Origin and distribution of the New World trypanosomes existing in livestock is briefly presented. Non pathogenic trypanosomes are T. theileri found in cattle and buffaloes, T. ingens found in cattle, and T. melophagium found in sheep in highlands. Only the Megabypanum species of cattle and buffaloes can slightly interfer in the parasitological diagnosis. T. vivax is pathogenic in cattle, sheep, goats and buffaloes; although it was found in deers, the existence of a wild reservoir as never been demonstrated. Whether the South American T. vivax can infect dogs, cats and horses still needs to be determined. T. evansi is pathogenic for horses, buffaloes, dogs, cats and part of the wild fauna including vampire bats which act as reservoir and vectors. It can also be found in cattle, sheep, goats and pigs; T. evansi has a large domestic and wild reservoir and is robustly implanted in Latin America. T. equiperdum is restricted to horses ans donkeys due to its sexual transmission, it is sporadic in Latin America. T. cruzi, the agent of Chagas disease in humans, is also pathogenic in part of the wild fauna; it can be found in all domestic and wild mammalian hosts. The study of trypanosomiasis, mostly based on serological surveys, encounters problems of specificity. Problems of sub-genus or species-specificity depend on the Trypanosoma species present in the area and the hosts under investigation. Infections with the four pathogenic Trypanosoma species described cannot be properly distinguished with antibody-detection; interferences should be carefully taken into consideration for sero-epidemiological surveys.