Piroplasmosis can spread via ticks and direct blood transmission. | Alexandra Beckstett/The Horse

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