Infection with Theileria equi and Babesia caballi (Equine piroplasmosis)

Acute cases, the most common form, show fever usually above 40 degrees C, poor appetite, malaise, raised breathing and pulse rates, congested mucous membranes and small, dry faecal balls. Subacute cases add weight loss, intermittent fever, pale to yellow membranes with petechiae, mild colic and swelling of the lower limbs. Chronic cases show mild inappetence, poor performance and weight loss, with an enlarged spleen on rectal examination. A rare peracute form leaves horses dead or dying. Transplacental infection usually ends in abortion.

Equine piroplasmosis is caused by two tick-borne protozoa, Theileria equi and Babesia caballi. Babesia caballi sporozoites enter red blood cells directly and divide into paired merozoites. Theileria equi sporozoites first invade lymphocytes, develop into schizonts and then release merozoites into red cells, where four may form a 'Maltese cross'. Destruction of parasitised red cells causes haemolytic anaemia, with a falling packed cell volume, haemoglobinuria and jaundice. Theileria equi has also been found in liver, spleen, lungs and bone marrow. Infected equids remain carriers for long periods: usually for life with T. equi, whereas B. caballi infection tends to clear within a few years.

Horses, donkeys, mules and zebras are affected; parasite DNA has been detected in camels and dogs without disease, and the infection is not zoonotic. About fourteen hard tick species in Dermacentor, Rhipicephalus and Hyalomma act as vectors, and contaminated needles, syringes, surgical or dental instruments and blood products spread infection iatrogenically. Mares can pass infection to the foal across the placenta. The disease is endemic in tropical and temperate regions including Africa, the Middle East, southern Europe and Central and South America; surveys have found infected horses in Egypt, Saudi Arabia, the United Arab Emirates and Iran. It is WOAH-listed and restricts international movement of positive horses.

Other causes of fever, anaemia and jaundice in horses: equine infectious anaemia, anaplasmosis, immune-mediated haemolytic anaemia, anaemia of chronic infection or inflammation, and chronic kidney failure. These findings are nonspecific, so confirmation relies on seeing parasites in Giemsa-stained blood smears during acute disease, species-specific PCR, or serology by IFAT or competitive ELISA, which is also used to find carriers.

Blood (red cells and lymphocytes), spleen, liver, bone marrow, lungs, placenta

No vaccine exists. Control ticks, never reuse needles or syringes, use only tested blood donors and licensed blood products, and test horses before movement or import.

In peracute and acute disease, mortality is 5-10% in endemic areas and over 50% in newly imported naive equids; APHIS cites up to 50%. Treatment clears most horses over time, but T. equi is not always eliminated and survivors often remain carriers.