APHIS Factsheet

Veterinary Services January 2008

Equine Viral Arteritis

Equine viral arteritis (EVA) is an infectious viral 

disease of horses that causes a variety of clinical 

symptoms, most significantly abortions. The disease 

is transmitted through both the respiratory and reproductive systems. Many horses are either asymptomatic or exhibit flu–like symptoms for a short period of 

time. An abortion in pregnant mares is often the first, 

and in some cases, the only sign of the disease. EVA 

has been confirmed in a variety of horse breeds, with 

the highest infection rate found in adult Standardbreds. 

 Breeders, racehorse owners, and show horse 

owners all have strong economic reasons to prevent 

and control this disease. While it does not kill mature 

horses, EVA can eliminate an entire breeding season 

by causing numerous mares to abort. In addition, 

U.S. horses that test positive for EVA antibodies and 

horse semen from EVA-infected horses can be barred 

from entering foreign countries. As the horse industry becomes increasingly internationalized, nearly all 

major horse-breeding countries are including in their 

import policies measures to reduce the risk of EVA. 

The U.S. Department of Agriculture’s (USDA) Animal 

and Plant Health Inspection Service’s (APHIS) 

Veterinary Services (VS) program provides the equine 

industry with EVA diagnostic and surveillance support.

History

More than a century ago, a disease fitting the clinical 

description of what we now call EVA was reported in 

European veterinary literature. However, the virus 

was not isolated from horses in this country until 1953 

during an epidemic of abortions and respiratory 

disease. 

 The most recent EVA epidemic occurred in 1984 

when this disease affected 41 thoroughbred breeding farms in Kentucky. This outbreak brought to light 

two very important findings about EVA: the efficiency 

with which an acutely infected stallion could venereally 

transmit the virus and the high carrier rate that immediately occurred in stallions following natural infection 

with the virus. 

Transmission

EVA is primarily a respiratory disease. Particles from 

acutely infected horses’ nasal discharges are inhaled, 

often during the movement of horses at sales, shows, 

and racetracks. Horses are herd animals that tend to 

commingle, and this close contact facilitates the 

spread of the virus.

 However, unlike other respiratory diseases, EVA 

can also be transmitted venereally during breeding, 

either naturally or by artificial insemination. When a 

mare, gelding, or sexually immature colt contracts the 

disease, the animal will naturally eliminate the virus and 

develop a strong immunity to reinfection. In contrast, 

infected stallions are very likely to become virus carriers 

for a long time. Once stallions are in the carrier state, 

they transmit the virus to mares during breeding. 

 While the mare will shed the virus easily, a pregnant mare infected with EVA may pass the virus to her 

unborn fetus. Depending on the stage of pregnancy, 

the fetus can become infected, die, and be aborted. If 

the infected foal is born, it will only live for a few days. 

Symptoms

Many horses infected with EVA are asymptomatic. 

When symptoms do occur in the acute stage of the 

disease, they can include any or all of the following: 

fever, nasal discharge, loss of appetite, respiratory 

distress, skin rash, muscle soreness, conjunctivitis, and 

depression. Other clinical signs in infected animals are 

swelling around the eyes and ocular discharge, swollen 

limbs, swollen genitals in stallions, and swollen 

mammary glands in mares. 

 Abortion in pregnant mares is also a symptom of 

EVA. Abortion rates in EVA-infected mares can be as 

low as 10 percent or as high as 70 percent.

Diagnosis

Horse owners should suspect EVA when respiratory 

symptoms accompany an abortion in a mare. Since 

the clinical signs of EVA are similar to those of other 

respiratory disease, and no characteristic lesions are in 

EVA-aborted fetuses, only diagnostic tests can confirm 

the disease. Virus isolation can be attempted from 

swabs of the nose, throat, or eyes; semen, placentas, 

or fetal tissue; and blood samples. However, the most 

common method of diagnosis is testing blood for the 

virus’ neutralizing antibodies that cause EVA. While the 

presence of these antibodies alone does not indicate 

active infection, it does indicate EVA exposure has 

occurred. Very high levels of antibodies on a single 

sample or a rising antibody titer from paired blood 

samples collected 14 to 28 days apart indicate active 

infection.

Treatment

While there is no specific treatment for EVA, treatment 

should include rest and in selected cases, antibiotics, 

which may decrease the risk of secondary bacterial 
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infection. Adult horses recover completely from the 

clinical disease. However, the virus commonly persists in the accessory glands of recovered stallions, 

so these carrier stallions continue to shed the virus for 

years and remain a significant source of infection.

Prevention and Control

Fortunately, there is a way the industry can work to 

prevent and control EVA. A safe, effective, and lowcost avirulent live virus is now available. Combining 

this vaccine with isolation of the vaccinated animal 

from noninfected horses can prevent the spread of 

EVA. Since properly vaccinated EVA-negative stallions do not become carriers, all EVA-negative colts 

less than 270 days old should be vaccinated. The 

vaccine is not approved for use in pregnant mares. 

 Blood samples for EVA testing should be collected 

from all horses before breeding, and virus isolation 

should be performed on imported semen before use. 

Strict hygiene and disinfection of instruments and 

equipment are essential to minimize spread of the 

virus. EVA-negative mares should be bred only to 

EVA-negative, noncarrier stallions. 

 If blood test results are positive in a stallion, but 

there is no official documentation of negative EVA 

status prior to vaccination, the stallion must be tested 

for the presence of a carrier state. Virus isolation can 

be attempted on the semen from two separate ejaculations, or by mating two EVA-negative mares with 

the stallion. Twenty-eight days after breeding, mares’ 

blood should be tested for the development of the 

neutralizing antibodies to the EVA virus. 

 Carrier stallions should be bred only to EVApositive mares or mares that are properly vaccinated. 

When breeding an EVA-positive or carrier stallion 

to an EVA-negative, vaccinated mare, isolate both 

horses for 24 hours after breeding to prevent mechanical spread of EVA from voided semen. If this is the 

first time the mare has been bred to a carrier stallion, 

she should be isolated from other horses for an 

additional 21 days due to potential virus shedding.

 All vaccinated horses should receive yearly boosters to protect against infection and, for the stallions, to 

prevent the development of a carrier state. In a generation or two, these practices could all but eliminate 

the population of carrier stallions.

Additional Information

For more information about EVA, contact

USDA, APHIS, Veterinary Services

National Animal Health Programs

4700 River Road, Unit 43

Riverdale, MD 20737

Telephone (301) 734–6954

Fax (301) 734–7964

 Current information on animal diseases is also 

available on the Internet at www.aphis.usda.gov/.