Equis ISSN 2398-2977

Respiratory: EHV infection

Synonym(s): EHV-1, EHV-4

Contributor(s): Sarah Binns, Christopher Brown, Rachael Conwell, Timothy Mair


  • Five equine herpesviruses   Equine herpesvirus  have been identified, each with a different clinical presentation; EHV-1, 2, 3, 4 and 5.
  • Cause: rhinopneumonitis.
  • Signs: pyrexia, depression serous   →   mucopurulent nasal discharge; rarely secondary bacterial bronchopneumonia.
  • Diagnosis: virus isolation; serology; histopathology.
  • Treatment: control secondary bacterial infection; quarantine newly introduced or stressed animals; vaccination.
  • See also Abortion: EHV 1    Abortion: EHV-1  , CNS: myeloencephalopathy   CNS: myeloencephalopathy - EHV  , Lung: multinodular fibrosis   Lung: multinodular fibrosis  .
  • Prognosis: good.
Print off the Owner factsheet on Equine herpesvirus - EHV to give to your clients.



Predisposing factors

  • Young age.
  • Stress.
  • Season (autumn/winter).


  • Waning maternally derived immunity.
  • Reactivation of latent infection.


  • Spread by close contact or aerosol transmission   →   primary replication, usually in respiratory epithelium   →   viremia   →   localization in predilection site, eg lymphoid tissue, epithelium   →   latent infections may develop in neural or lymphoid tissues   →   recrudescence under conditions of stress.
  • Replication in respiratory tract   →   viremia   →   respiratory tract infection.
  • Damage to endothelium of central nervous system vasculature   →   myeloencephalitis   →   ataxia, posterior paresis.
  • Vasculitis affecting placenta or direct viremic spread to foal   →    abortion.


  • Ubiquitous, therefore difficult to prove pathogenicity.
  • Associated with pharyngeal lymphoid hyperplasia   Pharynx: lymphoid hyperplasia  , pharyngeal ulceration, and respiratory tract disease.


  • Primary replication in respiratory epithelium   →   viremia   →   pyrexia and malaise   →   relocalization in respiratory tract   →   epithelial necrosis, congestion, and petechiation   →   nasal discharge and conjunctivitis.
  • May localize in lymphocytes   →   necrosis of germinal centers of lymph nodes and reactive hyperplasia   →   local lymphadenopathy.
  • May cause bronchitis or interstitial pneumonia in young animals.
  • Secondary bacterial infection   →   mucopurulent nasal discharge, pneumonia.


  • Recently described form of fibrotic interstitial lung disease, pathogenesis still unclear.
  • Chronic respiratory disease and weight loss in older horses.


  • Primary viral replication and viremia over initial 2-5 days.
  • Course of acute disease may last several weeks.
  • Latent infections may persist for life of animal.


  • Horses: 50% seropositive to EHV-1; almost 100% of adults seropositive to EHV-2.
  • Subclinically or latently infected horses may act as reservoirs of infection.
  • Virus transmitted by direct or indirect contact or erosol (EHV-3 transmitted venereally).
  • Short-lived immunity (2-6 months), although mares which have aborted rarely abort again.


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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
  • Soare T, Leeming G, Morgan R et al (2011) Equine multinodular pulmonary fibrosis in horses in the UK. Vet Rec 169 (12), 313 PubMed. 
  • Pusterla N et al (2011) Surveillance programme for important equine infectious respiratory pathogens in the USA. Vet Rec 169 (1), 12 PubMed.
  • 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.
  • Wong D M, Maxwell L K & Wilkins P (2010) Use of antiviral medications against equine herpes virus associated disorders. Equine Vet Educ 22 (5), 244-252 VetMedResource.
  • Pusterla N & Mapes S (2008) Evaluation of an air tester for the sampling of aerosolised equine herpersvirus type 1. Vet Rec 163 (10), 306-308 PubMed.
  • Luce R et al (2007) Equine herpesvirus-1-specific interferon gamma (IFNy) synthesis by peripheral blood mononuclear cells in Throughbred horses. Equine Vet J 39 (3), 202-209 PubMed.
  • 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.
  • Wilson W D (1997) Equine herpesvirus 1 myeloencephalopathy. Vet Clin N Am Equine Pract 13 (1), 53 PubMed.
  • Smith K C (1997) Herpes viral abortion in domestic animals. Vet J 153 (3), 253-268 PubMed.
  • Donaldson M T & Sweeney C R (1997) Equine herpes virus myeloencephalopathy. Comp Cont Educ Pract Vet 19 (7), 864 VetMedResource.
  • Whitwell K E & Blunden A S (1992) Pathologic findings in horses dying during an outbreak of the paralytic form of equid herpesvirus type 1 (EHV-1) infection. Equine Vet J 24 (1), 13-19 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: enquiries@hblb.org.uk; Website: http://codes.hblb.org.uk.