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Equine Piroplasmosis on the Rise

As the demand for English sporthorses grows, so does their value. Many prospects are imported from Europe and other countries to fulfill growing demand. Normally, a horse imported into the United States is tested for a number of foreign animal diseases, including equine piroplasmosis (EP). However, it is becoming more apparent that this disease is on the rise in the United States. Let’s look at what piroplasmosis is and why it has the potential to become more prevalent in the horse world. 

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What is Piroplasmosis and How is it Spread?

Equine piroplasmosis is a protozoan that infects the blood of exposed horses or other equids. It comes as two strains: Theileria equi or Babesia caballi. These protozoa are transmitted to a horse through the bite of a tick that has ingested blood from an EP-positive horse. The disease primarily occurs in tropical and subtropical climates conducive to tick vectors. While there are 14 tick species capable of spreading this disease to horses, at this time none are prevalent in the United States.  

The current predominant means of transmission is iatrogenic (illness caused by medical procedures) through blood or serum transfusions or from blood-contaminated equipment, such as needles, syringes, IV sets, contaminated multiuse drug vials of sedatives, vitamins, nonsteroidal anti-inflammatory drugs, and medical, dental or tattooing equipment, to name a few examples. 

Poor hygienic practices are responsible for infection transmission. A large proportion of EP infections occur in bush track Quarter Horse racing stables where blood doping is common with one IV set and the same needle used between horses. Blood doping involves transfer of 1-2 liters of blood directly from one horse to another in an attempt to improve a racehorse’s performance. While bush track racing horses are primarily Quarter Horses, once these horses finish their racing careers, they often end up in the general equine population, housed amongst other sporthorse breeds. 

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Mismanagement practices by horse owners and trainers, particularly in the Quarter Horse racing world in both bush tracks and sanctioned Quarter Horse races, are responsible for the bulk of disease cluster outbreaks of EP. To date, only 12 states test for EP at sanctioned Quarter Horse racetracks before the horses are allowed onto the property. Testing is not required in any other states, nor does it address the huge issue of infectious diseases (EP and equine infectious anemia) amongst bush track racing facilities where there is no testing.  

Infection may extend to Thoroughbred horses when trainers take on both Quarter Horses and Thoroughbreds in their racing stables. Use of blood-contaminated equipment crosses breed lines in these cases. Thoroughbred horses that don’t excel well on the track often end up in the general population and find a career as an English sporthorse. 

A Current Source of the Problem

Besides clusters of EP-positive horses in bush track racing, attempts to bypass import requirements from other countries—the EU or South American—are fueling a new source of EP. The USDA-APHIS regulations (VS Guidance 13407.2) state: “Veterinary Services (VS) personnel test horses presented for import to exclude four diseases from the United States: Dourine, glanders, equine infectious anemia (EIA), and equine piroplasmosis (EP). Horses must test negative on official tests to these four diseases before VS will release them from quarantine and allow entry into the United States.” 

Popular imported breeds for English sport activities include Andalusians, Luisitano horses, Friesians and Warmbloods. Due to increasing values of these breeds, some buyers and sellers try to circumvent the requirements and expenses to move a horse to the United States by bringing the horses in illegally from the southern border, through Mexico. Unfortunately, Mexico is endemic (at a rate of 30%) for equine piroplasmosis due to its favorable climate for appropriate tick vectors. Not only are such breeds of horses from EP-endemic countries entering illegally but also horses spending time in Mexico are at risk of infection through tick vectors. Horses brought into the USA in this manner may be “silent” carriers that are infected with EP although not yet showing clinical signs. 

Health Issues from Piroplasmosis

A horse infected with piroplasmosis experiences a number of relatively nonspecific clinical signs after an incubation period lasting up to three weeks: Fever, poor appetite, anemia, jaundiced mucous membranes, weight loss, poor performance and exercise intolerance, peripheral edema and/or swollen abdomen from edema fluid accumulation, labored breathing, diarrhea, colic, and even sudden death due to multiorgan failure. Outbreaks of piroplasmosis can cause significant economic harm to the equine industry due to costs of treatment and potential loss of performance or life of valuable animals.  

Treatment is possible rather than having to resort to one of three other options: a) exportation of the affected horse from the country; b) life-long quarantine; or c) euthanasia. Treatment relies on placing the horse in strict quarantine in a USDA-APHIS-approved EP treatment program that administers high doses of imidocarb. In some cases, treatment must be repeated until the horse reaches a negative antibody status. Successful treatment can take up to one to two years to accomplish. 

Prevention of Piroplasmosis

Of critical importance for prevention of spread of EP is implementation of sound hygienic strategies. Medical equipment should be disinfected appropriately between each horse use. No needles, syringes, or IV sets should be re-used or shared between horses.  

Blood products used on horses should come from USA-based commercial laboratories that test for contaminants or infectious disease. Natural transmission via ticks in endemic Mexico results in life-long infections in horses that donate blood for production of blood and serum products that are shipped illegally into the USA from Mexico. These products are not subject to any health screening or controls and they bypass USDA import restrictions on equine biologic products.  

Tick control is important, particularly for horses brought in from countries endemic for EP. Careful monitoring for ticks on the horse is necessary, and the use of insecticide shampoo baths help to eliminate insect vectors. Mow grass and brush around stabling areas and pastures to reduce tick habitat and then fence horses away from transition zones of deep brush that harbor ticks.  

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The best strategy an owner or buyer can do is to have a horse tested for equine piroplasmosis much in the same way a horse is surveilled with a blood test for equine infectious anemia to be able to travel interstate, participate in riding clinics, to enter a new facility, or for change of ownership. Horses change hands all the time and it is not always possible to know if a horse was illegally imported through Mexico. Having a horse test negative for EP completely eliminates the concern that a horse from an unknown source may be infected with the protozoa. 

At this time, USEF and FEI-sanctioned horse events require only EIA testing, influenza vaccines for FEI, and influenza and rhinopneumonitis immunizations for USEF. There is no requirement for surveillance of other infectious diseases. Concerned horse owners might request horse-show managers to include EP testing as part of competition entry requirements. 

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