In many cases, infectious diseases of horses are highly preventable with appropriate immunization practices and well-followed biosecurity practices. But not all equine infectious disease has a vaccine that protects a horse against illness.
Two such problems include equine piroplasmosis (EP) and equine infectious anemia (EIA). Both diseases are caused by blood-borne transmission—EIA of a virus and piroplasmosis of a protozoan parasite. The situation with equine piroplasmosis expansion in the United States mirrors the increased incidence of equine infectious anemia in recent years.
Angela Pelzel-McCluskey, DVM, MS, is an equine epidemiologist for the USDA-APHIS-Veterinary Services. She is very involved in tracking infectious disease cases in the equine industry, and she explained that one of the big sources of both piroplasmosis and EIA infection rate increases in the United States has been horses involved in Quarter Horse bush track racing.
Bush Track Racing
There have been notable changes in the bush track industry, which has expanded wildly in recent years through social media. Not only has the number of tracks increased in the United States, but there has been an explosion of the number of participants and the fan base, said Pelzel-McCluskey.
She explained that an increasing number of EIA cases started popping up in 2008. Since 2017, the majority of EIA cases have been occurring in Quarter Horse racehorses with bush track connections. Before 2016, EIA was identified in only a few horses in the U.S. each year. Today, the majority of U.S. horses are untested or under-tested.
From a historical vantage point, in the 1970s, 50,000 horses tested positive for EIA, but the availability of Coggins testing helped to eliminate positive cases (which were euthanized). By the 1990s, only 1,000-4,000 horses tested positive for EIA. In 2021, of 1.4 million EIA tests, 103 tested positive with 87 of them Quarter Horse racehorses compared to the previous average several years ago of 40 EIA positives per year.
Testing for piroplasmosis of 485,000 horses since 2009 has yielded 562 positives, the bulk of which were Quarter Horse racehorses with some Thoroughbred racehorses and a number of other breeds of horses moving illegally from Mexico into the United States. Most cases are caused by the protozoal parasite Theileria equi, but some are caused by Babesia caballi. Many of the Thoroughbred horses infected were those stabled with Quarter Horse racehorses and managed by those trainers.
Of about 35,500 horses tested in 2021, 36 were diagnosed with piroplasmosis. Of those, 31 were Quarter Horse racehorses, most with ties to bush track racing. Five were horses illegally imported from Mexico, and 15 were infected with piroplasmosis and EIA.
Routes of Transmission
There are two primary routes of transmission for piroplasmosis in the United States:
- Tick bite
- Iatrogenic infection due to use between horses of contaminated blood products, needles, syringes, IV sets, equipment and multi-use drug vials. As an example of how piroplasmosis and EIA might be spread through a natural insect vector, Pelzel-McCluskey described a scenario of a generational family ranch along the Gulf Coast, where horses had never been tested for EIA until an elderly family member passed away. The kids or grandkids took the horses to a sale barn, where the herd was found to be EIA-positive due to naturally occurring transmission on the ranch for years.
A similar occurrence happens when tribal or feral horses that are untouched on the range for years are gathered and tested to be sold.
Another scenario is the more common one and is based on illegal movement of horses from Mexico, where piroplasmosis is endemic.
In Mexico, piroplasmosis has a 30% prevalence in part due to limited (or no) testing and no control, as well as the presence of natural tick vectors conducive to spreading the protozoal infection. Mexican-based horses are moving into the United States, often smuggled across the border. The goal of smuggling these horses is to participate in either sanctioned or bush track Quarter Horse racing in the United States.
Other horses are also smuggled into the United States via Mexico from South America and Europe. Those horses include Quarter Horse saddle horses that are good quality ranch horses, plus Andalusians, Lusitanos, Friesians and warmbloods that fetch higher prices in the United States.
“The big problem,” said Pelzel-McCluskey, “is that the Quarter Horse racehorses move back and forth between the bush track and sanctioned races. We have catalogued 84 bush tracks in 24 states in the United States; more are present, although not yet identified. In contrast, the American Quarter Horse Association (AQHA) only has 40 sanctioned racetracks.
“Often an owner will initially try a horse out on the bush tracks to see if it is good enough to make it at sanctioned racetracks,” she said. “In addition, the horses are moved back and forth to and from Mexico year-round, adding to exposure risk.”
Sanctioned Quarter Horse racetracks in 12 states now require EP testing. That means piroplasmosis-positive horses are found through this procedure.
Once a horse is identified as positive with piroplasmosis, the USDA contacts the owner/trainer and tests all horses
on the site of the piroplasmosis-positive horse and any horses on other premises that are determined to have an epidemiologic link to the positive horse. In this way clusters are found, typically due to unhygienic practices, including the use of performance-enhancing drugs and techniques such as blood doping.
Blood doping involves transfer of 1-2 liters of blood directly from one horse to another in hope of improving the racer’s performance. The blood might be obtained on-site or shipped in from another favorite horse in the United States or in Mexico. The typical practice is to use a single IV tubing set between horses, sometimes using that same IV line for years. Blowback of blood into the IV set line leaves material contaminated with piroplasmosis or EIA virus. Not only that, but bush track horses often receive injections every day of vitamins, NSAIDs, stimulants and performance-enhancing medications, often through the same needle used on other horses and/or from blood-contaminated drug vials.
A sobering thought is that EIA virus is transmissible from the mouthparts of a fly containing only 0.0001 virus particles, yet a needle carries 100-1,000 times that volume. Blood on the mouthparts of a fly in flight desiccates when traveling past 200 yards, but blood in the tip of a needle or syringe stays moist with liquid media to enable virus survival for a considerable time.
In addition, blood and serum products coming in illegally from Mexico to the bush tracks are not subject to any health monitoring or controls. Natural transmission via ticks in Mexico results in life-long infections in horses that donate blood for production of blood and serum products that are shipped into the United States and which circumvent USDA import restrictions on equine biologic products.
There are a few options to deal with a positive EP case:
- Export to a country willing to receive a piroplasmosis-infected horse. The USDA has to endorse an international health certificate, but the required statements about the horse not being known to carry a contagious disease cannot be endorsed. Rather, it is necessary to obtain a waiver from the receiving country for that horse to enter.
- The horse is put into a life-long, permanent quarantine or the horse is put into long-term quarantine with enrollment in the piroplasmosis program with the USDA. The horse is then treated to clear the infection, but it can take up to two years for the horse to finally test piroplasmosis-negative and qualify for quarantine release. Most owners with piroplasmosis-positive horses elect this option, says Pelzel-McCluskey.
The response to treatment is very favorable. Pelzel-McCluskey reported that of 356 Quarter Horse racehorses tested positive for piroplasmosis, most have resolved infection with high-dose imidocarb chemotherapy given as four injections spaced 72 hours apart. A couple dozen horses required a two-treatment course to clear infection.
It is a lengthy process to finally reach a negative cELISA test because the horse must lose all its antibodies after removal of the protozoal parasite. Treatment is hard on the horse’s system, especially due to side effects of the drug that causes spasmodic colic and diarrhea for a few hours following administration. This can be controlled reasonably with pre-treatment with Buscopan.
Once the horse reaches negative EP status, it will remain negative unless exposed again.
Unfortunately, there is no treatment for EIA.
Pelzel-McCluskey reported, “A piroplasmosis-positive horse is not always overtly sick. But despite looking clinically normal, most show chronic levels of anemia. Some may have a low-grade fever that goes unnoticed, and some may die if they receive a whopping dose of contaminated blood products. While a chronic carrier may possibly clear Babesia caballi in some cases, they cannot clear Theileria equi without specific treatment.”
Fortunately, in the United States there is no natural tick reservoir to transmit piroplasmosis, so the country is still considered non-endemic and remains free of natural tick-borne transmission. However, there are now Asian Longhorn ticks in the United States that could possibly transmit piroplasmosis.
Equine piroplasmosis is still considered a foreign animal disease. More than a decade ago in 2009 in a closed herd on a ranch in Texas, the cayenne tick, likely brought in with piroplasmosis-infected horses from a sister ranch in Brazil in the 1980s, caused a seropositive outbreak of piroplasmosis in 292 of 360 horses on that ranch.
Additional traceback and testing of horses that were sold from the ranch over many years uncovered a total of 413 horses that were infected with T. equi. Pelzel-McCluskey explained, “Transmission on the ranch occurred over a period of at least 30 years, but intensive tick control and treatment of positive horses finally eradicated the infection successfully from the ranch. No natural tick-borne transmission of piroplasmosis in the U.S. has been found since.”
High Risk of Infected Horses
The number of high-risk, piroplasmosis-infected horses has come from several sources:
- Imports from an endemic country prior to 2005, after which time testing switched to cELISA test, which is better than the complement fixation testing (CFT) to identify chronic reactors. It takes about 30 days post-exposure for a horse to test positive on the cELISA test. All horses imported into the U.S. undergo cELISA and CFT testing for both EP organisms.
- Horses smuggled in from Mexico, including European horses such as Lusitanos, Andalusians, Friesians
and warmbloods coming in through Mexican channels. Currently, these horses are not tested unless they are intercepted by law enforcement while being smuggled across the border or years later when tested to participate in international FEI or Olympic events.
- Quarter Horse racehorses that circulate between non-tested bush tracks and sanctioned racetracks that require testing. Not all states require piroplasmosis testing at sanctioned racetracks. Some Quarter Horse race trainers also manage and train Thoroughbreds and might perform the same non-hygienic practices on them that cause iatrogenic spread of piroplasmosis.
The take-home message for veterinarians is that testing should be done for both EIA and piroplasmosis on any current or former racing Quarter Horses. Plus, testing should be done on Andalusian, Lusitano, Friesian and warmblood horses that claim to be imported but do not have official USDA documentation of importation.
Quarter Horses change hands often throughout their racing careers. They might have been tried out on the bush tracks prior to entering sanctioned races, or they might never enter anything other than bush track races. That puts them at risk for unhygienic practices from a different culture and philosophy of horse management that leads to iatrogenic infection. An otherwise seemingly normal horse that is diagnosed as anemic should be suspect for piroplasmosis and/or EIA.
Education is essential in English and Spanish. It is important to explain to an owner/trainer that any horse testing positive triggers a premises quarantine for 30-60 days, depending on the disease. Testing will be done on the horses as the quarantine is implemented and again for quarantine release.
In addition, another way to target improvement in hygiene practices might be by capitalizing on social media postings for bush track racing.
Equine veterinarians are in a unique position of seeing some of these patients, which gives them an opportunity to discuss biosecurity and details about blood-borne disease transmission. Describing what happens to a horse’s performance due to piroplasmosis might stimulate bush track racehorse owners/trainers to forego their standard practice of using contaminated blood-borne products and equipment. A horse with no parasite infection and without anemia achieves more optimal racing performance and speed.
Veterinarians have an opportunity to recommend testing, especially when a horse is changing ownership. The piroplasmosis test is inexpensive, with both Babesia and Theileria testing costing a client less than $50. Such interactions provide an opportunity to distribute new, unused IV sets, needles and syringes to these clients.
With a concerted effort of equine professionals throughout the industry, it is possible to limit the incidence of blood-borne diseases. While this might seem like a problem only of Quarter Horse racing (and other imported breeds),
it could be a problem that translates into heartbreak for an equestrian who unknowingly purchases a horse that
has been through the bush track and/or Mexico experience.
Awareness by equine veterinarians can help ensure testing takes place and put focus on eradicating these blood-borne diseases. It’s also a time to educate those responsible for iatrogenic transmission and encourage them to change their ways.