during a post-arrival outbreak of dis ease. Bovine herpesvirus-1 (BHV-1), the causative agent of IBR, was isolate d from 6 out of 15 tissues. Of these 6 isolates, 5 failed to react with a m onoclonal antibody specific for one of the epitopes on glycoprotein D, one of the most important antigens of BHV-1. The ability of one of these mutant BHV-1 isolates to cause disease in calves vaccinated with a modified-live IBR vaccine was assessed in an experimental challenge study. After one vacc ination, the majority of the calves developed humoral and cellular immune r esponses. Secondary vaccination resulted in a substantially enhanced level of immunity in all animals. Three months after the second vaccination, calv es were either challenged with one of the mutant isolates or with a convent ional challenge strain of BHV-1. Regardless of the type of virus used for c hallenge, vaccinated calves experienced significantly (P < 0.05) less weigh t loss and temperature rises, had lower nasal scores, and shed less virus t han non-vaccinated animals. The only statistically significant (P < 0.05) d ifference between the 2 challenge viruses was the amount of virus shed, whi ch was higher in non-vaccinated calves challenged with the mutant virus tha n in those challenged with the conventional virus. These data show that cal ves vaccinated with a modified-live IBR vaccine are protected from challeng e with either the mutant or the conventional virus.