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To Vaccinate or Not to Vaccinate?

There has been increased interest in equine viral arteritis (EVA) recently due to an outbreak in 2006, in the Quarter Horse population. While the primary route of infection for EVA is respiratory, the greatest economic impact of the disease results from infected mares losing their pregnancies. Infection may be inapparent or clinical signs may appear three to seven days after infection. Those signs include fever; swelling of limbs, sheath or udder; inappetence; depression; skin rash; and nasal discharge. Abortions occur and in broodmare bands, this may lead to significant economic losses. The abortion rates may be as high as ninety percent of the mares. Stallions often experience a period of temporary infertility and then go on to be a carrier stallion and serve as a reservoir for the disease. The disease is harbored in the stallion’s accessory sex glands, and while the stallion is fertile, they transmit the infection to na?ve mares at a rate of 85 to 95 percent. Occasionally, carrier stallions spontaneously clear the infection. There is no known carrier state in mares or geldings.

It is possible to breed successfully and safely to carrier stallions, as there is a vaccine available for EVA. Strategies for control include blood testing all new breeding stallions prior to the breeding season. For non-carrier stallions, their negative state should be documented, and then they should be vaccinated thirty days prior to the breeding season. The non-carriers should be re-vaccinated annually. All male foals should be vaccinated prior to 270 days of age. Mares not previously exposed to EVA should be vaccinated a minimum of three weeks prior to breeding to carrier stallions. Keep in mind these mares should be isolated from other pregnant mares for a minimum of three weeks after vaccination. Vaccinating pregnant mares is currently not recommended.

It is important to consider whether or not a horse might be shipped outside the United States before vaccinating. Some countries, including Japan and Iceland, will not accept horses testing positive for EVA, even if that positive test is due to vaccination rather than actual disease.

In order to control this disease, basic hygiene procedures should be followed. New arrivals and horses returning (from shows, breeding farms, etc.) should be isolated for four weeks. Pregnant mares should be kept separated from other horses, and they should be maintained in small bands.

The AQHA is monitoring EVA activity currently, and information about that may be found on their website, www.aqha.com. Also, the USDA is monitoring EVA activity. More information on their surveillance program may be found at the USDA’s website.

If you have questions regarding EVA the disease or its control, please contact your veterinarian. We know the vaccination protocol is very complex, and we are happy to assist you.

July 08, 2009

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