Horse owners are familiar with a Coggins test—that piece of paper required for entry into most horse shows and sales and/or for interstate horse transport. But many overlook the fact that this piece of paper, which serves as proof that the horses has tested negative for equine infectious anemia (EIA), is essential to protecting the health of the national equine population. A number of recent EIA positives in racing Quarter Horse populations in California and Texas has increased the need for awareness about this potentially fatal blood borne disease of horses, donkeys, and mules.

Also known as swamp fever, EIA is caused by a virus closely related to the human immunodeficiency virus (commonly known as HIV). The equine condition might not cause any outward clinical signs, or they can be apparent, ranging from fever and lethargy to weight loss; anemia; and swelling of the legs, chest, and abdomen.

Natural disease transmission occurs when a deerfly or horsefly bites an infected horse, consumes a blood meal, and transfers the virus to another horse. People can also introduce the virus to a naive horse via the use of infected blood or blood products or through the use of blood-contaminated needles, syringes, surgical instruments, dental equipment, tattooing equipment, or any other equipment. Infected horses become lifelong carriers that pose a risk of infection to other horses. There is no known treatment for EIA. The options for managing an EIA-positive horse are euthanasia or lifelong quarantine of the individual horse at least 200 yards from unaffected horses.

Recently, the California Department of Food and Agriculture’s (CDFA) Animal Health Branch has been investigating the largest EIA outbreak in that state in years. Since 2012 diagnostic testing confirmed 34 racing Quarter Horses, ranging in age from 3 to 8 years, as positive for EIA. The investigation identified approximately 250 exposed horses on 19 California premises that tested negative on the initial and 60-day retest after removal of the positive horse. Thirty-three of the EIA-positive horses were euthanized and one was moved to a premises for isolation.

Disease investigations indicate the positive horses were involved in Quarter Horse racing and potentially were exposed to high-risk practices such as sharing needles and other medical equipment and the use of contaminated blood products. Due to change of ownership, lack of lip tattoos, and difficulty reading lip tattoos, the training and racing histories of positive horses were hard to obtain.

Although difficult to verify, evidence suggests that some of the positive horses participated in unsanctioned “bush track” racing. Evidence points to disease spread via contaminated multi-dose drug vials. In this situation, the contamination occurred when a new needle and used syringe are used for drug administration—infected blood in the hub of the used syringe contaminates the drug vial, resulting in disease spread with subsequent withdrawal of the drug and administration.

So should area horse owners be worried about EIA? The recent upswing in positive EIA cases in the racing Quarter Horse population is a cause for concern, as many retired racehorses with the potential to spread EIA enter second careers in the breeding shed, rodeo world, show arena, or as backyard pleasure, trail, or ranch horses. Given the right conditions, one undetected case can lead to a large regional, national, or international outbreak. All horse owners are encouraged to establish and adhere to good biosecurity practices and avoid sharing needles, syringes, blood, or blood products among horses.

Owners concerned with potential exposure to EIA should contact their veterinarian to discuss serologic (blood) testing. The two most commonly used serologic tests are the agar gel immunodiffusion (AGID) and the enzyme-linked immunosorbent assay (ELISA), the former of which is known as the Coggins test. A positive test indicates the presence of EIA-specific antibodies in the horse’s blood. The ELISA test can detect antibodies earlier than the AGID test but, because the ELISA test can produce false positive results, the confirmatory test for EIA is the AGID.

For more information visit or contact Katie Flynn, BVMS, equine staff veterinarian for the CDFA Animal Health Branch, at 916/900-5039 or

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