Equine piroplasmosis (EP) is a blood-borne disease spread by 

ticks. It can affect horses, donkeys, mules, and zebras. 

EP is endemic in most tropical and temperate areas of the world 

with ticks capable of carrying the disease. In the United States, 

EP is endemic only in Puerto Rico and the U.S. Virgin Islands. 

However, isolated outbreaks have occurred in horses on the 

U.S. mainland. Also, many areas of our country have climates 

and tick vectors suitable for the disease to establish itself here. 

If you’re a horse owner or veterinarian, you need to know about 

the risks of EP transmission in the United States. The following 

information describes the disease and how to help protect the 

U.S. horse industry from it.

EP is a foreign animal disease in the United States. 

You must report any detection of EP to your State 

Veterinarian (usaha.org/saho) and/or USDA Area 

Veterinarian in Charge (aphis.usda.gov/animal_health/

contacts/field-operations-districts.pdf). 

How USDA Protects Equine Health

The U.S. Department of Agriculture (USDA) protects our Nation’s 

horse industry from EP in several ways:

• USDA regulates equine imports and maintains tick control 

and surveillance programs. 

• We require a negative EP test for most imported horses to 

enter the United States. EP-positive horses can only enter 

the country temporarily under special arrangements, such 

as for international equine events like the World Equestrian 

Games.

• USDA has laboratories approved to test horses for EP. We 

notify the World Organization for Animal Health (WOAH) if 

an outbreak occurs here. 

• We also offer a supervised EP-treatment program. See 

more detail on the next page under “Treatment.”

Transmission

Two parasites, Babesia caballi and Theileria (formerly Babesia) 

equi, cause this disease. In most cases, horses that survive the 

acute phase of infection continue to carry the parasites. These 

“chronic carriers” become potential sources of infection for 

other horses. 

Equine Piroplasmosis

The disease spreads naturally via tick bites. It can also spread 

through contact with blood-contaminated equipment or blood 

products. 

Tick Bites 

Ticks become infected with EP when they ingest blood from 

infected equines. The ticks then spread it by biting uninfected 

equines. 

EP is carried by Ixodid or “hard” ticks of various genera. The United 

States is home to several tick species capable of carrying this 

disease: 

• Cayenne tick (Amblyomma [cajennense] mixtum)

• Tropical horse tick (Dermacentor nitens) 

• Winter tick (D. albopictus) 

• American dog tick (D. variabilis)

• Southern cattle tick (Rhipicephalus [Boophilus] microplus)

• Asian longhorned tick (Haemaphysalis longicornis)

Although these ticks could host and transmit EP, current 

surveillance shows no natural tick-borne transmission of EP on 

the U.S. mainland.

Contaminated Blood 

Equipment contaminated with EP-infected blood can also spread 

the disease. This may include injectable, surgical, dental, or 

tattoo equipment. 

The most common routes of transmission in recent U.S. cases 

include: 

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racing career. Former Quarter Horse racehorses should also be 

tested as part of prepurchase examinations. 

Clinical Signs

In most cases, an EP-infected horse can take 10 to 30 days to 

show signs of the disease. Acute cases of EP may be mild or 

more severe. Mild forms of the disease cause horses to appear 

weak and show lack of appetite. More severe clinical signs 

include fever, anemia, yellowish mucous membranes, a swollen 

abdomen, and/or labored breathing. Other signs of EP can 

include central nervous system disturbances, roughened hair 

coats, constipation, colic, and hemoglobinuria (a condition 

which gives urine a red color). In some cases, death may occur. 

Yet, the disease may not affect all horses equally. Some may 

show few or no clinical signs in the acute phase and no obvious 

decrease in performance. Horses in the chronic phase of 

infection usually show no clinical signs other than anemia, or 

they may exhibit weight loss, poor performance, and loss 

of condition.

Diagnosis

Testing at a USDA-approved laboratory can diagnose EP. Some 

U.S. equine events or racetracks may require a negative EP test 

before allowing horses to compete. 

Veterinarians: If you suspect EP, contact State or Federal animal 

health officials before you collect any samples. 

Treatment

Historically, EP-infected animals would be euthanized, exported 

from the United States, or quarantined for the rest of their lives. 

Today, USDA offers a treatment program for EP-infected horses. 

Imidocarb dipropionate is the drug used to treat EP infection. 

An accredited veterinarian performs the treatment, with 

oversight from State and Federal animal health officials. 

This program has been successful in clearing most horses 

of infection over time. Treated horses are released from 

quarantine once they test negative for EP. 

Prevention

There is no vaccine for EP. To prevent infections, avoid (1) 

bites by infected ticks, (2) contact with blood-contaminated 

equipment, and (3) administration of contaminated blood 

products. 

For More Information

Call USDA Veterinary Services at (970) 494-7391 or send an 

email to equine.health@usda.gov. You can also visit us online 

at www.aphis.usda.gov/animal-health. 

• Reuse of needles, syringes, or intravenous administration 

tubing between horses

• Blood-contamination of multidose drug vials

• Administration of contaminated blood or plasma products

• Direct blood transfusion between horses 

Rarely, in utero transmission from mare to foal has also been 

reported.

History

The United States identified natural tick-borne transmission 

of EP in Florida in the early 1960s, primarily due to B. caballi.

In response, we began a State-Federal EP control program in 

Florida to eradicate the disease and tropical horse ticks. The 

program used quarantine and drug treatment for infected 

equines, tick treatment for infected and exposed animals and 

premises, and movement controls to prevent EP spread. As a 

result of this lengthy eradication campaign, the United States 

was declared EP-free in 1988.

Another outbreak of tick-transmitted EP was not identified in 

the United States until 2009. Over 400 horses from a large ranch 

in south Texas were found to be infected with T. equi. Ticks 

linked to the outbreak included cayenne tick (A. [cajennense] 

mixtum) and American dog tick (D. variabilis). The Texas ranch 

had another site in Brazil, where EP was endemic. Movement 

of horses between these two ranches was likely the source of 

the outbreak. In cooperation with State, federal, and academic 

partners and ranch personnel, we eradicated T. equi from the 

Texas ranch by quarantining the premises long-term, culling 

infected horses or treating them with imidocarb dipropionate, 

and mitigating tick vectors with acaricide. 

Epidemiology

In recent years, EP transmission in the United States has 

been from:

• Illegal imports of infected horses or blood products from 

countries where the disease is endemic. 

• Unhygienic practices that transfer blood from an infected 

horse to uninfected horses. 

This has caused clusters of infection in epidemiologically 

related populations, particularly Quarter Horses involved in 

unsanctioned horse racing. 

We can avoid these modes of transmission by: 

1. Testing horses for EP, as required during legal import, or 

before their use as blood donors. 

2. Sanitizing dental and surgical equipment between horses, 

and never reusing needles and syringes. 

Current Quarter Horse racehorses are at increased risk for EP. 

Owners should routinely test these horses for EP and equine 

infectious anemia (another blood-borne disease) during their