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Canine hepatozoonosis, an emerging tick-transmitted protozoal disease of dogs in the United States, was first recognized in Texas in 1978. Since 1978, the geographic distribution of the disease has expanded to include Louisiana, Alabama, Georgia, and Oklahoma. The expansion of hepatozoonosis maybe associated with spread of the tick vector, Amblyomma maculatumThe causative agent of North American hepatozoonosis was initially thought to be a particularly virulent strain of Hepatozoon canis,the parasite that causes hepatozoonosis in the Old World. However, because of notable differences between the disease and the morphologic features of asexual and sexual stages of the parasites in the USA and the Old World, the organism responsible for canine hepatozoonosis in North America was designated a new species, Hepatozoon americanumThe clinical course of canine hepatozoonosis in the USA is chronic, progressive, and generally fatal. The disease manifests with intermittent fever, gait abnormalities, marked lethargy, weight loss, muscle wasting, and pain.