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Nature of the disease
Equine rhinopneumonitis or Equine viral abortion is caused by infection with equine herpes virus type 1 (EHV1) which causes rhinopneumonitis, abortion, neonatal mortality and occasionally encephalomyelitis in horses and donkeys.
OIE List B disease
Susceptible species
Horses (Equus caballus) and donkeys (Equus asinus)
EHV1 occurs world-wide, in all countries with significant horse industries. Equine rhinopneumonitis is quite common in Australia and New Zealand although abortions are less commonly seen. It has never been recorded in Vanuatu.
Clinical signs 
Abortions and rhinopneumonitis appear to be caused by slightly different strains of EHV1.
The rhinopneumonitis form of EHV1 is usually seen as an outbreak of upper respiratory disease (‘colds’) in young horses during autumn or winter. Affected horses have:
  • A sudden fever lasting 1-7 days
  • Intermittent serous nasal discharge
  • Congestion of the nasal mucosa and conjunctivae
  • Inappetence
  • Cough
  • Slight enlargement of the submaxillary lymph nodes,
  • Occasionally oedema of the legs
  • Sometimes constipation followed by diarrhea

Recovery usually occurs within a week. Occasionally the nasal discharge becomes mucopurulent and continues for 1-3 weeks, accompanied by a cough. Adult horses seldom show clinical signs, although they may undergo sub-clinical infection.

Abortions caused by EHV1 usually occur in the last 4 months of gestation, but may occur as early as the fourth month. Infection usually occurs 1–4 months prior to abortion and infected mares show no signs of respiratory or other illness. There are usually no complications after the abortion. The foetus is expelled dead, with the foetal membranes intact. Occasionally, when mares are infected late in gestation, infected foals are either stillborn or die within a few days of birth. The conception rate of mares, 30 days after abortion, is high. Abortion occurs most commonly in mares during their first pregnancy. Having once aborted mares usually breed normally in subsequent years, and only rarely abort again.

Sometimes horses develop neurologic disease with incordination, loss of bladder and tail function, loss of sensitivity of perineal and inguinal region.

Post-mortem findings 
In cases of rhinopneumonitis there is:
  • Inflammation of the upper respiratory tract
  • An initial leucopaenia that is replaced later by a leucocytosis
  • Pulmonary oedema and fibrinous infiltration of interstitial tissues

Foetuses aborted before 6 months show widespread necrosis with intranuclear inclusion bodies, especially in the liver and lungs. Those aborted after 6 months show :

  • Jaundice
  • Petechiation of mucous membranes
  • Enlargement of the spleen
  • White to cream-coloured necrotic foci in the liver
  • Accumulation of fluid in the pleural cavity

In stillborn foals, or those dying soon after birth the main gross lesions are enlarged, plum to purple coloured lungs that are oedematous and only partly aerated. There is fluid and froth in the bronchi, and subepicardial and subendocardial haemorrhages.

Differential diagnosis 
Other infectious causes of abortion in horses include:
Specimens required for diagnosis 
Diagnosis of EHV1 abortions is made by isolation of virus from foetal tissues, histological demonstration of typical intranuclear inclusions or antigen demonstration.
Fresh and preserved samples of foetal tissues — lung, liver, spleen and kidney — should be collected.

For serology blood samples should be taken from affected mares.

EHV1 infection spreads mainly by the respiratory route, although infection by ingestion and transmission by fomites is also thought to occur. Aborted foetuses contain large amounts of virus and may be responsible for the introduction of the abortogenic strain to previously uninfected premises.

Previously infected adult horses may undergo subsequent subclinical infection, although it is unclear whether this is due to re-infection or re-activation of a latent infection. It is not known whether a latent carrier state occurs with EHV1, although this is common with other herpes viruses.

Risk of introduction   
EHV1 is most likely to be introduced with the importation of affected horses.
Control / vaccines  
Prevention of disease caused by EHV1 is mainly by the use of measures to minimise the risk of introducing the virus.

Live vaccines exist but they are banned in some countries, inactivated vaccines are also available and licensed in most countries.

  • Equine Herpes Virus 1 Infection, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 1081-1084
  • Office International des Epizooties, 2002