Testing a new herpes abortion vaccine

trial vessel inflammation (vasculitis) with its associated tissue damage is the mechanism primarily responsible for allowing further, transplacental spread of the virus to the fetus. Two tightly adjoined, interdigitating layers of epithelium (chorionic and endometrial) located at the intersection of the maternal and fetal blood circulations are the main components of the mare's placental "barrier" which normally imposes an effective physical constraint to the passage of viruses into the fetal compartment. As a result of the immunoinflammatory cellular injury within and around endometrial blood vessels, this epithelial continuity is disrupted. The localized loss of the physical integrity of uteroplacental epithelial at sites of virus-elicited inflammatory pathology is sufficient to allow the unimpeded spread of EHV- l infection through the placental barrier to reach the fetal circulation. Thus, a second critical epithelia] barrier of the mare becomes breached by the herpesvirus pathogen. The question of why the EHV-1 infected fetus is aborted is less well understood. The consensus is that either uterine pathology or fetal distress from viral infection, or a combination of both, initiates an incompletely understood, physiological cascade that leads to sudden, premature separation of the placenta from the endometrium, with subsequent anoxic death of the fetus and expulsion of the separated fetoplacental unit. From this updated understanding of the pathogenesis of equine herpesvirus abortion, an important practical ramification has emerged. Two layers of immune protection are necessary to effectively prevent EHV-1 abortion: (1) blocking viral infection of the respiratory mucosal epithelium (mucosal immunity) and (2) curtailment of the generation and systemic spread of viremic lymphocytes (cellular immunity).

From Equine Disease Quarterly, October 1998, Volume 7, Number 1 CONTACT Dr. George Allen, (606) 257-3663 Maxwell H. Gluck Equine Research Center

Testing a new herpes abortion vaccine Pregnant mares were vaccinated three times during gestation with a single dose of inactivated vaccine containing EHV- 1, EHV-4 and a carbomer-based adjuvant. Control pregnant mares were left unvaccinated and served as sentinels and as controls for the challenge method. At four weeks after the third vaccination, all mares were challenged intranasally with the AB4 strain of EHV-1. The challenged mares were observed daily for clinical signs up to foaling and any effects on gestation or the offspring were noted. Virus shedding, and cell-associated viremia were monitored daily. Mares were bled at regular intervals to determine complement fixing (CF) and virus neutralizing (VN) antibody levels. All mares became infected with EHV-1 after challenge as judged by serological and virological responses. One-hundred per cent of the control mares aborted between days 15 and 65 post infection. In contrast, only 20% (1 of 5 horses) of the vaccinated mares aborted on day 16 post challenge. The remaining mares delivered healthy foals between day 347 and day 362 of gestation. Post mortem examination showed that all abortions were EHV-l-induced. The incidence of abortions in the vaccinated group was significantly reduced when compared to the incidence in the control group.

Volume 18, Number 12, 1998

A study to investigate the efficacy of an inactivated equine herpesvirus-1 (EHV-1) and EHV-4 vaccine in reducing abortions caused by EHV-1 in pregnant mares, was described at the 8th International Conference on Equine Infectious Diseases by P.H. Flore, et al (Fort Dodge).

From these data they conclude that an inactivated, adjuvanted vaccine based on EHV-1 and EHV-4 antigens is safe for pregnant mares and significantly reduces the incidence of EHV-l-induced abortions.

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