A 7-year-old Quarter Horse gelding in Jackson County, Mo., is positive for equine piroplasmosis, according to a report issued to the World Organization for Animal Health (Office International des Epizooties, or OIE) by John Clifford, DVM, deputy administrator of the USDA’s Animal and Plant Health Inspection Service. See the OIE report.  

According to the report, on June 2 the horse was presented to a veterinary hospital in Kansas for acute illness consistent with infection of a blood-borne pathogen. The horse was placed under quarantine and isolation. On suspicion of piroplasmosis, a foreign animal disease investigation was initiated June 3.



Equine Piroplasmosis Facts




  • An infectious, tick-borne disease caused by two types of protozoal parasites, which attack the red blood cells.
  • 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 potent type of chemotherapy that can have serious side effects in some horses.
  • Disease is spread by ticks, the use of contaminated needles, and possibly through blood-contaminated semen of infected stallions.
  • Can be spread by some tick species in the United States, and a few species can pass the parasite transovarially (from mother to offspring).
  • Infected ticks can live for two years without feeding. They pass the parasite while feeding.
  • The United States has screened all imported horses for piroplasmosis for nearly 30 years.

The Missouri Department of Agriculture placed the premises where the horse had resided, a facility with 63 other equids, under quarantine on June 6. The National Veterinary Services Laboratory confirmed piroplasmosis (Theileria equi) in the affected horse on June 10.

Results of tests on the other equids on the property are pending. Of these, 44 animals are stabled, 18 are on a common pasture, and one is on an isolated pasture.

The report noted that the affected horse was purchased six months ago. The origin of this infection was listed as unknown or inconclusive.

Florida’s Department of Agriculture and Consumer Services in February wrapped up on investigation into equine piroplasmosis in that state. During the investigation, officials quarantined 25 premises and tested more than 200 horses for the disease. In all, 20 horses on seven premises were euthanized after they were found to be infected with the organism that causes equine piroplasmosis.

Evidence uncovered during that investigation indicated that transmission of the piroplasmosis organism occurred due to management practices and not by natural transmission, which occurs via ticks.

Equine piroplasmosis was eradicated officially from Florida (and, thereby, the United States) in 1988, so one of the big questions when the Florida cases popped up was that of its source. Investigators determined the positive horses were closely epidemiologically linked to two horses that entered Florida from Mexico

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