Dealing with herpes in horses

Quick implementation of vaccination and biosecurity can stop an outbreak, says Dr Mac.

Dealing with herpes in horses
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Improved diagnostic capability has shown that equine herpes is a more important cause of abortion in small studs than previously thought.

There are five strains of the disease; the most important are 1 and 4. Both can cause abortions, although Strain 1 is the main culprit. The disease is transmitted by the slimy nasal mucus of infected horses or the birth fluids after abortion. It can also be carried by infected tack or feeding buckets, or by people working with an infected horse.

An outbreak usually starts after the introduction of a carrier mare to a stud. If a mare aborts, you should presume the cause is equine herpes. The virus has become fairly widespread, as many small breeders now take their horses to shows, where they come into contact with carrier horses and bring the infection home.

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The incubation period is two to 10 days. The first signs of infection is usually a fever, nasal discharge and coughing lasting about seven days. Sometimes horses can show serious neurological signs and young foals can die from pneumonia.  Abortions usually occur late in pregnancy between the seventh and 11th month, about two weeks after respiratory symptoms are seen in the herd. Mares often abort without showing any other signs of disease.

The first step is to obtain a diagnosis from a vet. The virus can be isolated from nasal swabs, an aborted foetus, or paired blood tests in a mare that has aborted. Aborted materials should be placed into a plastic bag, kept cool and submitted to a laboratory as soon as possible.

The mare that has aborted should be isolated and the stable or stud quarantined for three weeks. Those handling infected horses must wash their hands, disinfect gumboots and change their clothes. In South Africa, a killed virus vaccine (Zoetis pneumabort) is available; mares should be injected with it every two months during pregnancy after the fifth month.


Vaccination is advised in any mare moved to new premises or if other horses on the property go to shows. Vaccination, however, does not prevent the carrier state developing in infected horses, so biosecurity measures are essential. These include electric paddocks for small groups of pregnant mares, separated by at least 20m from other horses.

Infected or suspected carrier mares should be kept together. Show horses, yearlings, stallions and young stock should be kept as far away as possible from broodmares. Research has shown that quick implementation of vaccination and biosecurity can stop an outbreak and prevent further abortions.