Piroplasmosis Testing

Piroplasmosis is an infectious, tick-borne disease caused by one of two parasites, Babesia equi or Babesia caballi, which attack and destroy red blood cells in horses. The mortality rate can be as high as 20% among susceptible animals. Recovered horses become chronic carriers without clinical signs. The only treatment (a type of chemotherapy) is not always succesful, and it can have serious side effects in some horses.

Piroplasmosis is found in nearly every country in the world except the United States, Canada, Australia, England, Ireland, Japan, and Iceland. It is estimated that only 10% of the world's horse population is naïve (has not been exposed to or developed antibodies to one or both parasites). Therefore, it is crucial to the naïve U.S. horse population to screen horses from around the world before allowing them into the United States.

A new blood test for piroplasmosis was unveiled by the USDA in early 2004. The competitive enzyme-linked immunosorbent assay (CELISA) was described as more sensitive that the complement fixation test (CFT), which has been the screening test for piroplasmosis for many years. (There is one other test for piroplasmosis, an indirect immunofluorescent antibody test, but it is complicated and does not lend itself to testing large numbers of samples.)

The CFT does give a percentage of false negatives; a carrier horse might test negative even when it has antibodies in the blood. Therefore, the concern is that the United States has allowed (or continues to allow) carrier horses to enter the country and become focal points for spreading the disease. The CELISA, which detects a different class of antibody to the CFT, was said to have a greater chance of identifying chronically infected horses. Furthermore, it is more sensitive than the CFT and can pick up smaller amounts of antibody that the horse produces to fight off the parasite.

There are means by which carrier horses can be treated and, for a short time, become seronegative on the CFT. This is doubly scary since piroplasmosis can also be spread by contaminated needles. Also, several leading virologists have suggested that piroplasmosis can be passed in blood-contaminated semen from chronically infected stallions that might show no outward sign of disease.

The new CELISA can be used to test for antibodies to the two different parasites that cause piroplasmosis. The USDA's National Veterinary Services Laboratory (NVSL) spent two years validating the CELISA, which was developed by the USDA Agriculture Research Service. The test was also evaluated by several countries prior to being introduced in the United States. The OIE (Organization Internationale des Epizooties) has approved the CELISA as an alternative prescribed test for international trade.

Once the CELISA was taken out of the experimental testing phase and used in routine testing for piroplasmosis, problems showed up. Horses testing negative prior to importation were testing positive upon post-entry screening in the United States. So, in November 2004, the USDA discontinued using the CELISA and went back to the CFT.

What happened that resulted in this unprecedented action by USDA/ APHIS? No one seems willing to acknowledge what went awry. The American Horse Council has written a letter to NVSL asking for an explanation. It is in the best interest of the U.S. horse population to have a more sensitive test for piroplasmosis; one was developed and adequately validated. Yet it now has been discontinued, apparently because of technical problems that occurred once USDA/APHIS NVSL began producing the test reagents on a large scale and using the test on a routine basis.

This has caused confusion around the horse world, prompting Tattersalls sales company in England to issue a notice to consignors that described the discrepancies when horses were pre-tested negative for export, then tested positive upon import to the United States and turned away.

Currently, testing for piroplasmosis is back to the old CFT, so owners should not have problems importing horses. However, there is no guarantee that some horses will not give a false negative reaction for piroplasmosis (and could have the parasite).

Horse owners with questions or comments about piroplasmosis testing can contact Jay Hickey at the American Horse Council (AHC), 1616 H Street NW, 7th Floor, Washington, DC 20006; 202/296-4031; fax 202/296-1970; ahc@horsecouncil.org.


  • An infectious, tick-borne disease caused by two protozoal parasites (Babesia caballi and B. equi).
  • These parasites attack the red blood cells.
  • It is characterized by fever, anemia, weight loss, jaundice, and in some cases, death.
  • Case fatality rate can be up to 20% in naive horses.
  • The only treatment is a type of chemotherapy that is only effective on B. caballi and can have serious side effects in some horses.
  • The disease is spread through ticks, contaminated needles, and possibly through blood-contaminated semen of infected stallions.
  • Some tick species in the United States can spread the disease, and a few species can pass the parasite transovarially (from mother to offspring).
  • Ticks can live for two years without feeding, and they pass the parasite during feeding.
  • The United States has screened horses for piroplasmosis for nearly 30 years.


Information for this column was supplied by AAEP members Peter Timoney, FRCVS, PhD, of the Gluck Equine Research Center in Kentucky; and Richard Mitchell, DVM, of Fairfield Equine Associates in Connecticut, who is chairman of the AHC Health and Regulatory Committee.

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