Introduction

• 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.

Pathogenesis

Etiology

• Equine herpesviruses   Equine herpesvirus  :
  • EHV-1: causes abortion   Abortion: EHV-1  , respiratory tract disease and neurological disease   Abortion: EHV-1  .
  • EHV-2: associated with upper respiratory disease, pharyngeal lymphoid hyperplasia   Pharynx: lymphoid hyperplasia  , pharyngitis and pneumonia in foals   Lung: pneumonia - interstitial (foals)  although its exact role is unproven.
  • EHV-3 is the cause of equine coital exanthema   Reproduction: coital exanthema - EHV 3  .
  • EHV-4 (formally EHV-1 subtype 2): also associated with respiratory tract disease   Respiratory: EHV infection  .
  • EVH-5 has been linked to fibrotic type of interstitial pneumonia   Lung: multinodular fibrosis  .

Predisposing factors

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

Specific

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

Pathophysiology

• 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.

EHV-2

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

EHV-4

• 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.

EHV-5

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

Timecourse

• 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.

Epidemiology

• 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.

Diagnosis

Treatment

Prevention

Outcomes