Equine Herpesvirus 1

VIRUSES HERPESVIRUSES

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