ISSN 2398-2977      

Abortion: EHV-1


Synonym(s): Equine herpesvirus-1 infection


  • Cause: equine herpesvirus-1 (EHV-1)   Equine herpesvirus  - the leading infectious cause of abortion.
  • Signs: abortion between 5 and 11 months gestation, abortion, stillbirth, weak neonates.
  • Diagnosis: post mortem, microbiology/virology.
  • Treatment: disinfection and quarantine.
  • Prognosis: good for future breeding, but latent infection is common.
  • See also Abortion: overview Abortion: overview.
Print off the Owner factsheet on Equine herpesvirus - EHV to give to your clients.



Predisposing factors

  • Stress (weaning, transportation, mixing groups, introduction of outside animals).
  • Stocking brood mares with young horses (either in buildings or at pasture).
  • Young horses: respiratory disease   Respiratory: EHV infection  .
  • Recently foaled mares: recrudescence of latent virus, infectious placental material from aborting mares.


  • Immunosuppression in latent carriers (possibly as high as 60% in some populations).


  • EHV-1 is transmitted by inhalation or direct contact with infected fomites (aborted fetus, placental tissues). Infection of respiratory tract epithelial cells is followed by cell necrosis and an acute inflammatory response resulting in viral shedding.
  • Virus spreads rapidly through the body by both cell-to-cell spread and in leukocytes, infecting the endothelium of blood vessels within 2-4 days after infection.
  • Virus attacks vascular endothelium where it replicates, causing widespread vasculitis, thrombosis and ischemic damage to the microcotyledons of the placenta, resulting in premature placental separation.
  • Endothelial vasculitis and ischemic necrosis of endometrium + placental separation    →   starves fetus of oxygen and nutrients   →   acute abortion.
  • Direct infection of the fetus from leakage of infected cells or free virus across damaged placenta results in viremia with localization in lungs, liver and lymphoid tissue    →   abortion.
  • All products of abortion are highly contagious sources of the virus.
  • Infected foals shed particles by respiratory aerosols.
  • Older infected horses may exhibit respiratory   Respiratory: EHV infection  or neurologic signs   CNS: myeloencephalopathy - EHV  or they may maintain a latent infection.
  • Immunity following infection or vaccination   Equine herpesvirus vaccine  is transient.


  • Incubation 3-16 weeks.
  • Virus does not persist in the genital tract following abortion   →   can cover mare following second heat provided she is kept separate from pregnant mares for 8 weeks post-abortion.


  • It is estimated that 80-90% of horses have been exposed to EHV-1 or 4 by 2 years of age.
  • Exact annual incidence of EHV-1 infections is not known.
  • EHV-1 is typical of herpes viruses   →   latent infection recrudesces under stress.
  • Latent EHV-1 infection identified in 54% of broodmares in Kentucky at post-mortem examination; in >60% of horses at an abattoir in the UK.
  • Duration of protective immunity is short-lived   →   regular and frequent vaccination is necessary   Equine herpesvirus vaccine  .
  • No difference in prevalence of latent infection in vaccinated versus unvaccinated horses.
  • Virus survives for 14 days outside the body. Occasional survival up to 40 days if dried on horse hair.
  • Young stock and horses in training may be infected with the respiratory form of the virus and may infect mares via respiratory route.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Dunowska M (2016) How common is equine herpesvirus type 1 infection?Vet Rec 178 (3), 67-69 PubMed.
  • Bazanow B A et al (2014) Abortogenic viruses in horses. Equine Vet Educ 26 (1), 48-55 VetMedResource
  • Kydd J H, Slater J, Osterrieder N, Antezak D F & Lunn D P (2010) Report of the second Havermeyer EHV-1 workshop. Steamboat Springs, Colorado, USA. September 2008. Equine Vet J 42 (6), 572-575 PubMed.
  • Pusterla N et al (2009) Prevalence of EHV-1 in adult horses transported over long distances. Vet Rec 165 (16), 473-475 PubMed.
  • Paillot R, Case R, Ross J, Newton R & Nugent J (2008) Equine herpes virus-1: virus, immunity and vaccine. Open Vet Sci J 2, 68-91 VetMedResource.
  • Manning F (2006) Equine herpesvirus-1: An update (part 2). UK Vet 11 (7), 18-20 VetMedResource.
  • Foote C E, Love D N, Gilkerson J R & Whalley J M (2004) Detection of EHV-1 and EHV-4 DNA in unweaned Thoroughbred foals from vaccinated mares on a large stud farm. Equine Vet J 36 (4), 341-345 PubMed.
  • Smith K C et al (2004) Equine herpesvirus-1 abortion: atypical cases with lesions largely or wholly restricted to the placenta. Equine Vet J 36 (1), 79-82 PubMed.
  • Studdert M J, Hartley C A et al (2003) Outbreak of equine herpesvirus type 1 myeloencephalitis: new insights from virus identification by PCR and the application of an EHV-1-specific antibody detection ELISA. Vet Rec 153 (14), 417-423 PubMed.
  • McCartan C G, Russell M M, Wood J N L & Mumford J A (1995) Clinical, serologic and virologic characteristics of an outbreak of paresis and neonatal foal disease due to equine herpes virus-1 on a stud farm. Vet Rec 136 (1), 7-12 PubMed.
  • Smith K C, Whitwell K E et al (1992) Abortion of virologically negative fetuses following experimental challenge of pregnant pony mares with Equid herpesvirus 1. Equine Vet J 24 (4), 256-259 PubMed.

Other sources of information

  • Horserace Betting Levy Board (2016) Codes of Practice. 5th Floor, 21 Bloomsbury Street, London WC1B 3HF, UK. Tel: +44 (0)207 333 0043; Fax: +44 (0)207 333 0041; Email:; Website:
  • Colahan P T et al (1991) Equine Medicine and Surgery. Vol 2. 4th edn. American Veterinary Publications, Inc. ISBN: 0 939674 27 0. pp 1070 (concise summary of main points).

Further Information 


  • Center for Preventive Medicine, Animal Health Trust, Lanwades Park, Kentford, Newmarket CB8 7UU, UK.

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