Equine Viral Arteritis

What is EVA?

Equine viral arteritis (EVA) is a contagious disease caused by the equine arteritis virus (EAV). The virus occurs worldwide including mainland Europe.

There is no risk to human health or species other than eqidae.

How does the infection spread?

A. Routes of infection - All horses

Infection spreads through transmission of the virus between horses in 4 main ways:

• venereal infection of mares by stallions during mating
• artificially inseminating mares with semen from infectious stallions
• contact with aborted foetuses and other products of parturition
• direct contact in droplets (eg from coughing and snorting) from the respiratory tract

B. The Shedder Stallion

The stallion is a very important source of the virus. On infection, the virus localises in his accessory sex glands and the virus may be shed in his semen for several weeks afterwards, or for many months or years and possibly for life. After recovery from acute illness, his fertility is not affected and he will show no further clinical signs of infection even though he may still be infectious. Shedder stallions will infect susceptible mares during mating, or after insemination with the stallion's semen, and these mares may, in turn, infect in-contact animals via the respiratory route.

It is important to note that the shedder stallion is always seropositive (ie past or existing infection indicated in a blood test) but that a seropositive stallion is not necessarily a shedder.

Breeders using AI must note that the virus can survive in chilled and frozen semen.

C. Teasers

Teasers are also a potential source of the virus and should be subjected to the same precautions as stallions.

D. Mares

Available evidence indicates that the 'carrier' state does not occur in mares.

What are the clinical signs?

The variety and severity of clinical signs of EVA vary widely. Infection may be obvious or there may be no signs at all. Even when there are no signs, infection can still be transmitted and stallions might still become shedders.

EVA can cause abortions. Other signs include fever, depression, lethargy, stiff movement, runny nose, conjunctivitis, (‘pink eye’), swelling of the lower parts of the legs, around the eye and of the reproductive organs.

How is the Disease diagnosed?

Because of the variability or the possible absence of symptoms, clinical diagnosis is not always possible.
Laboratory diagnosis is therefore essential. This requires appropriate samples, which are nasopharyngeal swabs, heparinised or EDTA blood, semen, serum and possibly urine, to be taken by a veterinary surgeon and sent to a specialist laboratory. In blood samples, laboratories look for antibodies to the virus (serological test); in blood and other samples, they look for the virus itself (virus detection tests).

Where abortion may be EVA-related, detailed clinical information must be sent to the laboratory with the foetus and its membranes.

How is EVA treated?

A. There is no treatment available for EVA itself, although there may be treatments to alleviate some of its symptoms.

Is EVA is a notifiable disease?

In Great Britain, EVA is a notifiable disease under the Equine Viral Arteritis Order 1995.
It is a legal requirement to notify a Divisional Veterinary Manager (DVM) of the Department for Environment, Food and Rural Affairs (Defra) when:

• it is known or suspected that a stallion has the disease or is a carrier of the virus
• it is known or suspected that a mare which has been mated or subjected to Artificial Insemination in the last 14 days has become infected with the virus.

Full details of the exact notification requirements are in the EVA Order 1995.

EVA Vaccine

A vaccine (Artervac, Fort Dodge) is available on an Animal Test Certificate (or similar licence) in the UK, Ireland, France and Germany, but with different national restrictions on use. Consult your veterinary surgeon on uses allowed in your country. In the UK, the vaccine can be used for all horses.

There are data to show that Artervac is safe. There is also experimental efficacy data to show that it can protect horses 3 weeks after the second dose of vaccine is given. However, it is not yet clear how the vaccine will perform in the field. If it performs well, it will be an effective way of protecting stallions. Veterinary advice should be sought on vaccination timings and administration.

Vaccinated horses will become seropositive. Horses should therefore be blood tested immediately before vaccination to show that they were previously seronegative.

Details about the prior blood test and the vaccination, both primary and booster, should be recorded in the horse's passport.

Pre-Vaccination Blood Test

If the passport includes a section titled Laboratory Health Test, details about the test should be recorded in that section by a veterinary surgeon.

If the passport does not include a section titled Laboratory Health Test, or the horse does not have a passport, the following records, certified by a veterinary surgeon, should be kept:

1. Date when the pre-vaccination blood sample was taken.
2. Type of test.
3. Certificate from the laboratory showing the blood test result, and the identity of the laboratory that carried out the test.

Vaccination

Details should be recorded by a veterinary surgeon in the section of the passport titled Vaccinations Other Than For Equine Influenza. The details are the date and place when the vaccination was given, and the name and batch number of the vaccine.

If these details cannot be entered in a passport, a record of the same details, certified by a veterinary surgeon, should be kept.

A post-vaccination blood sample may be taken to confirm seroconversion as a result of either primary or booster vaccination. The same records as set out above for pre-vaccination tests should be kept.

The above records are important as evidence of previous seronegativity for breeding and export purposes. Some importing countries require this information for vaccinated horses, either in passports or official export certificates.

For official export certification purposes, samples for EVA blood testing must be sent to a Government laboratory, i.e. the Veterinary Laboratories Agency, Addlestone.

As the vaccine's efficacy is not yet fully known, owners should monitor the horse's antibody response after vaccination, in consultation with the veterinary surgeon.

Vaccination is not an alternative to good management.

The code of practice remains essential to prevent EVA.