Equine Piroplasmosis: Is Your Horse at Risk?

Certain groups of U.S. horses are at risk of acquiring this blood- and tick-borne foreign animal disease.

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vet pulling blood for test
Piroplasmosis can spread via ticks and direct blood transmission. | Alexandra Beckstett/The Horse
Equine piroplasmosis (EP) is a blood- or tick-borne disease caused by blood parasites Theileria equi or Babesia caballi. Clinical signs in the acute phase of infection can include fever, inappetence, depression, elevated respiratory and heart rates, colic, anemia, and sudden death. If the horse survives acute disease, he becomes a chronic carrier of the parasite and can serve as a lifelong transmission risk to other horses. Clinical signs in a chronically infected horse can include anemia, weight loss, and reduced performance, but most chronic carriers appear outwardly normal.

While EP is considered endemic in many countries, and certain tick species around the world can actively transmit T. equi or B. caballi while feeding on horses, the U.S. mainland is currently free of natural tick-borne transmission of EP, and the disease is officially classified as a foreign animal disease. Equids imported to the U.S. from other countries must test negative for both T. equi and B. caballi at entry to prevent incursion of the disease. Veterinarians who suspect EP in a horse are required to report the possible case to state and federal animal health officials.

So, if EP is supposed to be a foreign disease, why are we talking about it? Every year since 2008, veterinarians have identified cases of EP in the U.S. in specific high-risk groups of horses. The largest high-risk group includes current and former Quarter Horse racehorses. In this population some owners and trainers have spread the disease among horses by direct blood transmission through unhygienic practices. These practices (called iatrogenic transmission) include reusing needles, syringes, and intravenous tubing among horses, administering illegal blood products from other countries, giving direct blood transfusions to increase athletic performance (blood doping), and administering multidose drug products that have become blood-contaminated by nonsterile handling techniques between horses.

It’s important to note these methods of iatrogenic transmission also spread other blood-borne diseases, such as equine infectious anemia (EIA). Many of the Quarter Horse racehorses found infected with EP (or EIA) by iatrogenic transmission are also being used for unsanctioned, or “bush-track,” horse racing. Equine piroplasmosis testing required to enter sanctioned racetracks in certain states has led to the discovery of some of these cases, but if a horse is racing exclusively at tracks that don’t require a test or has left unsanctioned racing, the animal might be infected with EP and undetected.

Another high-risk group of horses includes those moved into the U.S. illegally across the border with Mexico. Horses presented for legal import from Mexico undergo a required quarantine and import testing process that includes EP testing, because Mexico is an EP-endemic country. In recent years high demand for certain breeds of horses in the U.S., such as Andalusians, Lusitanos, Warmbloods, and Friesians, has increased the number of these breeds being presented via Mexico for import. Horses testing EP-positive at import quarantine are rejected from entry and returned to Mexico. Some of these horses have been subsequently found to have entered the U.S. illicitly by crossing undetected in areas of open borderland. These illegally imported horses present an ongoing risk of disease transmission to domestic horses in the United States.

So, is your horse at risk? If you have a current or former Quarter Horse racehorse and you’ve never had him tested for EP, contact your veterinarian and request EP testing. If you have a horse that originated from another country, but you were not the owner that imported it, or if you never received USDA documentation of your horse’s official importation, talk to your veterinarian about EP testing. The best test to detect chronically infected horses is the competitive enzyme-linked immunosorbent assay (cELISA), which is available for T. equi and B. caballi at approved U.S. laboratories. If your horse tests EP-positive, you can enroll him in the USDA treatment program, which has been successfully used to permanently clear the infective organism from hundreds of horses since 2010.

For more information visit the equine piroplasmosis page on the USDA-APHIS website: bit.ly/3tc3KTO.


Written by:

Angela Pelzel-McCluskey, DVM, is the equine epidemiologist for the USDA Animal and Plant Health Inspection Service (APHIS) Veterinary Services. She is based in Fort Collins, Colorado. Pelzel-McCluskey obtained her veterinary degree in 2001 from Texas A&M University, in College Station. She worked in equine private practice in both Texas and Colorado and has served as an epidemiologist with state and federal animal health agencies since 2004. Pelzel-McCluskey currently oversees the federal response to reportable equine disease outbreaks nationwide and has been the lead epidemiologist for more than 25 state, regional, and national disease outbreak responses during her combined state and federal service.

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