Hepatozoonosis is caused by apicomplexan haemoparasites of the genus Hepatozoon, which are closely related to Plasmodium spp. and piroplasms. Recent research revealed that two tick-borne Hepatozoon spp. infect dogs and cause distinct syndromes. Comparisons of these related species illustrates that whereas Hepatozoon canis appears to be well adapted to its canine host, Hepatozoon americanum, an emerging pathogen producing severe and frequently fatal myositis, is highly virulent and might have recently crossed the species barrier from a wild host.