Relationship between equine herpesvirus-1 viremia and abortion or equine herpesvirus myeloencephalopathy in domesticated horses: A systematic review.
Authors: Soboll-Hussey Gisela, Dorman David C, Burgess Brandy A, Goehring Lutz, Gross Peggy, Neinast Claire, Osterrieder Klaus, Pusterla Nicola, Lunn David P
Journal: Journal of veterinary internal medicine
Summary
# Equine Herpesvirus-1 Viremia and Clinical Outcomes: What the Evidence Actually Shows EHV-1 infection causes significant morbidity in horses, yet fundamental questions remain about whether the severity and persistence of viraemia predict the development of abortion or neurological disease—knowledge that would help clinicians stratify risk and guide intervention decisions. Researchers performed a systematic review of peer-reviewed literature published before January 2021, identifying 34 studies (30 examining neurological outcomes, 8 examining abortion) that specifically measured viraemia levels and duration alongside clinical consequences, with the latter group predominantly using experimental challenge models with Ab4 and OH03 virus strains. A consistent finding emerged: detectable viraemia preceded both equine herpesvirus myeloencephalopathy (EHM) and abortion in the studies reviewed, suggesting viral replication in the bloodstream is a necessary precursor to systemic complications. However, the analysis revealed no reliable relationship between the magnitude or duration of viraemia and whether individual horses subsequently developed neurological signs or aborted—a critical gap highlighting that viraemia detection alone cannot currently predict clinical outcomes. For equine practitioners, this underscores that whilst virological monitoring remains valuable for outbreak confirmation and epidemiological tracking, it cannot yet serve as a clinical predictor of severe disease, and management decisions should continue to rely on clinical presentation, risk assessment, and biosecurity rather than viraemia titres.
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Practical Takeaways
- •EHV-1 viremia typically precedes clinical signs of neurological disease or abortion, suggesting early detection of viremia may help identify at-risk horses before clinical manifestation
- •Current evidence cannot establish a threshold viremia level or duration that predicts abortion or EHM, limiting ability to use viremia measurements alone for prognostic purposes
- •Veterinarians should continue monitoring for clinical signs of EHM and reproductive loss in EHV-1 infected horses regardless of viremia levels, as viremia presence alone is insufficient to predict outcomes
Key Findings
- •34 studies met inclusion criteria from 189 identified studies, with 30 evaluating viremia and neurologic outcomes and 8 examining viremia and abortion
- •Viremia was generally detectable before the onset of either EHM or abortion in both experimental and observational studies
- •Incidence rates for EHM and abortion varied considerably among studies, with no clear correlation established between viremia magnitude or duration and clinical outcomes
- •Risk of bias was low to moderate across studies, but sample sizes were small and multiple studies reported negative outcome data