Monday, 16 August 2010 14:52

“Doc – Is There A New Disease of Horses, Called Equine Viral Arteritis?”

Written by  A.J. Neumann, D.V.M.
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You are partially correct. There is a disease of horses known as Equine Viral Arteritis, "EVA" for short, but it is not new to this country or hemisphere. The virus likely caused a large outbreak of sickness in horses in the United States in 1872-'73. It has occurred sporadically in the United States since that time. Cases have been identified and reported from New York, Ohio, Indiana, California, Pennsylvania and Kentucky. EVA became a very hot topic in the latter months of 2006 due to an outbreak on a New Mexico Quarter Horse facility.

The Spring 2007 issue of The Draft Horse Journal carries an article, which I wrote, about the Equine Herpesvirus and its effect on pregnant mares. This is not to be confused with Equine Viral Arteritis, as this virus infection will often duplicate many symptoms seen in Herpesvirus infections. The lesions and symptoms of both diseases have been recorded for a century and a half where the cases have been called distemper, influenza, pinkeye, shipping fever and epizoatic cellulitis. The arteritis virus has apparently been responsible for the pinkeye, cellulitis and acute septicemic conditions while the herpes virus has mainly caused influenza-type diseases. Both types of viruses have caused abortions. In 1957, the name, Viral Arteritis, was given to a disease resulting from a virus recovered from an equine fetus which was different clinically, pathologically, hematologically and immunologically from the herpesvirus.

EVA, or Equine Viral Arteritis, is caused by a togavirus and only infects equidea. Cases are sporadic in occurrence although it is thought that some outbreaks may be missed as the symptoms are mild and therefore confused with those caused by the herpesvirus and equine influenza respiratory infections.

The incubation period of EVA is from 2 to 14 days. Early signs of EVA are like those seen in any respiratory infection such as fever, nasal discharge, “runny” eyes and conjunctivitis. As the disease progresses, frequent signs such as general weakness, depression, loss of appetite, colic and diarrhea may be seen. Edema of the rear legs, abdomen, udder or scrotum and sheath may occur along with edema of the eyelids. In typical cases these symptoms will disappear in 7 to 14 days after their onset, with the exception of the edema of the eyelids, legs and ventral portions of the abdomen.

In natural outbreaks of EVA, about 50 to 80% of mares will abort in 7 to 14 days after the onset of clinical symptoms. The abortion will occur in infected mares regardless of their stage of pregnancy. The disease seems to be more severe in pregnant mares than in open or barren mares. If a horse is poorly nourished or heavily parasitized, EVA will exact a heavy toll on these animals. If a mare should contract EVA late in pregnancy, her foal may be born alive but will die very quickly from viral pneumonia.

The disease is spread by exposure to contaminated equipment or via direct inhalation of contaminated water droplets from a coughing sick horse, as well as semen from a persistently infected stallion. The virus can also be transmitted by blood transfusions or by needles contaminated with blood from a sick animal during the fibril period.

The diagnosis of EVA is not particularly difficult. The virus can be cultured from nasopharyngeal swabs, aborted fetuses, urine and the semen of some infected stallions. A blood serum antibody test is also available and is widely used to diagnose the disease and to identify affected stallions.

70% of stallions that contract the disease will become chronically infected and will pass the virus in their semen. It makes no difference if the semen is fresh, cooled or frozen. Experiments have shown that infected stallions can shed the virus for an indefinite length of time, often for weeks, months or years. The virus infects the stallion’s testicles and can remain there as an infective agent for the rest of the animal’s life. This factor makes the chronically infected stallion the single most important source of infection.

Research has shown that the carrier state is “testosterone dependent.” Therefore mares, geldings and sexually immature stallions do not become persistent or chronic carriers of EVA. They will only transmit the disease during the first 14 days after they become infected and exhibit signs of the disease.

Many foreign countries, in an effort to prevent the spread of EVA, will not accept horse’s semen or embryos that test positive for the virus. Owners who expect to export stallions or semen to foreign countries should realize that a positive test of the animal, or semen, will severely limit the sale.

There is a vaccine available to use in horses to produce an immunity against the EVA virus. The use of this modified live virus vaccine has been found to be a safe and effective means of controlling EVA. A single dose of the vaccine (Arvac, Fort Dodge Labs) injected intramuscularly will give some immunity to the horse in 4 days and complete resistance to the virus infection after 10 or 12 days. Vaccination of stallions and non-pregnant mares, 3 weeks before breeding is recommended. If a mare is pregnant she should not be vaccinated.

The blood serum test for EVA cannot differentiate between blood titers caused by the disease and those due to the vaccination of the animal. Therefore, before vaccination, the stallion or mare should be blood tested to determine its status. This is especially essential in dealing with the stallion since an EVA negative stallion, who is then vaccinated properly, will not become a carrier. Those stallions found to be positive for EVA and shedding the virus in their semen, should only be used to breed those mares who test positive or who are properly vaccinated against the disease.

The United States Department of Agriculture recommends that: “All EVA-negative intact colts younger than 9 months be vaccinated. They should then receive an additional booster 6 months later and an annual booster thereafter.” This proper vaccination program would prevent the stallions from becoming infected and shedding the EVA virus.

In some states the state veterinarian may have to be contacted to approve the use of the vaccine. Your local veterinarian would know about this factor.

I suppose reading this article written about a disease of horses, which you probably haven’t heard much about, is boring. However, all things change with time and EVA seems to be on the rise. At least the next time you read about an outbreak of EVA you can say to yourself, “I know about that disease because I read about it in The Draft Horse Journal.”

For more information contact the USDA at or the University of Kentucky’s Gluck Center at

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