Safety and efficacy of a thymidine kinase negative equine herpesvirus-1 vaccine in young horses.

A drug induced equine herpesvirus-1 (EHV-1) mutant lacking thymidine kinase inducing activity was developed and evaluated as a vaccine. The safety and effectiveness of the vaccine to protect against experimentally induced EHV-1 respiratory disease were evaluated in weanling horses free of EHV-1 neutralizing antibody. The vaccine was safe when administered either intramuscularly or intravenously, and EHV-1 was not shed intranasally during the 12 days following administration. Intranasal challenge with virulent EHV-1 was used to evaluate vaccine efficacy. Following challenge, there was a significantly (p less than 0.05) greater increase in peak body temperatures and duration of nasal virus shedding in the nonvaccinates, and a significant (p less than 0.05) increase in serum neutralizing antibody titers in the vaccinates.