Equine Herpesvirus 1

Introduction

Equine Herpes virus 1 has a number of isolates which vary in virulence depending on tropism. EHV4 is serotypically identical to EHV1, but can be distinguished via monoclonal antibodies, PCR and RE (restriction enzyme) profiling.

Infection is by aerosol transmission and this can occur even if there is a maternal antibody present in the body. Initial replication occurs 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 then occurs. Abortion can follow; the 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. Any foals that are born will be weak and virus-positive.

Latency always follows infection, and the virus can be reactivated under stress at any point in later life. Over 60% of horses are latently infected and show antibody as yearlings. The greatest threat of reinfection is to mares in late-term pregnancy and because of this,mares in this phase of gestation should be isolated, especially from any newly brought in horses (that may be stressed).

Infected horses (coughing) can shed virus for up to 10 days.

Clinical Signs

Respiratory signs such as serous nasal discharge, coughing, sneezing, lacrimation and upper respiratory noise.

Abortion can occur in the third trimester and this is characteristic of this disease. Please note; genital pustules are caused by EHV3.

Paresis is a rare clinical symptom caused by lesions in the CNS and resulting thrombosis.

Diagnosis

In the case of abortion, the whole fetus should be sent for testing. At a specialist laboratory, immunostaining of fetal tissues will take place to diagnose the virus. Nested PCR for envelope glycoproteins in nasal swab can be used and this is more sensitive than virus isolation. Paired serum samples should also be taken on day one and then around two weeks later to show increase in complement fixation test (CFT) titre.

Treatment and Control

Control measures should include the isolation of pregnant mares in last trimester and no movement for at least 1 month after last abortion.

Vaccines are available and the horse should be vaccinated every 6 months. Inactivated vaccine may reduce respiratory disease but cannot protect against abortion.

References

Brown, C.M, Bertone, J.J. (2002) The 5-Minute Veterinary Consult- Equine', Lippincott, Williams & Wilkins

Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition) Elsevier Science

Bridger, J and Russell, P (2007) Virology Study Guide, Royal Veterinary College

Knottenbelt, D.C. A Handbook of Equine Medicine for Final Year Students University of Liverpool

Knottenbelt, D.C, Pascoe, R.R. (2003) Colour atlas of Diseases and Disorders of the Horse Elsevier Health Sciences

Mair, T., Love, S., Schumacher, J. and Watson, E. (1998) Equine Medicine, Surgery and Reproduction WB Saunders Company Ltd

Pasquini, C, Pasquini, S, Woods, P (2005) Guide to Equine Clinics Volume 1: Equine Medicine (Third edition), SUDZ Publishing

 Rose, R. J. and Hodgson, D. R. (2000) Manual of Equine Practice (Second Edition) Saunders