Equine piroplasmosis results from infection by the protozoa Babesia caballi or
Theileria equi (formerly Babesia equi). Both organisms belong to the phylum
Apicomplexa and order Piroplasmida. They can infect an animal concurrently.
Other related protozoa such as Babesia bovis (the organism that causes bovine
babesiosis) have been reported rarely in horses.