Equine viral arteritis

Most horses that suffer an equine arteritis infection show no clinical symptoms. In the disease’s acute stages, the following symptoms may occur:

• fever
• nasal discharge
• lack of appetite
• breathing issues
• skin rash
• muscle ache
• conjunctivitis

Other symptoms that have been reported are swelling around the eyes, discharge from the eyes, swollen limbs and swollen genitals. In infected gestating mares, abortion may also be included in the symptoms.

It is difficult to clinically discern between EVA and a number of other equine infectious diseases, including equine influenza virus, equine herpesvirus 1 and 4, equine rhinitis A and B virus, equine adenovirus and streptococcal infections.

Equine arteritis virus can be subdivided into three different genetic sub-groups:

• EAV-1 which occurs mainly in Europe
• EAV-2 which occurs mainly in America
• EAV-3 which occurs in both Europe and North America

EAV is transferred mainly through breathing; viral particles in the infected animal’s nasal discharge are inhaled. The virus is, however, also transmitted sexually.

The relocation of animals for the purpose of sales, shows and races forms a transmission risk. Intensive contact facilitates the spreading of the virus.

After initial infection, the virus is generally entirely neutralised by the antibodies within 28 days, after which no renewed infections occur. From that moment, seropositive mares and geldings are no longer a threat to their herd.

Stallions prolonged carriers

Despite the production of antibodies, stallions may carry the virus for a prolonged period (months and even years) after their initial infection. The virus settles in the animals’ sexual glands and is transferred through semen. Infected semen may cause the disease in mares. Unlike seropositive mares and geldings, a seropositive stallion is a potential source d EAV-infections. After an infection with the virus, between 30 and 60 per cent of the stallions will become persistently infected. Please note: Persistently infected stallions are always seropositive, but not all seropositive stallions are persistently infected.

Transfer through mares

In general, infected mares will be able to neutralise the virus easily. Gestating mares, however, may infect the unborn foal with the EAV virus. In most cases, the infected foetus perishes and is aborted. If the foal is carried to term, it will die within days after birth.

To exclude or confirm a (subclinical) EAV infection, throat or nose swabs or sperm samples can be used for isolation of the virus in its acute stages. WVBR also offers a PCR test that can be applied to sperm samples or throat/nose swabs. The virus may also be diagnosed by determining seroconversion in paired serum samples taken during the virus’ acute stages and analysed 21 to 28 days later.

Virus isolation

If a stallion is found to be seropositive for EAV-VNT, its semen must be tested for EAV. The recommended approach is to isolate the virus in two sperm samples taken on the same day, on two consecutive days, or at intervals of days or weeks. Gathering the semen must be done without using antiseptics or disinfectants on the animal’s genitalia, as these may contaminate the sperm sample and cause false-positive results. Samples thus obtained are unsuitable for research. Sperm samples (semen rich in sperm) must be cooled at 4º C immediately after extraction and be delivered to the diagnostics laboratory on the same day. Alternatively, the samples may be frozen at -20º C. In this case, the samples must remain frozen during transportation till, and including, arrival at the laboratory.

For the virus isolation, the diluted sperm sample is incubated at a temperature of 37º C with a cell culture sensitive to EAV. After two to six days, the cells are assessed under a microscope to check for any changes resulting from EAV {so-called cytopathogenic effect (cpe)}. If no changes are seen, new virus isolation is conducted with the abovementioned culture liquid.

If the cell culture shows microscopic changes, a virus characterisation is conducted to determine whether the changes are the result of EAV. If EAV is determined, the virus is present in the stallion's sperm, making it a persistently infected carrier.

PCR

The virus can also be shown by detecting genetic material through a polymerase chain reaction (PCR). Wageningen Bioveterinary Research (WBVR) developed a PCR test specifically for this disease.

Serological test

EAV-specific antibodies can be shown through a virus neutralisation test (VNT). In some cases, the serum that is being studied can cause a toxic reaction with the cell culture, rendering the VNT moot. This may be caused by vaccinations administered to the animal that sparked the presence of antibodies that react to the cell culture and destroy the cells. Another possible cause may be found in the quality of the sample (infected, not fresh, contaminated with disinfectants). In the latter case, a repeat test with a sample of the proper quality should yield a viable test result.

In the VNT, various solutions of the research sample are introduced to a fixed amount of EAV. After an incubation period of one hour at 37º C, the mixture is introduced to a cell culture that is sensitive to the virus. If there are sufficient EAV-neutralising antibodies present in the serum, the cell culture will remain unaffected. If there are insufficient antibodies, the culture will be affected. The animal is considered seropositive if the solution with a ratio of 1:4 or higher neutralises the EAV.

Recent years have shown a global increase in seropositive equines. This increase is likely the result of an increase in equine transport and the use of infected sperm.

Legislation in many countries bans the export of EAV-infected sperm. Moreover, some countries disallow the import of seropositive horses.