Equine Herpesvirus 1

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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.

There are three forms of the disease, respiratory, reproductive and neurological. The neurological form is rare but most severe.

Infection is by aerosol and nasal secretions 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

The respiratory form includes signs such as serous nasal discharge, coughing, sneezing, lacrimation and upper respiratory noise.

The reproductive form is characterised by abortion in the third trimester. Please note: genital pustules are caused by EHV3.

The neurological form starts with inappetance, change of behaviour and progresses to ataxia, urine dribbling and decreased tail tone. Paresis is a rare clinical symptom caused by lesions in the CNS and resulting thrombosis.

Intermittent fever is a common clinical sign.


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

No treatment other than supportive is currently available.

Control measures should include the isolation of pregnant mares in last trimester and no movement for at least 1 month after last abortion. In case of an outbreak, isolation is the most important control measure.

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


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

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