Trypanosoma evansi, the causative agent of 'surra', is a flagellated hemoprotozoan parasite. It kills thousands of animals every year and causes significant animal morbidity and loss of productivity. T. evansi originated from Trypanosoma brucei through deletion of the maxicircle kinetoplast DNA which conferred the capacity for mechanical transmission by flies and allowed T. evansi to expand beyond the tsetse belt. Presently, it is the most widely distributed pathogenic trypanosome in Africa, Asia, and Latin America, but its potential for geographical extension is not limited, as shown by recent sporadic cases in Spain and France. The most susceptible animals are horses, dogs, camels, and buffaloes, but other domestic livestock and wild mammals (including Australian marsupials, experimentally) can also be infected. Test-and-treatment of infected hosts and mass treatment programs are recommended in high-prevalence settings. International trading of potential carriers is a risk.