Katie Hennessy

Equine Piroplasmosis: What You Need to Know

Equine Piroplasmosis (EP) is a foreign animal disease caused by the protozoan parasites, Theileria equi and Babesia caballi. This blood-borne disease of horses, mules and donkeys is spread by ticks but can also be spread between horses with contaminated shared equipment such as needles, dental/surgical or tattoo equipment. While it is unlikely to have an infected horse or donkey because the United States is officially “free” from piroplasmosis, any positive cases must be reported to both state and federal animal health officials. 


It can take 5-30 days for horses to show signs of the disease. Clinical signs are similar to other diseases and can include anemia, fever, swollen limbs, jaundice and labored breathing. Some horses survive this acute phase, but they will remain a carrier of the parasite posing a risk of infection to other horses. 


The cELISA (competitive enzyme-linked immunosorbent assay) is the standard test for screening chronic EP infection and the complement fixation test (CFT) is used for more acute infections. If a horse tests positive for piroplasmosis, there are three options: euthanasia, quarantine for life or enrollment in the USDA APHIS Equine Piroplasmosis Treatment Program. This program is under state supervision and owner expense, the animal is treated with imidocarb and is maintained in quarantine during this time until a negative test is obtained up to two months afterward.


Testing is usually done as surveillance when shipping horses or when required by racetracks but should always be considered in horses with similar clinical signs.


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