Taking Piroplasmosis Seriously

Taking Piroplasmosis Seriously

One way piroplasmosis spreads is via reused needles and blood-contaminated equipment.

Photo: Erica Larson, News Editor

In most parts of the world, equine piroplasmosis (EP) is endemic, routine, even commonplace. In the United States, however, this foreign animal disease is reportable and subject to regulatory action to mitigate disease spread. It is a parasitic disease of all equids, caused by Theileria equi and/or Babesia caballi. In endemic (native) countries ticks or other arthropods transmit EP from one infected horse to another. However, people can spread disease agents as well, by reusing needles or syringes or using blood-contaminated instruments such as dental or surgical equipment that have not been cleaned and disinfected appropriately between uses.

Pathogens and Pathology

Upon introduction to the horse’s body, both T. equi and B. caballi invade the horse’s red blood cells and can cause severe anemia (low red blood cell count) in some infected animals, evidenced by lethargy, reduced performance, and pale mucous membranes. Horses might also develop fever, icterus (jaundice), anorexia, and digestive problems including colic, constipation, or diarrhea. And because these clinical signs can also accompany other diseases, pursuing proper diagnostic testing is crucial. Veterinarians can detect EP by identifying the parasites on blood smears in clinically ill horses or through serologic testing using the competitive enzyme-linked immunosorbent assay (cELISA) or complement fixation (CF) tests. A third type of serology test, the immunofluorescent antibody (IFA) test, is also available to veterinarians, and scientists have developed polymerase chain reaction (PCR) tests for research purposes.

Picking Up on EP

In EP-endemic countries most horses become infected within their first year of life, and their fatality rate (5-10% of affected horses) is significantly lower than that in naive horses transported to endemic areas (which can exceed 50%). 

Australia, Canada, England, Iceland, Ireland, Japan, and the United States are the only countries worldwide where EP is not endemic. Cases of the disease do, however, appear sporadically in the United States. Veterinarians screen arriving imported horses to detect T. equi or B. caballi infection, but the screening process might have missed some infected horses imported prior to 2005. Still others might cross into the United States without going through the proper channels. Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, a professor at Colorado State University’s College of Veterinary Medicine & Biomedical Sciences and researcher in infectious diseases, explains how veterinarians might have overlooked an infected horse under the old screening process:

“The complement fixation test that was used for importation from the 1970s to 2005 detects the antibodies to the blood parasites in a different way than the cELISA test, which is now used for import testing,” she says, adding that while the CF test is optimal for detecting horses that have been infected within the past week or two, it might miss chronically infected carrier animals. The current cELISA test, on the other hand, does detect chronic cases. 

Angela Pelzel-McCluskey, DVM, a regional epidemiologist for the USDA’s Animal and Plant Health Inspection Service Veterinary Services (USDA-APHIS-VS), and other veterinarians believe the CF test should be added back to the screening regimen; they have concerns that some horses entering the United States from endemic countries might be very recently exposed and, thus, missed upon cELISA testing. Requiring both tests would allow detection of infection in both acutely and chronically infected horses.

Currently, T. equi is the primary agent causing EP in the United States. Pelzel-McCluskey explains that two U.S. tick species are known T. equi vectors—the cayenne tick (Amblyomma cajennense), found in South Texas, and the American dog tick (Dermacentor variabilis), which is more widespread throughout most of the southern and eastern United States.

The only tick in the United States capable of spreading B. caballi is the tropical horse tick (Anocentor nitens) from deep south regions of Texas. Traub-Dargatz says these ticks can transmit B. caballi to the next generation of vectors, and they can spread the agent to and pick it up from horses. With T. equi, however, the horse is the reservoir host, meaning A. cajennense and D. variabilis cannot pass it from one tick generation to the next.

If you go ride your horse in the woods and he picks up a tick, he's probably not going to get piroplasmosis.

Dr. Benjamin Buchanan

Outbreaks in the United States

In 2008 an outbreak among horses participating in unsanctioned racing in Florida was traced to iatronic spread, which means through human intervention. In this situation humans transferred blood from one horse to another (e.g., reusing needles, along with blood boosting prior to racing called “blood doping”). A similar outbreak occurred in June 2009 among a group of horses involved in unsanctioned racing in Missouri and Kansas.

In October 2009 veterinarians diagnosed a horse with EP on a Quarter Horse ranch in Texas. Ultimately, 413 horses associated with that ranch were discovered to be T. equi-infected after epidemiologic investigation and testing of 2,500 potentially exposed horses in 2009 and 2010. Investigators determined that natural tick-borne transmission (likely from the cayenne tick and the American dog tick) caused the disease spread on the ranch. That particular outbreak led to several discoveries regarding the nature of the disease and treatment options.

Benjamin Buchanan, DVM, Dipl. ACVIM, AVECC, of Brazos Valley Equine Hospital, in Navasota, Texas, worked with the Texas Veterinary Medical Association at the time of this outbreak. He and his practice colleagues did not treat the affected horses, but he says he gained a fresh understanding of the disease and a professional respect for handling outbreaks. 

Buchanan credits open communication, for instance, as a huge part of the success in quelling EP spread in Texas and improving understanding of the disease. The state “put together a group of industry stakeholders to come sit down and talk about how the disease is actually transmitted, what the risks are, and what their (control) approach has been,” he says. “They’ve maintained that group.”

Disease Transmission

In a natural setting, ticks transmit EP parasites by feeding on an infected horse and transferring the parasites to another horse through their saliva. Because the horse is T. equi’s natural reservoir host, tick transfer requires populations of both “competent” ticks (meaning they can pick up and transmit EP) and infected horses. “We learned from the Texas ranch outbreak that you need very specific high-risk factors to come together for EP transmission—a large number of infected horses, competent tick vectors present in high density, possibly even year-round maintenance of those tick vectors (meaning the ticks overwinter in the mild climate), and long periods of time with horses exposed in these conditions,” says Pelzel-McCluskey. She admits this scenario is not one that would play out in many areas of the United States.

Buchanan agrees. “If you go ride your horse in the woods and he picks up a tick, he’s probably not going to get piroplasmosis,” he says. “But, if you’re in an area where EP is a problem, you want to treat for ticks.”

With iatrogenic transmission, two-legged EP transmitters unwittingly pass the agent with the compromised needle hygiene/disinfection practices described. Blood-doping particularly has given rise to the occasional occurrence of disease agent transmission among horses used for unsanctioned racing, as in the Florida, Kansas, and Missouri outbreaks.


Historically, veterinarians have used several drugs to treat piroplasmosis, but the current treatment protocol involves administering the drug imidocarb dipropionate. 

Veterinarians didn’t know until the Texas outbreak whether horses could be cleared completely of infection or if recovered horses would always carry the disease-causing organism. The large number of Texas cases gave investigators the opportunity to find out if treatment could clear the organism completely and permanently, says Pelzel-McCluskey.

As of 2013, more than 170 of the Texas horses have been cleared completely of T. equi using imidocarb treatment. Of those horses, six failed to clear with the first round of treatment. Pelzel-McCluskey considers this a good thing. “It proves that post-treatment diagnostics (using a series of PCR tests right after the last treatment to detect antigen presence) worked very well to tell us if we had failed to clear the organism,” she says. “Most horses clear after three doses of the four-dose treatment, but we are keeping with a four-dose protocol. The diagnostics picked up horses that remained infected right away; we retreated, and they have now cleared subsequently.”

Despite the imidocarb treatment’s success at clearing infection, Pelzel-McCluskey cautions, “Treatment is not simple, not innocuous. The treated horses quickly become uncomfortable; they sweat, drink a lot of water, have watery diarrhea, and can have colic signs for several hours after each injection.” Although pretreating with antispasmodic drugs such as N-butylscopolammonium bromide (Buscopan) can mitigate some of these signs, most horses still react post-injection.

Prevention and Regulation

In addition to import screening of horses, several states and a number of racetracks have instituted EP testing requirements for horses entering their jurisdictions. Both Traub-Dargatz and Pelzel-McCluskey call this ongoing surveillance critical. “We worry that some tracks may tire of requiring EP testing,” says Pelzel-McCluskey. She pointed out that Indiana discontinued its racetrack testing requirements for EP at the end of 2012. By January and February of 2013, veterinarians detected 11 new cases in horses that had been negative on previous tests but were due for annual testing. “This will continue to be a place where we have a gap,” she says. “Horses move from unsanctioned to sanctioned racing, and there will be previously untested horses that have at some point had an exposure.”

Traub-Dargatz recommends always controlling ticks because of the variety of diseases they can spread, though it becomes especially critical in areas with naturally infected livestock EP reservoirs and competent tick vectors. Her general tick control steps include: 

  • Mowing tall grass and weeds;
  • Fencing or removing brush areas; and
  • Checking horses for ticks, especially at or around their ears, at the base of the mane, tailhead, sheath, udder, and under the tail.

Traub-Dargatz and Pelzel-McCluskey both caution that there are some difficulties inherent to using topical sprays or spot-on tick-control products in horses: Because horses sweat all over their bodies, products need to be applied frequently. Also, because ticks are not insects, general insect-repellent sprays are ineffective, so owners must read the label indications for the product they purchase.

However, consider the human factor first when considering EP prevention for “indoor” horses or those residing in non-tick-endemic areas. “It’s amazing how many outbreaks in nonendemic regions are iatrogenic,” says Traub-Dargatz. She references key prevention methods from the USDA-APHIS-VS’s information sheet on EP control:

  • Not sharing needles, because some infected horses appear normal/healthy;
  • Not reusing syringes among horses; 
  • Cleaning and disinfecting instruments that could be blood-contaminated, such as dental or tattooing equipment, after every use;
  • Not using horses as a source for blood or blood products unless they have been properly disease tested; and
  • Throwing away or disinfecting anything contaminated with blood. 

Because state and federal agencies consider EP a foreign disease in the United States, they regulate the management of positive cases. A veterinarian or lab that diagnoses EP must report it to the proper veterinary regulatory authorities. Government veterinarians regulate and supervise quarantine, tick control protocols, and treatment procedures when cases arise.

Traub-Dargatz describes four options owners have if their horses test EP-positive:

  1. Humane euthanasia;
  2. Legal export of the horse to any country that will accept it;
  3. Lifetime quarantine (Traub-Dargatz says protocols can be used on a premises to prevent exposing the infected horse to ticks and to negative horses); and
  4. Enrollment in a USDA-APHIS/USDA--Agricultural Research Service treatment program designed to clear T. equi from the horse while in long-term quarantine.

Take-Home Message

While EP is a serious and potentially fatal disease in the horse, current diagnostic, regulatory, and therapeutic methods can prevent it from gaining a permanent foothold in the United States. Horse owner and trainer commitment to tick prevention, legal transportation of horses, and proper biosecurity measures is necessary to control disease spread.

About the Author

Christy Corp-Minamiji, DVM

Christy Corp-Minamiji, DVM, practices large animal medicine in Northern California, with particular interests in equine wound management and geriatric equine care. She and her husband have three children, and she writes fiction and creative nonfiction in her spare time.

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