A Clinical and Epidemiological Investigation of the First Reported Human Infection With the Zoonotic Parasite Trypanosoma evansi in Southeast Asia

Southeast Asia is a hotspot for emerging and remerging infectious diseases [1]. Over the last decade, Vietnam in particular has been an epicenter for such infections [2]. The country has a human population of >90 million, some of the highest densities of farmed livestock in Asia, and a substantial burden of infectious diseases, many of which occur without an etiological diagnosis [3]. More than half of Vietnam's population resides in rural areas, and most participate in small-scale animal production, which likely facilitates the transfer of pathogens from animals into humans (zoonotic transmission). Therefore, we predict that zoonotic infections occur frequently in rural locations but do not trigger onward human-to-human transmission and often fail to be diagnosed [4]. Furthermore, because of limited diagnostic capacity in provincial animal health facilities, conclusive epidemiological investigations of zoonotic infections in potential animal sources are expensive, challenging, and rarely performed. Trypanosoma is a genus of unicellular parasitic flagellate protozoa within the class Kinetoplastida. The genus has several members, the majority of which require transmission between different hosts to complete their life cycle. Trypanosoma infections are responsible for a large burden of infectious disease in humans globally [5]. The major zoonotic parasites, Trypanosoma brucei species (T. brucei subspecies gambiense and T. brucei subspecies rhodesiense) and Trypanosoma cruzi cause a substantial burden of sleeping sickness and Chagas disease in sub-Saharan Africa and Latin America, respectively. Other Trypanosoma species, such as T. lewisi, T. brucei subspecies brucei, T. congolense, and T. evansi, can also cause disease in humans, but are rare [6]. The majority of these atypical human cases of Trypanosoma infections are due to T. lewisi and T. evansi. Trypanosoma lewisi is a ubiquitous, nonpathogenic parasite of rats, and is considered to be a human pathogen, although there have been only a limited number of reported cases [7]. Trypanosoma evansi is associated with acute disease in camels and horses (surra) and chronic disease in cattle and buffalo, and can be found in South America, North Africa, the Middle East, and South and Southeast Asia [8]. The only human case of T. evansi with molecular confirmation of the infecting species occurred in India in 2005 [9]. This patient had a deficiency of apolipoprotein L1 (APOL1) [10], a component of human serum with trypanocidal activity [11]. Four other probable cases have been reported worldwide [8], 3 in India and 1 in Egypt, but molecular parasite speciation was not performed. Here we describe a clinical, parasitological, and epidemiological investigation of the first confirmed human infection of T. evansi in a previously healthy individual without APOL1 deficiency in Asia.