Equine viral arteritis is a contagious, viral disease that affects members of the horse family. The disease is caused by the equine arteritis virus and is of short duration. Signs can include fever; depression; swelling of the limbs, scrotum, and the covering fold of skin on the penis in stallions; inflammation of the conjunctival lining of the eye; nasal discharge; and abortion. Occasionally, it will cause death in young foals.
The virus is present in horse populations in many countries throughout the world; Japan and Iceland are notable exceptions. However, outbreaks of the virus are uncommon and are usually associated with the movement of horses or shipment of semen. While the virus is known to infect many breeds of horses, the rate of infection varies widely, usually being highest in Standardbreds and Warmbloods. To date, there is no evidence of infection in populations of wild horses.
The virus may be spread by the respiratory route (such as breathing in particles of virus exhaled by an infected animal) or may be transmitted by sexual contact, from mare to foal, or by indirect contact. Infected stallions can become carriers and readily spread the infection due to the fact that the virus is continually shed in the semen. Mares can be infected by either natural breeding or artificial insemination with infective semen. Viral transmission may be widespread at racetracks or on breeding farms. Transmission of the virus is not always associated with the appearance of the signs characteristic of equine viral arteritis. In fact, most infected horses show no signs of disease. The virus may persist in some otherwise healthy stallions for years.
Most cases of infection are without signs. When signs are observed, they may include any combination of the following: fever of 2 to 9 days duration, low blood white cell counts, depression, poor appetite, leg swelling (especially of the hind legs), and swelling of the covering fold of skin on the penis and the scrotum in stallions. Less consistent signs include inflammation of the conjunctival lining of the eye, tearing, abnormal sensitivity to light, swelling around the eyes, inflammation of the lining of the nose and nasal discharge, swelling of the ventral body wall (including the mammary glands of mares), a skin reaction visible on the sides of the neck or head (although it can sometimes occur elsewhere on the body), a stiff gait, difficulty breathing, diarrhea, jaundice, and lack of coordination. Signs are more severe in young, old, and weakened horses, but complete recovery is expected, even without symptomatic treatment. Death is rare but has been reported in foals up to several months old.
The reason equine arteritis virus causes abortion is unclear. Abortion may occur while the horse is displaying signs of disease or while it is recovering from the infection. It may also occur in infected mares that do not develop any signs of illness. Mares may abort any time from 3 months to over 10 months of gestation. Abortion rates in a herd can vary from less than 10% to as high as 60%. Abortion is not the result of a mare being bred to a carrier stallion or inseminated with infective semen. Mares that abort are already pregnant at the time of exposure, and abortion typically occurs 1-4 weeks later. Mares infected late in gestation may not abort, but give birth to a congenitally infected foal.
Stallions affected with equine viral arteritis may undergo a period of short-term reduced fertility. This is believed to be the result of increased temperature in the testicles caused by prolonged high fever and severe scrotal swelling that can occur during infection.
The signs of equine viral arteritis are similar to those of other diseases. Laboratory testing is needed to confirm the cause. Blood or nasal swabs can be tested, but timing is important. Obtaining samples as soon as possible after signs develop is helpful to get a viable sample for testing. Later in the disease, the horse may not be shedding the virus. Stallions suspected to be chronic carriers can have blood tested for antibodies to the virus or their semen can be tested to detect the virus.
There is no specific antiviral treatment for equine viral arteritis. Because virtually all horses recover completely, supportive treatment (including fever reducing, anti-inflammatory drugs, and diuretic agents) is appropriate only in severe cases or for stallions in which prolonged fever and extensive scrotal swelling can result in short-term reduced fertility. Good nursing care and rest, with a gradual return to normal activity are usually sufficient. As yet, there is no proven treatment that will successfully eliminate the carrier state in stallions.
Equine viral arteritis is a preventable disease that can be controlled by good management practices and selective use of vaccination. Minimizing or eliminating direct or indirect contact of unprotected horses with infected animals or with virus-infective semen is critical to success. All national, state, and local guidelines should be followed regarding testing of horses or semen for the equine arteritis virus and appropriate vaccination. Pregnant mares should not be vaccinated except in certain circumstances.