“EVA” is a highly contagious disease caused by equine arteritis virus. It is most notable as a reason for spontaneous abortions in pregnant mares. More subtle symptoms include fever, respiratory problems, reduced appetite, eye swelling and discharge, and edema of the limbs, mammary glands, and male genitals. The virus may be spread by contact with nasal and respiratory secretions as well as transmitted sexually. While most adult horses develop high immunity to re-infection after the illness runs its natural course, stallions often become silent carriers of EVA for years after exposure, thus becoming reservoirs of disease. Infected broodmares rarely bear live offspring; infected foals delivered alive rarely survive beyond a few days.
Numerous outbreaks of equine viral arteritis have occurred in breeding facilities over the past years. It is in these concentrated populations of animals that the disease sustains the highest morbidity. Abortion rates vary considerably but may occur in as many as 70 percent of infected broodmares. Stallions who become carriers may spread the virus during natural breeding or through artificial insemination (AI) techniques with EVA positive semen. Respiratory transmission of EVA occurs rapidly at horseshows, racetracks, and other locations where horses may come together in large numbers.
EVA does not usually lead to fatality in an adult horse. In fact, many infected horses are clinically asymptomatic. Horses that do exhibit signs of illness will typically run a high fever for 2 to 10 days, combined with any of the following symptoms: clear nasal discharge which may become colored after secondary bacterial infection; swelling of the conjunctiva (tissue surrounding the eyes); skin rashes and hives; depression and lameness; respiratory distress; and edema with tenderness in the limbs and reproductive organs. Horses with respiratory symptoms will typically lose interest in food as well.
The mechanism by which arteritis virus causes disease is associated with its tendency to localize in the arterioles, the branches of arteries that narrow into capillary vessels. Vasculitis, or inflammation of the vessel walls, causes fluid (serum) leakage into surrounding tissues. The most severely affected tissues correspond with the symptoms mentioned above. The virus is shed along with the fluid leakage into almost all secretions and wastes from the body, including semen, tears, respiratory mucous, urine, and feces, for up to 3 weeks following infection.
While most adult horses make a full recovery, untreated stallions can remain infertile for several months following severe edematous damage in the reproductive tract. Once a stallion becomes a carrier of EVA, the only known method to clear the virus is castration. Occasionally, but not predictably, a carrier stallion may eliminate the virus spontaneously. These stallions are no longer considered a reservoir of infection, and none have been shown to begin shedding equine arteritis virus at a later date.
Foals born live should be considered a potent source of infection to naïve (previously unexposed), unvaccinated foals and horses in the herd. Even with aggressive supportive treatment, infected foals rarely survive more than 2 to 3 days, during which they may spread the virus to healthy animals despite efforts to quarantine and disinfect. There is no specific treatment for EVA in foals, and humane euthanasia is preferred to the suffering the animal will endure.
Unprotected mares will contract EVA from infected stallions by sexual contact or through EVA positive semen during artificial insemination (AI) more than 75% of the time. Because of the low survival rate of infected foals, all broodmares should be vaccinated prior to breeding. Pregnant mares should not receive the vaccine, especially during the last two months of gestation, unless the risk of exposure to EVA outweighs the risk of possible adverse reactions (fetal infection, abortion) associated with the vaccine, albeit rare.
The vaccine is also contraindicated in foals less than 6 weeks of age, therefore maternal antibody protection of the foal through colostrum is the only way to protect them from contracting arteritis virus.
Stallions should be confirmed negative with a blood test for EVA prior to the initiation of the EVA vaccine.
The symptoms of equine viral arteritis closely mimic those seen in equine herpesvirus infections; therefore, confirmation of the disease by diagnostic testing is necessary to institute appropriate containment and prevention measures. The testing method that achieves the quickest results (PCR assay) should be utilized where available.