Original Article: The outcome of vaccinating five pregnant mares with a commercial equine viral arteritis vaccine

摘要

Integral to current programmes for the prevention and control of equine viral arteritis (EVA) in North America is the availability of a safe and effective vaccine against the disease. This is a modified live virus (MLV) vaccine (Arvac)1, which has been in commercial use since 1985 (Timoney and McCollum 1993). It was derived from an experimental vaccine against EVA that was developed earlier (Doll ef al. 1968). In safety and efficacy studies of their vaccine, Doll ef al. (1968) demonstrated that equine arteritis virus (EAV) serially passaged 131x in primary horse kidney followed by 32x in primary rabbit kidney (HK131-RK32) was not fully attenuated for pregnant mares in that it induced abortion in 2 mares in the 8th to 9th month of gestation when inoculated directly into the fetus or into the amniotic sac. Furthermore, they isolated virus from the lungs and spleen of a foal killed within a few hours of birth whose dam had been vaccinated 12 days earlier with virus of higher passage history in cell culture (HK131-RK80). Subsequently, McCollum (1970) demonstrated that a modified strain of EAV of still higher passage history (HK131-RK111) was safe when tested in pregnant pony mares. Neither abortion nor any other adverse effect was observed in 18 maresinoculated by the intramuscular route when 30 days pregnant to near term. Although the current MLV vaccine against EVA has a significantly higher passage history in cell culture (HK131-RK111-Eq dermis 24) than the earlier vaccines evaluated by Doll et al. (1968) and McCollum (1969, 1970, 1986), the vaccine manufacturers considered it prudent not to recommend its use in pregnant mares, especially in the last 2 months of gestation, unless in situations of high risk of natural exposure to the virus. Notwithstanding this label restriction, the vaccine, in fact, was used to vaccinate pregnant mares on the occasion of several major occurrences of EVA over the past 20 years. Use of the vaccine under such circumstances did not apparently result in outbreaks of vaccine-induced abortion or illness and death in neonatal foals. However, there is one report implicating the MLV vaccine strain as the cause of an isolated case of equine abortion (Moore et al. 2003). The dam, a recently vaccinated mare, aborted in the course of an extensive outbreak of EVA on a Thoroughbred breeding farm in Illinois in 1994. Viral identification was based on the nucleotide sequence homology of open reading frame 5 of the isolate and that of the vaccine strain of EAV.