Risk Factors and Outcomes in Horses After the 2021 Outbreak of Equine EHV-1

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

J Vet Intern Med. 2025 Mar-Apr;39(2):e70040. doi: 10.1111/jvim.70040.
Authors
María de la Cuesta-Torrado 1 , Ana Velloso Alvarez 1 , Isabel Santiago-Llorente 2 , Lara Armengou 3 , Federico Nieto 4 , José Ríos 5 , Fátima Cruz-López 6 , Eduard Jose-Cunilleras 3 7
Affiliations

1 Dept. Animal Medicine and Surgery, Universidad Cardenal Herrera-CEU, CEU Universities, Alfara del Patriarca, Spain.
2 Hospital Clínico Veterinario Complutense, Universidad Complutense, Madrid, Spain.
3 Unitat Equina, Fundació Hospital Clínic Veterinari, Barcelona, Spain.
4 Equihealth Veterinarios, Barcelona, Spain.
5 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.
6 VISAVET Health Surveillance Centre, Universidad Complutense, Madrid, Spain.
7 Dept. Animal Medicine and Surgery, Universitat Autònoma de Barcelona, Barcelona, Spain.

PMID: 40055829
DOI: 10.1111/jvim.70040

Abstract

Background: Equine herpesvirus myeloencephalopathy (EHM) is a persistent threat to horses, with unclear risk factors and disease severity.

Objectives: 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 Rt value underscores the contagious nature of EHV-1. Keywords: EHV‐1; horse; quarantine; risk factor; vaccine. © 2025 The Author(s). Journal of Veterinary Internal Medicine published by Wiley Periodicals LLC on behalf of American College of Veterinary Internal Medicine.

• This topic was modified 38 minutes ago by Robert Oglesby DVM.