Investigations into the occurrence of equine piroplasmosis, caused by Theileria (babesia) equi, continue and as of June 10 a total of 457 horses have been found positive to T. equi in the United States.

Data indicates three sources of introduction and/or associated spread. Veterinarians believe 410 positives are epidemiologically linked to an outbreak that has apparently spanned many years in Southwest Texas. These 410 positive horses have been discovered in no fewer than 16 states.

The remaining 47 positives were discovered when testing to meet various regulatory testing requirements imposed for interstate movement and/or exhibition requirements. In each of these cases, epidemiological information supports the source of infection was either undetected at time of importation or iatrogenic spread (in this case via blood doping) associated with nonsanctioned Quarter Horse racing