Equine piroplasmosis (EP) is an infectious disease of international importance. While some parts of the world deal with it as a fairly routine equine health problem, other areas are piroplasmosis-free and want to keep it that way.

A significant issue arises with horses in international competition when a positive athlete is barred entry into a negative country hosting a major event. Piroplasmosis was at the center of debate prior to the 1996 Olympic Games in Atlanta.

Because the U.S. Southeast region has the appropriate climate and the specific type of tick population to support the establishment and transmission of piroplasmosis, the Georgia Department of Agriculture initially issued a "no piroplasmosis" policy for potential Olympic horses.

Photo 1: Horses on the three-day event course or competing in the 50- or 100-mile endurance championships are exposed to local vegetation and the species of ticks present in the area surrounding the event. (Photo: courtesy of Dr. Kenneth Marcella)

Two years later, after much debate, discussion, research and international pressure, this hard-line stance was softened, and horses from piroplasmosis-endemic countries and even positive horses were allowed to enter the United States and to compete in limited events in Atlanta.

Restrictions were placed on the positive horses, and many types of prevention and surveillance measures were undertaken at the games. No problems developed during or after the Olympics, the positive horses left the United States and conversation about piroplasmosis pretty much stopped.

But now, as the 2010 World Equestrian Games in Lexington, Ky., approach, issues about positive EP horses competing, tick removal and control and the overall threat to the country's horse population have resurfaced. This time around more information is available about the disease and research, and it is hoped that education will lead to better ways of dealing with piroplasmosis-positive horses and safer, more effective ways to stop the spread of EP in this country.

Disease transmission

Equine piroplasmosis also has been called babesiosis and was commonly referred to as horse tick fever, which provides a clue to its means of transmission and presenting clinical signs.

EP is a disease of equidae (horses, donkeys, mules and zebras) and is caused by one of two protozoan blood parasites, Babesia equi and Babesia caballi.

The active or acute phase of EP, which occurs within seven to 22 days of exposure, is marked by fever, anemia, jaundice, labored breathing, rough hair coat, swelling of the abdomen, chest or legs and colic in some cases.

Photo 2: Horses affected with piroplasmosis can exhibit anemia, weakness, lack of appetite, respiratory difficulty, constipation and intestinal irritation. Supportive therapy, including fluid replacement, may be necessary and piroplasmosis can mimic equine infectious anemia, colic (as in this photograph) and other diseases. (Photo: courtesy of Dr. Kenneth Marcella)

EP is transmitted from horse to horse primarily by a tick vector. Several species of tick have been identified as capable of transmitting the Babesia organisms. The primary one is Dermacentor nitens, the tropical horse tick. But Dermacentor albipictus, the winter tick; Dermacentor variabilis, the American dog tick; and Boophilus microplus, the southern or tropical cattle tick, have been shown to be capable of transmitting EP in experimental situations. Contaminated hypo-dermic needles and other management practices that allow the transfer of sufficient amounts of blood also can pass along EP.

Diagnosis of EP requires a negative test for equine infectious anemia because these two conditions present very similarly. Direct blood smears can confirm the presence of this parasite within the red blood cell but a competitive enzyme-linked, immunosorbent assay (cELISA) recently has been developed that should improve accurate diagnosis of EP.