Piroplasmosis and the 2010 World Equestrian Games

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.

John Quigley, the state veterinarian at that time, spoke before the FEI Samsung International Equine Sports Medicine Conference in March 1994, stating: "When a nation, country or state is faced with a disease in its livestock population, a decision must be made to eradicate, control or ignore it and live with it. Why should other nations and countries be expected to take risks with the possible introduction and spread of equine piroplasmosis within its equine population?"

Georgia's official position was that there would be no waiver of EP for the 1996 Olympics.

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.

Mild cases may exhibit lack of appetite and general weakness; severe cases result in death. Horses that survive this acute phase typically become carriers. They recover from the disease and show no symptoms but still harbor the infective organism and can pass the disease to other horses.

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.

Disease history

The only countries free from EP are consistently reported to be the United States, Canada, Australia, Japan, England, Ireland and Iceland, but the situation here in America may be slightly more complicated.

Historically, Walking Horses from Cuba are believed to have introduced EP into the Miami, Fla., area in 1959.

This "new" disease finally was confirmed in 1962, according to Dr. Ralph Knowles, retired Chief Staff Veterinarian at the U.S. Department of Agriculture, who explains: "It spread through that region of Florida, resulting in the death of 20 percent of affected animals."

A massive state-federal eradication program was begun in 1962 in South Florida. EP finally was removed as a threat to the equine population in 1988 at a cost of nearly $12 million.

In August 2008, however, a diagnosis of EP was made in a horse from Manatee County, Fla. This "outbreak" eventually resulted in almost 20 infected horses being identified on seven different premises. A statement from the Florida Department of Agriculture and Consumer Services, Division of Animal Industry, noted that, "evidence uncovered during the investigation indicated that transmission of the EP organism occurred due to management practices and not by natural transmission via ticks".

Still, it is the presence of the right types of ticks and the correct environment for development of the protozoan parasite that keeps agricultural officials vigilant and drives policy decisions concerning piroplasmosis in horses in the Southeast.

It was estimated that the Georgia International Horse Park contained 1.3 million ticks per hectare during Olympic competition.

Because of this staggering number, seropositive horses were permitted to compete in dressage and show jumping in arenas, but affected horses were not allowed out on the three-day course which contained significant vegetation.

Testing at the 2000 Sydney Olympics found, in contrast, no tick species implicated as vectors of EP on the competition site. Seropositive horses were allowed to compete in all phases of Olympic Equestrian Competition at this event.

Low risk in Kentucky

Risk assessment for the 2010 games at the Kentucky Horse Park shows a vastly different situation from Atlanta. There is a low prevalence of tick species capable of transmitting EP around the horse park, and the park itself consists of manicured grounds with short field grass.

Farms and pastures around the park share this type of grounds management, which will vastly reduce the percentage of horses being exposed to ticks. Computer models of the probability of one or more susceptible horses becoming positive for EP at the 2010 WEG Games in Kentucky put the chances at 0.00065, or seven horses in a million.

Additional measures being put into place in Lexington include continued tick surveillance (to determine number and species of ticks), treatment of the environment and treatment of individual horses if necessary.

Currently, other than the Olympics or World games, few waivers are given to competition horses that test positively for piroplasmosis. Developing research, however, soon may offer a treatment option that attempts to clear horses of their previously diagnosed Babesia infection.

Researchers report that horses responding to new therapy show a clearance rate of 95 percent to 97 percent for B. equi and rates of 97 percent for B. caballi.

Not all horses will be candidates for this treatment, but substantial improvement is noted in some horses and clearing the label of "piroplasmosis positive" from a nice jumper or dressage prospect can significantly increase the value, usefulness and international travel plans for that equine athlete.

Marcella is an equine practitioner in Canton, Ga.