That healthy, vigorous stallion you’re considering for your mare may infect her with a virus. She could become ill and continue to shed the virus at home for three weeks. Other stock at your barn will be at risk for the illness and pregnant mares may abort. And you may not even realize it.
Freedom from symptoms in previously exposed horses, including carrier stallions, is what makes equine viral arteritis (EVA) a threat to breeding operations. Cooled or frozen semen shipped for artificial insemination is just as dangerous as natural cover.
The risk of serious respiratory disease in previously unexposed horses is clear, as is the potential for abortion outbreaks and neonatal foal deaths. Fortunately, natural infection appears to provide long-lasting protection. However, affected stallions can remain lifelong shedders of the virus and thus infect other horses.
EVA was characterized in the 1950s, following an outbreak of respiratory disease and an abortion storm on an Ohio Standardbred breeding farm. The virus received more widespread attention following a 1984 epidemic among Thoroughbreds in Kentucky. EVA is found worldwide in horses, donkeys, mules and zebras.
What It Does
EVA symptoms are respiratory and venereal, although nonspecific symptoms of fever, loss of appetite and depression are also expected.
Respiratory symptoms, usually worse in young foals but also potentially severe in adults exposed for the first time, range from mild upper-respiratory signs and conjunctivitis to full-blown viral pneumonia. Swelling of the external genitalia can occur in both sexes and mammary swelling in mares.
The virus attacks the placental blood vessels, and mares with pregnancies of two months or more may abort. Foals may be infected in utero and, if not born dead from the placental involvement, may be born weak and ill and often die of pneumonia. In both forms, edema of the legs and the lower abdomen may also be a prominent feature.
Infected horses shed the virus in respiratory secretions, vaginal secretions, urine, semen and uterine fluids when there’s an abortion. When sexually mature stallions are infected, up to 50% may become long-term carriers with the virus setting up house in the reproductive organs. Stallion fertility isn’t affected, but non-immune mares bred to them can become ill.
There’s little documentation of higher rates of early pregnancy loss in these mares, but they will carry the virus back to the herd and pose a risk to other mares and to foals. Infection in horses that aren’t yet sexually mature clears in two to three weeks, but during that period they may infect other animals.
While the virus can sometimes be present in a large percentage of horses on a breeding farm, including shedding stallions, without causing problems, the consequences can be disastrous when it is introduced into a previously unexposed population.
Most major horse-breeding nations have strict policies with guidelines for such things as registering infected carrier stallions, protocols for breeding immune mares to carrier stallions, suggested vaccination programs for non-immume young and adult horses and strict regulations regarding importation of infected horses and testing of semen used for AI. The United States is an exception.
After the 1984 problem on Thoroughbred breeding farms in Kentucky, the Thoroughbred organizations of Kentucky and New York took it upon themselves to establish a control program. However, with the exception of similar measures in place for some warmbloods, no other breed or government control programs are in place for incoming stock or semen.
Programs to prevent the spread of EVA aren’t complicated and don’t require the elimination of EVA-positive stallions from the breeding population. Some aspects of effective control involve:
Testing of all breeding stallions to determine their EVA status.
Breeding, natural and artificial, of EVA carrier/shedder stallions only to naturally immune or properly vaccinated mares.
Isolation of mares bred to carrier stallions for three weeks after breeding to minimize any risk of them infecting non-immune animals through contaminated secretions.
Vaccination of young stock beginning at the age of nine months, the age studies show that any maternal antibodies are gone.
Vaccination of non-immune mares that are to be bred to carrier stallions three to four weeks in advance of breeding.
The vaccine available in the United States is a modified live vaccine, which means vaccinated horses will shed the virus for approximately 21 days. For this reason, many states require veterinarians obtain permission before using it.
In addition, special precautions are necessary after giving the vaccine. Any vaccinated horse must be isolated from non-immune and non-vaccinated animals for 21 days to avoid infecting them.
Pregnant mares should not be vaccinated. Mares that have recently foaled should be allowed to recover completely before vaccination, and foals at their sides should be at least two weeks old before the mare is vaccinated. There is a risk of causing illness in suckling foals when the dams are vaccinated.
EVA is far from the worst infectious disease threat facing our horses, but it shouldn’t be ignored, either. When outbreaks occur, the economic loss can be significant. Fortunately, the disease isn’t difficult to control. As a mare owner, all you need to do is ask if the stallion you’re considering is an EVA carrier/shedder and take proper precautions if he is.