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Canadian journal of veterinary research = Revue canadienne de recherche veterinaire2026; 90(1); 16-24;

Molecular prevalence of equine alphaherpesvirus-1 shedding in healthy broodmares in Ontario.

Abstract: Equine herpesvirus-1 (EHV-1) is ubiquitous in the horse population, but prevalence estimates have ranged from 3 to 88% depending on the population and method of sampling. No prevalence studies have been carried out in Ontario, Canada. The objective of this study was to measure the prevalence of EHV-1 shedding in healthy broodmares in Ontario. A total of 381 mares from 42 farms in Ontario were sampled, including pregnant and barren broodmares. Samples were collected from the nose, vagina, and blood of each mare up to 6 times from December 2016 through October 2017 using a cross-sectional study design. The EHV-1 glycoprotein B (gB) copy number was measured using droplet digital polymerase chain reaction (ddPCR). A survey was completed at time of sampling regarding signalment, pregnancy status, and vaccination. Overall, 85% of the mares sampled were positive for EHV-1 from at least one site, on at least one occasion. Samples were positive 8.1%, 15.8%, and 17.2% of the time from the nose, vagina, and white blood cells, respectively. Pregnant mares had increased odds of shedding virus from the nose [odds ratio (OR) = 1.50, 95% confidence interval (CI): 1.03 to 2.18, = 0.037]. Vaccination only reduced the odds of virus presence in blood (OR = 0.70, 95% CI: 0.49 to 0.99, = 0.043). Advanced gestation appeared to also have a sparing effect on virus presence in blood (OR = 0.89, 95% CI: 0.85 to 0.93, < 0.001). Most mares in Ontario were positive for EHV-1 despite being healthy and vaccinated, although the amount of viral DNA detected was extremely low. In addition, the vagina was identified as a source of viral shedding. L’herpèsvirus équin de type 1 (EHV-1) est omniprésent chez les chevaux, mais les estimations de prévalence varient de 3 à 88 % selon la population et la méthode d’échantillonnage. Aucune étude de prévalence n’a été menée en Ontario, Canada. L’objectif de cette étude était de mesurer la prévalence de l’excrétion d’EHV-1 chez les juments poulinières en bonne santé en Ontario. Au total, 381 juments provenant de 42 élevages ontariens ont été échantillonnées, incluant des juments gestantes et non gestantes. Des échantillons sanguins ainsi que des prélèvements à partir du nez et du vagin ont été effectués chez chaque jument, jusqu’à 6 fois, entre décembre 2016 et octobre 2017, selon un plan d’étude transversal. Le nombre de copies de la glycoprotéine B (gB) de l’EHV-1 a été mesuré par PCR numérique en gouttelettes (ddPCR). Un questionnaire portant sur les caractéristiques signalétiques, le statut de gestation et la vaccination a été rempli au moment du prélèvement. Au total, 85 % des juments échantillonnées étaient positives à l’EHV-1, à au moins un site et à au moins une reprise. Les prélèvements nasaux, vaginaux et leucocytaires étaient positifs respectivement dans 8,1 %, 15,8 % et 17,2 % des cas. Les juments gestantes présentaient une probabilité accrue d’excrétion virale nasale [rapport des cotes (OR) = 1,50, intervalle de confiance (IC) 95 % : 1,03 à 2,18, = 0,037]. La vaccination réduisait uniquement la probabilité de présence du virus dans le sang (OR = 0,70, IC 95 % : 0,49 à 0,99, = 0,043). Un stade de gestation avancé semblait également avoir un effet protecteur sur la présence du virus dans le sang (OR = 0,89, IC 95 % : 0,85 à 0,93, < 0,001). La plupart des juments de l’Ontario étaient positives à l’EHV-1 malgré leur bonne santé et leur vaccination, bien que la quantité d’ADN viral détectée fût extrêmement faible. De plus, le vagin a été identifié comme une source d’excrétion virale.(Traduit par Docteur Serge Messier).
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Publication Date: 2026-01-01 PubMed ID: 41585008PubMed Central: PMC12825682
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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 measured the prevalence of equine herpesvirus-1 (EHV-1) shedding in healthy broodmares in Ontario, Canada.
  • It found that a large majority (85%) of sampled mares shed EHV-1 virus from at least one bodily site despite being healthy and vaccinated, though the viral DNA levels were very low.

Background

  • EHV-1 is a common virus affecting horse populations worldwide.
  • Prevalence rates vary widely (3% to 88%) depending on population and sampling methods.
  • No prior studies had characterized EHV-1 shedding prevalence specifically in Ontario’s broodmare population.

Study Objectives and Design

  • The main goal was to determine how frequently healthy broodmares in Ontario shed EHV-1 virus.
  • The study used a cross-sectional design sampling 381 mares from 42 farms across Ontario over an 11-month period (Dec 2016 – Oct 2017).
  • Mares sampled included both pregnant and barren (non-pregnant) animals.
  • Sampling was done up to six times per mare from three sites: nose, vagina, and blood.
  • A survey was completed at the time of sampling recording factors such as age, pregnancy status, and vaccination history.

Methods: Detection of EHV-1

  • The presence of viral DNA was quantified by droplet digital PCR (ddPCR), targeting the EHV-1 glycoprotein B (gB) gene copy number.
  • This method is sensitive and allows for precise measurement of small amounts of viral DNA.

Key Results

  • Overall, 85% of mares were positive for EHV-1 at least once from at least one sampling site.
  • Positive detection rates by site were:
    • 8.1% of nasal swabs
    • 15.8% of vaginal swabs
    • 17.2% of blood (white blood cells) samples
  • Pregnant mares were significantly more likely to shed virus nasally (odds ratio (OR) = 1.50), suggesting pregnancy increases nasal viral shedding risk.
  • Vaccination decreased the odds of detecting virus in the blood (OR = 0.70), but did not significantly affect nasal or vaginal shedding.
  • Advanced gestation stage was associated with reduced virus presence in blood (OR = 0.89), indicating a potential protective effect as pregnancy progresses.
  • Despite high prevalence, the overall viral DNA loads detected were very low, indicating low-level or latent infections rather than active disease.

Novel Findings and Implications

  • This was the first study to document EHV-1 shedding prevalence in Ontario’s broodmare population.
  • The high proportion of apparently healthy and vaccinated mares shedding virus highlights the ubiquitous and persistent nature of EHV-1 in horse populations.
  • The vagina was identified as an important and previously under-recognized site of viral shedding, which has implications for understanding transmission dynamics.
  • Vaccination appeared to primarily reduce viral presence in blood but had limited impact on mucosal shedding sites, suggesting that vaccines may protect against systemic infection more than viral shedding or transmission.
  • Pregnancy status influences viral shedding patterns, which may inform management and biosecurity practices for broodmares.

Conclusions

  • Molecular evidence shows that healthy broodmares in Ontario commonly harbor and intermittently shed EHV-1 virus despite vaccination.
  • Vaginal, nasal, and blood samples all can harbor low levels of viral DNA.
  • Understanding the patterns of viral shedding can aid in controlling the spread of EHV-1 and protecting breeding populations.

Cite This Article

APA
Cooper CJ, Arroyo LG, Hammermueller JD, Botts MM, Pearl DL, Wootton SK, Lillie BN. (2026). Molecular prevalence of equine alphaherpesvirus-1 shedding in healthy broodmares in Ontario. Can J Vet Res, 90(1), 16-24.

Publication

Canadian journal of veterinary research = Revue canadienne de recherche veterinaire
ISSN: 1928-9022
NlmUniqueID: 8607793
Country: Canada
Language: English
Volume: 90
Issue: 1
Pages: 16-24

Researcher Affiliations

Cooper, Carina J
  • Canadian Large Animal Specialty Services, Sherwood Park, Alberta T8H 0T7 (Cooper); Department of Clinical Studies (Arroyo), Department of Pathobiology (Hammermueller, Botts, Wootton, Lillie), and Department of Population Medicine (Pearl), Ontario Veterinary College, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1.
Arroyo, Luis G
  • Canadian Large Animal Specialty Services, Sherwood Park, Alberta T8H 0T7 (Cooper); Department of Clinical Studies (Arroyo), Department of Pathobiology (Hammermueller, Botts, Wootton, Lillie), and Department of Population Medicine (Pearl), Ontario Veterinary College, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1.
Hammermueller, Jutta D
  • Canadian Large Animal Specialty Services, Sherwood Park, Alberta T8H 0T7 (Cooper); Department of Clinical Studies (Arroyo), Department of Pathobiology (Hammermueller, Botts, Wootton, Lillie), and Department of Population Medicine (Pearl), Ontario Veterinary College, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1.
Botts, Michaela M
  • Canadian Large Animal Specialty Services, Sherwood Park, Alberta T8H 0T7 (Cooper); Department of Clinical Studies (Arroyo), Department of Pathobiology (Hammermueller, Botts, Wootton, Lillie), and Department of Population Medicine (Pearl), Ontario Veterinary College, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1.
Pearl, David L
  • Canadian Large Animal Specialty Services, Sherwood Park, Alberta T8H 0T7 (Cooper); Department of Clinical Studies (Arroyo), Department of Pathobiology (Hammermueller, Botts, Wootton, Lillie), and Department of Population Medicine (Pearl), Ontario Veterinary College, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1.
Wootton, Sarah K
  • Canadian Large Animal Specialty Services, Sherwood Park, Alberta T8H 0T7 (Cooper); Department of Clinical Studies (Arroyo), Department of Pathobiology (Hammermueller, Botts, Wootton, Lillie), and Department of Population Medicine (Pearl), Ontario Veterinary College, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1.
Lillie, Brandon N
  • Canadian Large Animal Specialty Services, Sherwood Park, Alberta T8H 0T7 (Cooper); Department of Clinical Studies (Arroyo), Department of Pathobiology (Hammermueller, Botts, Wootton, Lillie), and Department of Population Medicine (Pearl), Ontario Veterinary College, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1.

MeSH Terms

  • Animals
  • Horses
  • Herpesviridae Infections / veterinary
  • Herpesviridae Infections / epidemiology
  • Herpesviridae Infections / virology
  • Female
  • Ontario / epidemiology
  • Virus Shedding
  • Herpesvirus 1, Equid / isolation & purification
  • Herpesvirus 1, Equid / physiology
  • Horse Diseases / virology
  • Horse Diseases / epidemiology
  • Pregnancy
  • Prevalence
  • Cross-Sectional Studies
  • Vagina / virology

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