Equine Herpesvirus 1



Antigenicity

  • Isolates vary in virulence based on tropism
  • Note:
    • Genital pustules are caused by EHV3
    • EHV4 is serotypically identical to EHV1, but can be distinguished via monoclonal antibodies, PCR and RE (restriction enzyme) profiling

Pathogenesis

  • Entry via aerosol
  • Initial replication in the upper respiratory tract
  • By 24hrs, coughing is induced once virus has reached bronchi and pulmonary lymph tissue
  • Cell-associated viremia, with virus predominantly in the T cells
  • Abortion can follow:
    • Virus transfers from leukocytes to placental endothelium, causing thrombosis and ischemia
    • 95% of abortions are in the last trimester, when chorionic placentomes have created an end-artery system vulnerable to ischemia
  • Foals born will be weak and virus-positive
  • Paresis is a rare clinical symptom caused by lesions in the CNS and resulting thrombosis
  • Latency always follows infection, and the virus can be reactivated under stress

Epidemiology

  • Aerosol infection occurs despite maternal antibody
  • Over 60% of horses are latently infected and show antibody as yearlings
  • Greatest threat or reinfection is to mares in late-term pregnancy
  • Infected horses (coughing) can shed virus for up to 10 days

Diagnosis

  • In the case of abortion:
    • Whole fetus should be sent for testing
    • Immunostaining of fetal tissues
  • Nested PCR for envelope glycoproteins in nasal swab: more sensitive than virus isolation
  • Paired serum samples to show increase in CFT titre

Control

  • Isolation of pregnant mares in last trimester
  • No movement for at least 1 month after last abortion
  • Vaccination every 6 months
    • Inactivated vaccine may reduce respiratory disease but cannot protect against abortion