In the early 1930s, researchers in the Department of Animal Pathology at the Kentucky Agricultural Experiment Station, namely Dimock, Edwards, Doll and others, together with colleagues at Vanderbilt University, Goodpasture and Randall, performed groundbreaking research on an equine abortion epidemic plaguing the industry. They showed in a series of studies that the causative agent was “filterable,” i.e., a virus, which they propagated in various experimental models, including infected tissue cultures taken from aborted fetuses, grafts on chorioallantoic membranes of fertilized eggs and Syrian hamsters. The specific cause, a herpesvirus, however, was not confirmed until the 1960s.
Equine abortion virus, as EHV-1 was originally called, caused so much damage that vaccination of horses was attempted before the final identification of the virus. The early trials were done with inactivated preparations after virus amplification in hamsters but were controversial because severe local and systemic reactions ensued. What is more, they were not sufficiently effective against abortion storms within the high-density horse populations of Kentucky. Therefore, live virus preparations were introduced soon thereafter in controlled infection programs in mares.
Finally, in the early- to mid-1960s, cell culture adaptation and weakening of EHV-1 strains resulted in the development of modified-live virus (MLV) vaccines, some of which are still in use. It was only in the late 1970s that the planned infection programs were abandoned, thanks to the demonstrable safety and efficacy, albeit limited, of developed vaccines.