Published December, 2012

Of the world's horse population, only about 10% live in countries deemed free of equine piroplasmosis (EP); the United States is one of those regions. However recent disease outbreaks have prompted further investigation into the re-emergence and control of EP in America.

At the 9th International Conference on Equine Infectious Diseases, held Oct. 21-26 in Lexington, Ky., Robert Mealey, DVM, PhD, Dipl. ACVIM, associate professor of immunology and infectious diseases in Department of Veterinary Microbiology and Pathology at the Washington State University College of Veterinary Medicine, discussed the current EP situation in the United States and recent research on the topic.

Naturally, the pathogens that cause EP--Theileria equi or Babesia caballi--are transmitted via certain species of ticks, Mealey said. Many of the cases seen in recent years in America, however, have been spread via the reuse of needles, syringes, and other blood-contaminated equipment that has not been sanitized between uses.

"We haven't seen a lot of severe clinical disease in the United States," he added, noting that subclinical infections are much more common.

Disease Re-emergence in America

Changing gears, Mealey discussed several recent U.S. outbreaks and the suspected or confirmed modes of disease transmission.

Florida, 2008--In 2008, a Florida racehorse with clinical disease tested positive for EP, and an investigation into the source of the disease found that several horses recently imported from Mexico likely carried the disease agent into the country. Investigators later determined that the likely means of transmission was the use of shared needles and syringes and the transfer of blood between horses.

Missouri, 2009--A similar outbreak occurred in Missouri in June 2009. The index case was a Quarter Horse racehorse; a total of eight horses were determined to be infected with EP, all of them sharing the same trainer as the index horse. Investigators believe the method of transmission of the disease agent was, again, the reusing needles and/or syringes among a group of horses and other less-than-optimal hygiene practices.

Texas, 2009--The only outbreak in recent years believed to involve natural tick-borne transmission is one that occurred on a single ranch in Texas. More than 290 of the 360 horses residing on the ranch tested positive for the disease, and a total of 413 EP cases were traced back to the ranch. Investigators learned that two types of ticks found on the ranch and on horses--the cayenne tick and the American dog tick--were capable to transmitting EP between horses.

Mealey said there were 189 cases of EP in the United States unrelated to the Texas outbreak. Of those, investigators determined that 179 were caused by T. equi and just 10 were caused by B. caballi. Some of these cases can be traced back to horses imported prior to 2005, before the newer, more sensitive test was introduced for use during import examinations.

Treatment and Control

One challenge EP presents is that it's a variable response disease, Mealey said, meaning that not all horses react in the same manner to infection. However, "treatment with the goal of parasite clearance is a reasonable approach for infected horses in the United States," he said.

One treatment option some veterinarians employ is using a drug called imidocarb dipropionate (marketed as Imizol), which is labeled for use in treating babesiosis--essentially the same disease--in dogs. While there are several research papers examining the drug's method of action in horses, Mealey said, it's not completely understood. Researchers know that the drug is rapidly absorbed and slowly eliminated after intramuscular injection; it has a narrow safety margin, meaning that a lethal dose isn't far from the therapeutic dose; and toxicity has occurred at the therapeutic dose. But the bottom line, Mealey said, is that imidocarb is effective in eliminating T. equi from the animal's system.

Mealey relayed that veterinarians used imidocarb to treat 163 of the 292 horses in long-term quarantine on the Texas index ranch, and all but five tested negative forT. equi after one round of treatment. Those five horses were treated a second time, he added, and all tested negative after the second treatment.

Despite these encouraging results, Mealey said, more research is needed into treating EP. Ideally, researchers will be able to find or develop less toxic drugs that effectively combat the causative agents. Additionally, a vaccine might be possible in the future with additional research, he noted.

Take-Home Message

Although it's still considered a foreign animal disease, EP has made a statement in the United States in recent years. Research is under way to learn more about the disease and possible treatment methods, in hopes that the disease can once again be eradicated from the country.

Erica Larson, News Editor at The Horse