Trypanosomes are principally responsible for two human diseases: human African trypanosomiasis (HAT) or sleeping sickness (caused by Trypanosoma brucei gambiense and T. b. rhodesiense), and Chagas disease, also called South American trypanosomiasis (T. cruzi). However, some trypanosomes that are natural parasites only of animals can sometimes infect humans and cause the so-called "atypical human trypanosomiases" (aHT). T. evansi, the agent causing surra in camels, horses, dogs, and bovines, and T. lewisi, a cosmopolite rat parasite, are the most frequently involved. These atypical infections involve no or only minor symptoms, but major symptoms are sometimes present. Parasite elimination is generally spontaneous, but can require treatment. Molecular tools, such as polymerase chain reaction, have improved the accuracy of parasite identification. Immunological techniques, mainly immunoenzymatic assays, can detect asymptomatic subjects. Several causes, most often concomitant, have been hypothesized, including immune immaturity, immunodeficiency, and close contact with infected animals. Innate immunity to animal trypanosomes depends on a trypanolytic factor called apolipoprotein L-I, present in human serum. A deficit in both apolipoprotein L-I alleles has been reported in an Indian patient infected by T. evansi. The prevalence of aHT is probably underestimated. Moreover, these trypanosomes might become potential emerging zoonotic pathogens, due to their ability to invade new hosts. An international network has been set up to survey these aHT (NAHIAT: Network on Atypical Human Infections by Animal Trypanosomes).
Keywords: Trypanosoma evansi; Trypanosoma lewisi; atypical trypanosomiasis; trypanosome; trypanosomiasis.