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. Learn more in this article from the May 2022 issue of The Horse.

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

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We at The Horse work to provide you with the latest and most reliable news and information on equine health, care, management, and welfare through our magazine and TheHorse.com. Our explanatory journalism provides an understandable resource on important and sometimes complex health issues. Your subscription will help The Horse continue to offer this vital resource to horse owners of all breeds, disciplines, and experience levels.


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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