Respiratory: EHV infection in Horses (Equis) | Vetlexicon
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Respiratory: EHV infection

ISSN 2398-2977

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


  • 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.

Presenting signs

Acute presentation

  • Cough.
  • Nasal discharge.
  • Depression.
  • Fever.

Geographic incidence

  • Worldwide: EHV-1, EHV-2, EHV-4.
  • USA, Europe, including the UK: EHV-3, EHV-5.

Age predisposition

  • Young (EHV-1, 2 or 4 respiratory disease).
  • Neonate (EHV-1 systemic infection).
  • Older adults (EHV-5).

Breed/Species predisposition

  • More common in groups of young, naive horses, such as TB racehorses.

Cost considerations

  • Respiratory tract disease in horses in training or competitive work.
  • Time out of work.
  • Diagnostic testing.

Special risks

  • Genral anesthesia not suitable for horses showing signs of respiratory compromise.



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.


Presenting problems

Client history

  • Nasal discharge.
  • General malaise.
  • Exercise intolerance.
  • Ataxia or posterior paresis.
  • Abortion.
  • Cough.

Clinical signs

  • Nasal discharge   Nose: discharge 03 - purulent    Nose: discharge 02 - froth  .
  • Conjunctivitis.
  • Pyrexia.
  • Submandibular lymphadenopathy.
  • Depression.
  • Dyspnea.
  • Ataxia, posterior paresis, urine retention (EHV-1).
  • Weight loss (EHV-5).

Diagnostic investigation

  • Virus isolation from nasopharyngeal   Pharynx: nasopharyngeal swab  or genital swabs, or fetal tissues (virus transport medium).
  • Virus identification using specific antisera to inhibit plaque formation.
  • Polymerase chain reaction has been used to distinguish EHV-1 and EHV-4.


  • Intranuclear inclusions on tissue sections from aborted fetuses.
  • Virus neutralization, complement fixation, immunofluorescence or ELISA tests to identify antibodies.
  • Must evaluate paired acute and convalescent phase sera.

The determination of serum antibody titer to EHV-1 is of no value in diagnosing the cause of abortion; clinically normal mares can have evidence of seroconversion.

Radiography  Respiratory: radiography - upper respiratory tract 

  • Nodular interstitial pattern (EMPF).
  • Pulmonary abscess.

Confirmation of diagnosis

Discriminatory diagnostic features

  • Clinical signs.
  • History.
  • Serology.

Definitive diagnostic features

  • Virus isolation.
  • Histopathology.
  • EHV-5 DNA identification in bronchoalveolar fluid/lung biopsy.

Gross autopsy findings

  • Pneumonitis   Lung: pneumonitis - pathology    Lung: EHV4 (foal) - pathology  .
  • Multiple fibroic nodules in lungs (EHV-5).

Histopathology findings

  • Intranuclear inclusion bodies.

Differential diagnosis

Respiratory disease


Initial symptomatic treatment

Standard treatment

  • Minimize environmental irritants (dust, ammonia).
  • Isolation of affected horses, restric movement, hygiene measures.
  • Potential future use of antivirals to reduce duration of clinical signs.
  • See HBLB  Codes of Practice .


  • Clinical signs.
  • Serology - evidence of seroconversion.

Subsequent management


  • Antibiotics if secondary bacterial infection.


  • Maintain isolation, movement restrictions and hygiene measures.
  • Resolution of clinical signs.



  • Minimize stress, especially in pregnant mares.
  • Keep pregnant mares separate from other youngtock.
  • Strict hygiene.

For further information in the UK see the Horserace Betting Levy Board's Codes of Practice on  Equine Herpes Virus .


  • Inactivated vaccine   Equine herpesvirus vaccine  has been developed against EHV-1 and EHV-4.
  • Vaccinate pregnant mares during the 5th, 7th and 9th month of gestation.
  • Other horses: primary course of 2 doses 3-4 weeks apart, 3rd dose 6 months later, then 6 monthly boosters.
  • Modified live vaccine also available in USA for herpes 1. Primary vaccination is 2 doses 4-8 weeks apart, and re-vaccination at 3 monthly intervals.

Group eradication

  • Avoid stress.
  • Isolate infected animals - further information in the Horserace Betting Levy Board's Codes of Pracice on  Isolation .
  • Hygiene during outbreaks.



  • Good for clinical recovery in cases of respiratory tract infection.
  • Highly contagious therefore may spread throughout yard, prolonging movement restrictions.
  • Clinical signs may recur due to reactivation of latent infections.
  • Paralytic form EHV-1 carries a guarded prognosis if recumbent, better if only mild signs.
  • Guarded for EMPF.

Expected response to treatment

  • Clinical signs resolve over 2-3 weeks in uncomplicated respiratory tract infections.

Reasons for treatment failure

  • Latent infection.
  • Failure of vaccination.
  • Secondary bacterial infection.

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:; Website:

Further Information