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Veterinary sciences2026; 13(2); 121; doi: 10.3390/vetsci13020121

Identifying Host-Characteristics and Management Risk Factors in a California Equine Herpesvirus Myeloencephalopathy (EHM) Outbreak.

Abstract: Equine alphaherpesvirus-1 (EHV-1) is a highly contagious virus that can cause the neurological form, equine herpesvirus myeloencephalopathy (EHM). Understanding transmission-related risk factors is crucial for improving prevention strategies and guiding effective control measures. In this study, we collected data from 63 horses that had previously participated in the February 2022 winter horse show season at the Desert International Horse Park (DIHP) (26 cases and 37 controls) to identify host and management factors associated with EHV-1 infection and/or EHM development during the February 2022 outbreak at the DIHP in Thermal, California. Risk factors were evaluated using multivariable logistic regression and a random forest model with conditional permutation importance. Greater age was associated with higher odds of becoming a case (OR = 1.33; 95%CI: 1.04-1.69, -value: 0.01). Compared with hunters, jumpers had greater odds of developing EHV-1 and/or EHM (OR = 7.37; 95%CI: 1.57-34.61, -value: 0.01). Sharing a barn was also strongly associated with EHV-1 and/or EHM case status (OR = 7.37; 95%CI: 1.79-30.29, -value: <0.01). The machine-learning-based rankings were concordant with the regression estimates. Age, main activity, and sharing a barn were the most influential risk factors associated with elevated odds of developing EHV-1 and/or EHM. These results highlight specific demographic and management-related risk factors that could inform targeted prevention strategies.
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Publication Date: 2026-01-27 PubMed ID: 41745915PubMed Central: PMC12944867DOI: 10.3390/vetsci13020121Google Scholar: Lookup
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  • Journal Article

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study investigated the factors related to horse characteristics and management practices that influenced the risk of equine herpesvirus myeloencephalopathy (EHM) during an outbreak at a horse show in California.
  • The goal was to identify which factors increased the likelihood of horses contracting EHV-1 and developing EHM, to help guide better prevention and control measures.

Background on EHV-1 and EHM

  • Equine alphaherpesvirus-1 (EHV-1) is a contagious virus affecting horses.
  • One severe manifestation is equine herpesvirus myeloencephalopathy (EHM), a neurological disease causing significant illness.
  • Understanding how the virus spreads and what host or management factors contribute to infection and disease development is critical for disease control.

Study Context and Data Collection

  • The study focused on a 2022 outbreak at the Desert International Horse Park (DIHP) in Thermal, California.
  • Data were collected from 63 horses who attended a winter horse show season in February 2022.
  • Among these horses, 26 were infected or developed EHM (cases), and 37 remained uninfected (controls).
  • Researchers collected detailed information on horse demographics and management factors to assess which factors were linked to infection and disease.

Analytical Methods

  • Two main quantitative approaches were used:
    • Multivariable logistic regression to estimate odds ratios (ORs) for risk factors.
    • Machine learning via a random forest model with conditional permutation importance to rank variable influence.
  • These complementary approaches allowed the researchers to validate findings and identify the most impactful factors.

Key Findings on Risk Factors

  • Age: Older horses had significantly higher odds of becoming infected or developing EHM.
    • Each incremental increase in age raised the odds by about 33% (OR = 1.33).
  • Main Activity: The type of equestrian activity was linked to risk.
    • Jumpers were over 7 times more likely to become cases compared to hunters (OR = 7.37).
  • Management – Barn Sharing: Horses sharing a barn had markedly increased odds of infection/EHM (OR = 7.37).
    • This highlights close contact through shared housing as a key transmission route.

Interpretation of Results

  • The concordance between the logistic regression and machine learning models strengthens confidence in these findings.
  • Age, type of equestrian activity, and barn sharing emerged as the most influential risk factors.
  • These factors suggest that older horses, those involved in jumping, and those housed closely with others face greater risk during an outbreak.

Implications for Prevention and Control

  • Targeted prevention strategies can focus on:
    • Minimizing close contact in barns during outbreaks, possibly by isolating horses or limiting shared housing.
    • Special monitoring and biosecurity for older horses and those involved in jump activities.
    • Adjustments in management practices during horse shows or competitions to reduce spread.
  • Understanding these risk factors helps veterinarians and horse managers tailor approaches to limit EHV-1 transmission and reduce the incidence of EHM.

Cite This Article

APA
Gordon S, Pusterla N, Finno CJ, Young A, Martínez-López B. (2026). Identifying Host-Characteristics and Management Risk Factors in a California Equine Herpesvirus Myeloencephalopathy (EHM) Outbreak. Vet Sci, 13(2), 121. https://doi.org/10.3390/vetsci13020121

Publication

Veterinary sciences
ISSN: 2306-7381
NlmUniqueID: 101680127
Country: Switzerland
Language: English
Volume: 13
Issue: 2
PII: 121

Researcher Affiliations

Gordon, Shadira
  • Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
Pusterla, Nicola
  • Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
Finno, Carrie J
  • Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
  • Center for Equine Health, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
Young, Amy
  • Center for Equine Health, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
Martínez-López, Beatriz
  • Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.

Grant Funding

  • 2021-68014-34143 / Agriculture and Food Research Initiative Competitive Grant

Conflict of Interest Statement

The authors declare no conflicts of interest.

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