Equine Herpes Virus Abortions Over Last 50+ Years

The monitoring of abortions attributable to equine herpes virus type-1 (EHV-1) initiated in 1957 among the Thoroughbred mare population of Central Kentucky has continued annually for the past 51 years. The number of abortions per 1,000 pregnant mares (as illustrated in Figure 2), based on accessions to the University of Kentucky Livestock Disease Diagnostic Center (LDDC) and confirmed by laboratory diagnosis, is considered an accurate reflection of the field incidence. This accurate number is due to the diligence of farm managers and attending veterinarians ensuring that all fetuses and neonatal foal losses are submitted for examination.

This is the correct version of Figure 2. Due to a printing error, the vertical axes were displayed incorrectly in the printed version. We apologize for this error.

The figure for the number of pregnant mares is derived annually from the number of foals registered by the Jockey Club, taken as 70% of mares considered in foal. The most recent Jockey Club 2007 Fact Book reported 9,903 foals registered in Kentucky for 2005—29% of the U.S. foal crop—which translates to 14,147 pregnant mares. This number represents a recovery from the decline in numbers recorded during 2001 and 2002 due to the effects of Mare Reproductive Loss Syndrome (MRLS). The number of EHV-1 abortions per 1,000 pregnant mares has remained below five since 1977 despite a doubling in size of the mare population. Since 1977 the overall number of EHV-1 abortions has ranged from a low of 11 in 1993 to a high of 47 in 1997, with 18 recorded in 2006 and 22 in 2007. The majority of cases in recent years are single events on individual farms among a population of mares that is routinely vaccinated against the disease. However, multiple abortions may still occur, as evidenced by one farm in 2007 that experienced six abortions. When multiple abortions occur, the initial or index case is usually the source of infection to subsequent losses. The aborted fetus and placenta are a potent source of virus that overwhelms the immunity of even a vaccinated mare that is a contact in the same barn or paddock. To prevent such an occurrence, the precautions outlined in Figure 3 should be strictly followed. The accumulated data confirms that sound management practices (as illustrated in Figure 4) combined with an annual vaccination program at five, seven, and nine months of pregnancy, have contributed to a low level of EHV-1 abortions over the past 30 years.

Source: Equine Disease Quarterly