Risk Factors and Long-Term Outcomes in Horses After the 2021 Outbreak of Equine Herpesvirus 1 Myeloencephalopathy, Valencia, Spain.
Authors: de la Cuesta-Torrado María, Velloso Alvarez Ana, Santiago-Llorente Isabel, Armengou Lara, Nieto Federico, Ríos José, Cruz-López Fátima, Jose-Cunilleras Eduard
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: EHV-1 Myeloencephalopathy Outcomes and Risk Factors During the 2021 Valencia outbreak, researchers retrospectively analysed 191 exposed horses to identify predictors of clinical disease severity and long-term athletic prognosis following equine herpesvirus 1 infection. Of those tracked, 73% developed clinical signs (89 with neurological disease), with an 12% fatality rate among affected horses, whilst vaccination status emerged as a significant and somewhat counterintuitive risk factor—vaccinated horses showed 4.5-fold greater odds of developing myeloencephalopathy and 3.9-fold higher fatality risk, suggesting either that vaccinated horses represented a higher-risk population or that vaccination status captured other unmeasured variables influencing disease outcome. Recovery from neurological disease carried meaningful long-term consequences: only 73% of horses with EHM returned to competition compared to 89% of those shedding virus without neurological involvement, indicating that even successful neurological recovery may leave residual deficits affecting athletic performance. The effective reproduction rate (Rt) peaked at 4.2 initially but dropped below 1 within two weeks, demonstrating the rapid spread potential of EHV-1 in congregated populations and the effectiveness of outbreak control measures. These findings underscore the necessity for robust biosecurity protocols at shows and competitions, prompt isolation of infected animals regardless of clinical signs, and realistic prognostic counselling for owners of horses recovering from EHM regarding return-to-work timelines and performance expectations.
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Practical Takeaways
- •The unexpected association between vaccination and EHM development warrants investigation of vaccine protocols and timing in outbreak situations; consult with your veterinarian about vaccination strategy at high-risk events
- •Horses recovering from EHM may have persistent performance limitations even after clinical recovery; plan conservative return-to-work protocols and consider performance monitoring post-recovery
- •EHV-1 spreads rapidly (Rt 4.2 initially); implement strict isolation protocols within 2 weeks of exposure to break transmission chains and prevent outbreak amplification
Key Findings
- •Among 191 horses exposed during the 2021 Valencia EHV-1 outbreak, 73% developed clinical signs with 89 cases of EHM and a case fatality rate of 12%
- •Vaccination status was associated with increased risk of EHM development (OR 4.54, p<0.001) and case fatality (OR 3.9, p=0.043)
- •Horses with EHV-1 infection without EHM had significantly higher return-to-competition rates (89%) compared to EHM survivors (73%, p=0.024)
- •Effective reproduction rate (Rt) was initially 4.2 and decreased to <1 within 2 weeks, confirming high contagiousness of EHV-1