Epidemiology and reproductive outcomes of EHV-1 abortion epizootics in unvaccinated thoroughbred mares in South Africa

Abstract

REASONS FOR PERFORMING STUDY : EHV-1 is one of the most common causes of infectious abortion in mares. Analyzing the demography of outbreaks and detailing subsequent reproductive performance of affected mares will assist in the management of future (threatened) epizootics. OBJECTIVES : To examine the epidemiology and reproductive outcomes of two EHV-1 abortion epizootics with very different patterns of morbidity. Study design: Epidemiological and reproductive data were analyzed retrospectively following abortion epizootics associated with EHV-1, but initiated via different routes, among unvaccinated mares on two Thoroughbred farms in South Africa. METHODS : Aborting mares were assigned to either the EHV-1 abortion cohort via positive immunostaining (farm 1 and 2) or qPCR (farm 2) on tissue samples, or to the non-EHV abortion cohort. RESULTS : During their respective epizootics, EHV-1 abortions affected 9/30 (30.0%) and 18/316 (5.7%) of the pregnant mares on farms 1 and 2 respectively; there were also 25 (7.9%) non-EHV abortions on farm 2. Epizootic differences included: durations (farm 1 = 135 d; farm 2 = 34 d), intervals between first and subsequent abortions (farm 1 = 39 d; farm 2 = 2 d), and intervals to confirmation of EHV-1 (farm 1 = 40 d; farm 2 = 2 d). The median age of EHV-1 abortion mares (8.0; 5-18 years) in both epizootics was similar, but significantly younger (P= 0.004) than the 25 non- EHV-1 abortion mares (11.0; 4-24 years) on farm 2. Gestation stage (median; range) of EHV-1 (291.5; 277-313) and non-EHV-1 (211.9; 61-339 d) abortions were significantly different (P=0.001). The post-abortion complications and subsequent reproductive outcomes had no significant association with EHV-1 abortion. CONCLUSIONS : The marked difference in morbidity between the two epizootics may be associated with routes of introduction or intervention strategy dictated by availability of molecular diagnostic techniques. Unexpectedly, EHV-1 was not more commonly associated with post-abortion complications.