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Home / Equine Research Bank / J Vet Intern Med / Risk Factors and Long-Term Outcomes in Horses After the 2021...
Journal of veterinary internal medicine2025; 39(2); e70040; doi: 10.1111/jvim.70040

Risk Factors and Long-Term Outcomes in Horses After the 2021 Outbreak of Equine Herpesvirus 1 Myeloencephalopathy, Valencia, Spain.

Abstract: Equine herpesvirus myeloencephalopathy (EHM) is a persistent threat to horses, with unclear risk factors and disease severity. Objective: To evaluate risk factors, effective reproduction rate (Rt), and long-term athletic outcomes of an EHM outbreak. Methods: Retrospective study of the 2021 EHM outbreak in Valencia, Spain, examining associations between risk factors (sex, age, breed, country of origin, and vaccination status) and case fatality rate, EHM development, and odds of returning to competition using odds ratios [95% CI] and Rt via the Robert Kochs Institute method. Results: Among 191 horses, 38 (20%) were clinically normal, 13 (7%) were subclinical, and 140 (73%) presented clinical signs (89 EHM, 64%). One hundred sixty horses were isolated at the show, while 47 were treated in hospitals. The mean age was 9.8 ± 3.0 years; 85 (45%) were mares, 79 (41%) geldings, and 27 (14%) stallions. The EHM case fatality rate was 11/89 (12%). Vaccination was associated with EHM development (4.54[2.23-9.27]; OR[95% CI]; p < 0.001) and case fatality rate (3.9[1.1-14.4]; OR[95% CI]; p < 0.043). EHV-1-infected horses without EHM were more likely to return to competition (54/61; 89%) than those recovering from EHM (65/89; 73%; p = 0.024). It was initially 4.2 and decreased to < 1 within 2 weeks of the outbreak. Conclusions: During the 2021 EHV-1 outbreak in Valencia, vaccination status appears to be associated with EHM development. Horses recovering from EHM had slightly lower chances of returning to competition than those shedding EHV-1 without EHM. The high R value underscores the contagious nature of EHV-1.
© 2025 The Author(s). Journal of Veterinary Internal Medicine published by Wiley Periodicals LLC on behalf of American College of Veterinary Internal Medicine.
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Publication Date: 2025-03-09 PubMed ID: 40055829PubMed Central: PMC11888933DOI: 10.1111/jvim.70040Google Scholar: Lookup
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Summary

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Overview

  • This study analyzed the 2021 outbreak of equine herpesvirus myeloencephalopathy (EHM) in Valencia, Spain to identify risk factors, assess how contagious the outbreak was, and evaluate the long-term athletic outcomes for affected horses.

Background and Objective

  • Equine herpesvirus myeloencephalopathy (EHM) is a neurological disease caused by equine herpesvirus type 1 (EHV-1), which poses ongoing health risks to horse populations.
  • There is limited knowledge regarding which factors increase the risk of horses developing EHM during an outbreak, how severe the disease is in terms of fatalities, and how it affects horses’ ability to return to athletic competition.
  • The objective was to investigate risk factors (such as sex, age, breed, country of origin, and vaccination status), estimate the effective reproduction rate (Rt) during the outbreak, and track the long-term outcomes of horses post-EHM.

Methods

  • A retrospective study design was used, analyzing data from 191 horses involved in the 2021 EHM outbreak in Valencia, Spain.
  • Horses were classified based on clinical signs into clinically normal, subclinical infection, and symptomatic with clinical EHM.
  • Associations between various risk factors and outcomes (development of EHM, death rate/case fatality, and return to competition) were tested using odds ratios and 95% confidence intervals.
  • The effective reproduction number (Rt), which measures how many animals one infected horse infects on average over time, was calculated using the Robert Koch Institute method.

Key Findings

  • Out of 191 horses:
    • 38 (20%) showed no clinical signs.
    • 13 (7%) were subclinical carriers.
    • 140 (73%) showed clinical signs, and among these 89 (64% of clinical cases) developed EHM.
  • Isolation and hospitalization:
    • 160 horses were isolated at the competitive event.
    • 47 horses required hospital treatment.
  • Demographics:
    • The average age was 9.8 years.
    • Sex distribution was 45% mares, 41% geldings, and 14% stallions.
  • EHM case fatality rate was 12% (11 deaths out of 89 EHM cases).
  • Surprisingly, horses that were vaccinated had higher odds of developing EHM (OR=4.54, highly significant) and of dying from it (OR=3.9, significant).
  • Regarding athletic recovery:
    • Horses infected with EHV-1 but without neurological disease (no EHM) had a high return-to-competition rate (89%).
    • Horses recovering from EHM had a significantly lower return rate (73%).
  • The effective reproduction number (Rt) started at 4.2, indicating a high contagion rate, but dropped below 1 within two weeks, showing containment of the outbreak.

Interpretation and Conclusions

  • This outbreak study underscores that EHV-1 is highly contagious, as indicated by the initial high Rt value.
  • The association of vaccination with increased EHM development and mortality is unexpected and may suggest complex interactions between vaccine-mediated immunity and disease progression or could be influenced by other confounding factors requiring further investigation.
  • Horses that developed EHM faced greater challenges in returning to competitive sport compared to those that were infected but did not develop neurological signs.
  • The study highlights the importance of surveillance, isolation methods, and careful management during EHV-1 outbreaks to limit spread and reduce impact on equine health and athletic performance.
  • Further studies are needed to clarify the role of vaccination and to develop better preventive and treatment strategies.

Cite This Article

APA
de la Cuesta-Torrado M, Velloso Alvarez A, Santiago-Llorente I, Armengou L, Nieto F, Ríos J, Cruz-López F, Jose-Cunilleras E. (2025). Risk Factors and Long-Term Outcomes in Horses After the 2021 Outbreak of Equine Herpesvirus 1 Myeloencephalopathy, Valencia, Spain. J Vet Intern Med, 39(2), e70040. https://doi.org/10.1111/jvim.70040

Publication

Journal of veterinary internal medicine
ISSN: 1939-1676
NlmUniqueID: 8708660
Country: United States
Language: English
Volume: 39
Issue: 2
Pages: e70040
PII: e70040

Researcher Affiliations

de la Cuesta-Torrado, María
  • Dept. Animal Medicine and Surgery, Universidad Cardenal Herrera-CEU, CEU Universities, Alfara del Patriarca, Spain.
Velloso Alvarez, Ana
  • Dept. Animal Medicine and Surgery, Universidad Cardenal Herrera-CEU, CEU Universities, Alfara del Patriarca, Spain.
Santiago-Llorente, Isabel
  • Hospital Clínico Veterinario Complutense, Universidad Complutense, Madrid, Spain.
Armengou, Lara
  • Unitat Equina, Fundació Hospital Clínic Veterinari, Barcelona, Spain.
Nieto, Federico
  • Equihealth Veterinarios, Barcelona, Spain.
Ríos, José
  • Department of Clinical Farmacology, Hospital Clinic and Medical Statistics Core Facility, Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), 08036 Barcelona, Spain; and Biostatistics Unit, School of Medicine, Universitat Autònoma de Barcelona, Barcelona, Spain.
Cruz-López, Fátima
  • VISAVET Health Surveillance Centre, Universidad Complutense, Madrid, Spain.
Jose-Cunilleras, Eduard
  • Unitat Equina, Fundació Hospital Clínic Veterinari, Barcelona, Spain.
  • Dept. Animal Medicine and Surgery, Universitat Autònoma de Barcelona, Barcelona, Spain.

MeSH Terms

  • Animals
  • Horses
  • Spain / epidemiology
  • Disease Outbreaks / veterinary
  • Retrospective Studies
  • Herpesvirus 1, Equid
  • Risk Factors
  • Female
  • Male
  • Encephalomyelitis, Equine / epidemiology
  • Encephalomyelitis, Equine / virology
  • Horse Diseases / epidemiology
  • Horse Diseases / virology
  • Herpesviridae Infections / veterinary
  • Herpesviridae Infections / epidemiology
  • Herpesviridae Infections / virology
  • Vaccination / veterinary

Conflict of Interest Statement

The authors declare no conflicts of interest.

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