Equine piroplasmosis is a disease caused by one of two parasites (Babesia caballi or Babesia equi).  Babesia equi is the more pathogenic of the two parasites.  They are found in South and Central America, Europe, Africa, Asia and the Middle East.  Areas that are not considered to be endemic include the United States, Canada, Australia, Japan, England and Ireland.  The parasite lives within the red blood cells and is transmitted between horses by ticks that live in warm weather climates.


  • After infection, it can take between 1 and 3 weeks before signs become apparent

  • Infection can be mild or acute
  • Clinical signs are secondary to damage to the red cells
  • Some horses are infected and show no signs
  • Infected horses become dull and lethargic with a fever
  • Mucous membranes may be pale or yellow
  • The abdomen can be distended and the horse may show signs of colic
  • The horse may have an increased respiratory rate


While transmission is mainly by infected ticks (Dermacentor variabilis and Rhipicephalus (Boophilus) microplus), the disease has been spread mechanically from animal to animal through contaminated needles.


Several laboratory tests are available for diagnosis of piroplasmosis.  The most common tests are blood tests that look at antibodies to the organism. The U.S. Department of Agriculture (USDA) standard test is the cELISA (competitive enzyme-linked immunosorbent assay). Specific laboratories have been identified to run the tests and report the results.

Occasionally, the parasite can be seen on microscopic examination of a blood smear.


Horses that test positive for equine piroplasmosis MUST be quarantined.  Local veterinarians can work with state and federal veterinarians to ensure that manageable quarantine guidelines are being followed and are in place.

Although there are some drugs that have been identified for treatment of piroplasmosis, the organisms can be difficult to treat, and it is often difficult to completely clear the organism even after apparent infection has resolved.  In addition to quarantine, there is a treatment research program available for positive horses at Washington State University.  If you have a positive case, you can contact Washington State to determine if your horse qualifies for the treatment program. 


Acute cases that survive the initial infection may become carriers for life.  By working with your veterinarian and local and federal USDA agents, quarantine and treatment protocols can be designed for each individual horse. Recently the USDA has successfully cleared several positive horses of the disease. These horses are no longer infected and do not show positives on any of the available tests and have been released from quarantine.