Epidemiology and reproductive outcomes of EHV-1 abortion epizootics in unvaccinated Thoroughbred mares in South Africa.
Authors: Schulman M L, Becker A, van der Merwe B D, Guthrie A J, Stout T A E
Journal: Equine veterinary journal
Summary
# Editorial Summary: EHV-1 Abortion Epizootics in Thoroughbred Mares Equine herpesvirus 1 remains a significant reproductive threat in unvaccinated populations, yet limited data exist on how outbreak characteristics and diagnostic approaches influence both disease spread and long-term mare fertility. Schulman and colleagues analysed two EHV-1 abortion outbreaks on South African Thoroughbred farms with markedly different epidemiological patterns: one affecting 30% of pregnant mares over 135 days (Farm 1) and another causing 5.7% abortion losses over just 34 days (Farm 2), with diagnostic confirmation ranging from 40 days to 2 days respectively. Notably, EHV-1 preferentially caused late-term abortions (median 291.5 days gestation) compared to non-EHV aetiologies (median 211.9 days), and affected significantly younger mares (median 8 years) than those aborting from other infectious causes, though post-abortion complications and subsequent reproductive performance were not significantly worse in EHV-1-affected mares. The substantial variation in outbreak severity and duration between farms likely reflects differences in routes of viral introduction and the availability of rapid molecular diagnostics to implement timely biosecurity measures—a finding with considerable practical implications for outbreak management and the case for implementing vaccination programmes before epizootics occur. The unexpected finding that EHV-1 abortion did not compromise future reproductive outcomes should provide some reassurance to breeders managing affected mares, though prevention through vaccination clearly remains the preferred strategy.
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Practical Takeaways
- •EHV-1 abortion epizootics can present with markedly different patterns even within the same geographical region; route of introduction and early diagnostic capability appear to influence outbreak severity and progression
- •Younger mares appear disproportionately affected by EHV-1 abortion; this demographic pattern may assist in early outbreak recognition on unvaccinated farms
- •EHV-1 infection does not appear to compromise long-term reproductive performance more than other abortion causes, which may inform counselling and management decisions for affected mares
Key Findings
- •EHV-1 affected 30.0% of pregnant mares on Farm 1 and 5.7% on Farm 2, demonstrating highly variable morbidity between epizootics
- •Epizootic duration and interval between first and subsequent abortions differed markedly (Farm 1: 135 days and 39 days vs Farm 2: 34 days and 2 days)
- •EHV-1 aborting mares were significantly younger (median 8.0 years) than non-EHV-1 aborting mares (11.0 years, P=0.004)
- •Post-abortion complications and subsequent reproductive outcomes showed no significant association with EHV-1 abortion status