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2021 Equine Piroplasmosis (EP) 

Annual Report

Background

Equine piroplasmosis (EP) is a foreign animal disease in the United States and is immediately 

reportable to state and federal animal health officials. The disease can be contracted by any equid 

(horses, donkeys, mules, or zebra) and is caused by infection with blood-borne parasites, either 

Theileria equi or Babesia caballi. While natural tick-borne transmission of the disease is not 

currently present on the U.S. mainland, sporadic cases of iatrogenic transmission (human-caused 

by medical procedures) in high-risk populations, such as Quarter Horse racehorses, are being 

documented. 

Unhygienic practices being used by horse owners and trainers in this high-risk population are 

contributing to disease spread and include the re-use of needles, syringes, and intravenous 

administration sets between horses, administration of illegal blood products from other countries, 

direct blood transfusion between horses to increase athletic performance (blood doping), and 

administration of multi-dose drug products that have become blood-contaminated by non-sterile 

handling techniques between horses. In addition to transmitting EP between horses, these 

practices are concurrently transmitting other blood-borne equine diseases, such as equine 

infectious anemia (EIA). A source of continued EP incursion into the U.S. is the illegal 

movement of horses from EP-endemic regions, such as Mexico. 

Active surveillance for EP occurs in the U.S. through testing to enter sanctioned racetracks, 

export, interstate movement, and diagnostic testing. Traceback and cohort testing on all exposed 

horses is conducted in response to any EP-positive case found. Horses confirmed as EP-positive 

have the following disposition options: lifetime quarantine, euthanasia, export from the country, 

or long-term quarantine with enrollment in the EP-treatment program. Horses enrolled in the 

treatment program must be confirmed as permanently cleared of the EP pathogen and test EPnegative on all available diagnostic tests to be released from quarantine. 

Summary of 2021 Testing and EP-Positive Cases

• There were 35,493 domestic U.S. horses tested for EP from January to December 2021 and a

total of 36 new T. equi-infected horses identified in 7 states. Thirty-one (31) of these horses 

were Quarter Horse (QH) racehorses with iatrogenic transmission involved in the 

epidemiology of the infection and 5 horses (2 Andalusians, 3 QH saddle horses) were 

suspected or confirmed to have been illegally moved from Mexico. Fifteen (15) of the 36

EP-positive horses were found to be dual infected with equine infectious anemia (EIA),

Animal and 

Plant Health 

Inspection 

Service

Veterinary 

Services
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which was likely transmitted by iatrogenic infection in the QH racehorses (12) and by natural 

infection in Mexico for the horses illegally moved from Mexico (3). 

2021 EP Cases by State: 36 EP-infected horses found in 7 states (Jan-Dec 2021)

State Found # T. equipositive# Dual infected 

with EIA

Risk Group

Florida 1 0 Andalusian originally from Spain,

illegally moved from Mexico

Georgia 1 0 QH racehorse

Iowa 2 0 QH racehorses

Louisiana 2 1 QH racehorses

Oklahoma 1 0 QH racehorse

Tennessee 8 6 QH racehorses

Texas 21 8 17 QH racehorses in several 

clusters; 4 horses (1 Andalusian, 3 

QH saddle horses) illegally moved 

from Mexico

Total 36 15 31 QH racehorses; 5 horses illegally 

moved from Mexico

• For questions regarding this report, please contact:

Angela Pelzel-McCluskey, DVM, MS

Equine Epidemiologist

USDA-APHIS-Veterinary Services 

(970) 494-7391

Angela.M.Pelzel-McCluskey@usda.gov