Piroplasmosis in horses can be caused by two species of protozoa (Babesia equi and B. caballi), transmitted from an infected horse to a susceptible horse via ticks.

The protozoa that cause the disease in horses are part of the same genus of protozoa that cause cattle tick fever (Texas Fever, or babesiosis), which the United States worked hard to eradicate during the early 1900s and must keep vigilant efforts to prevent re-entrance via cattle and wildlife from Mexico.

The protozoa are present in the bloodstream of animals in the active stages of infection, but may also persist in and spend part of their life cycle in the tick vector. B. caballi, for instance, may persist through several generations of ticks. Contaminated needles and surgical instruments may also transmit the infection physically from an infected horse to a susceptible horse, especially with B. equi.

Incubation period in horses is 10 to 30 days for B. caballi and 12 to 19 days for B. equi. Acute cases are most commonly found with fever, lack of appetite, malaise, labored breathing, blood in urine, sweating, conjunctival hemorrhages, swollen abdomen and posterior weakness.  Some horses may be unable to get up and die within 24 to 48 hours after becoming ill. Colic may occur in some individuals, and fecal balls may be covered with thick mucus.

The horse may be sick for eight to 10 days and then gradually recover. Chronic cases may survive for months. Some individuals never appear to be sick but are carriers. When adult horses become infected, they may act as carriers for variable periods of time or for life.