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The increasing traffic of sport horses around the world, particularly in the breeding season, means that the British horse industry needs to be extremely vigilant about equine viral arteritis (EVA), which is a highly contagious viral disease.
The virus causes a flu-like illness involving:
- fever
- depression
- swelling (especially of the limbs)
- inflammation around the eyes
It also causes weak foals or abortions, usually in the latter half of pregnancy.
Infection and diagnosis
Unusually, EVA can be spread by either a stallion with infected semen or in the air via the respiratory secretions of a recently infected horse, through close or direct contact.
After an average incubation period of seven days, EVA is shed in all bodily secretions for up to 21 days (possibly longer in urine). The virus may persist indefinitely in the stallion. Venereal transmission is believed to be the major cause of spreading the virus.
Particularly alarming is the fact that an infected stallion may shed the virus in his semen for the rest of his life. Also, the virus can be preserved in fresh, chilled or frozen semen and spread by artificial insemination. The fertility of shedder stallions is not affected and they show no clinical signs, but they can infect mares during mating or through artificial insemination.
A definite diagnosis cannot be made without lab tests because the clinical signs are vague. Tests can isolate the virus from nose and throat swabs, blood, urine or semen samples. Evidence of EVA exposure can be found by taking an initial serum sample as soon as possible after signs appear, followed by a second sample 10–14 days later to detect a rise in the EVA antibody levels.
Testing and treatment
In the UK, the two main risks are EVA-infected stallions which are shedding the virus in their semen, and semen imported from shedding stallions.
Stallions which have been used for breeding or have been competing overseas and are returning to the UK for breeding activities should be tested on their return and not be used until the results are known.
Two blood samples are taken at least 14 days apart and sent to a DEFRA-approved laboratory for testing and the stallion isolated during this time. If the results indicate no exposure to the virus (seronegative), breeding may proceed.
If the stallion shows evidence of exposure to the disease (termed seropositive), DEFRA should be informed and further tests done to confirm that the stallion is not shedding. Many stallions can be seropositive for EVA, but if they are not shedding it in the semen, there is no problem.
Vaccination will cause a stallion to be seropositive and this is why it is vital to have the blood tested negative before vaccinating him. The negative result should be certified and recorded by a vet in the passport before administering the vaccine.
UK-based stallions should also be blood tested every year to monitor their EVA status, and should be vaccinated whenever a suitable, proven vaccine is available. A killed vaccine (Artervac, Fort Dodge Animal Health) is available in the UK and is now widely used here. Certain countries will not accept the importation of seropositive animals.
Mares imported from overseas should be blood tested within 28 days of import. The mare should be isolated for three weeks on arrival, with blood tests carried out immediately and repeated at least two weeks later. Many studs in the UK are requiring confirmation that mares are negative for EVA prior to arrival at stud. This would be the preferable option for control.
At present, there is no effective treatment for a chronically infected stallion. Such animals can remain persistently infected with the virus in the reproductive tract for variable amounts of time, from several months to a period of years and, in some cases, the lifetime of a particular stallion. There is no evidence that mares, geldings or foals which acquire the infection congenitally become carriers.
EVA risks in Britain
The virus is found throughout the world, but its prevalence varies between countries. In mainland Europe, it is widespread, resulting in high horse population immunity and low-grade signs of disease.
Except for a controlled outbreak in 1993, the UK has been relatively free from EVA. This means that, in contrast to mainland Europe, the UK horse population has little immunity to the disease and the consequences if the virus were imported again could be catastrophic.
In countries with a high prevalence of the virus, the horse population eventually gains immunity to the disease, which may explain why breeders in some countries adopt a different, more relaxed attitude to EVA, but this wouldn’t work in the UK.
In Britain, EVA is a notifiable disease, which means that, by law, anyone who owns, manages or examines a horse must notify DEFRA if they suspect it may be infected.
Health papers
Further information about EVA and its prevention is available in:
This article first appeared in Horse & Hound (24 June, ’04)
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