Atypical human infections by animal trypanosomes

Abstract

The two classical forms of human trypanosomoses are sleeping sickness due to Trypanosoma brucei gambiense or T. brucei rhodesiense, and Chagas disease due to T. cruzi. However, a number of atypical human infections caused by other T. species (or sub-species) have been reported, namely due to T. brucei brucei, T. vivax, T. congolense, T. evansi, T. lewisi, and T. lewisi-like. These cases are reviewed here. Some infections were transient in nature, while others required treatments that were successful in most cases, although two cases were fatal. A recent case of infection due to T. evansi was related to a lack of apolipoprotein L-I, but T. lewisi infections were not related to immunosuppression or specific human genetic profiles. Out of 19 patients, eight were confirmed between 1974 and 2010, thanks to improved molecular techniques. However, the number of cases of atypical human trypanosomoses might be underestimated. Thus, improvement, evaluation of new diagnostic tests, and field investigations are required for detection and confirmation of these atypical cases.

Figure 1. T. evansi on a Giemsa-stained blood smear from an experimentally infected rat (credit, Marc Desquesnes).

Size of the parasite (15–34 µm), small (diameter 0.6–0.7 µm) and subterminal kinetoplast, sharp posterior end, central nucleus, large undulating membrane, and free flagellum are the most striking features of the slender forms of the sub-genus Trypanozoon.

Figure 2. T. lewisi (adult form) on a Giemsa-stained blood smear from an experimentally infected rat (credit, Marc Desquesnes).

Size of the parasite (21–36 µm), kinetoplast rode shaped, large (1×0.7 µm) and subterminal, very sharp posterior end, posterior nucleus, large undulating membrane, “C shape” of the parasite, and free flagellum are the most striking features of the adult forms of the sub-genus Herpetosoma, to which belong T. lewisi-like parasites.

Figure 3. T. lewisi and T. evansi on a Giemsa-stained blood smear from a rat submitted to a mixed experimental infection (credit, Marc Desquesnes).

Smeared and stained in the same conditions, the parasites exhibit here their striking characteristics: the size of the kinetoplast (large in T. lewisi and small in T. evansi) is the most reliable criteria to distinguish these sub-genera; however, other characteristics can be observed, such as: the size of the parasite (T. lewisi>T. evansi), the nucleus position (central for T evansi, posterior for T. lewisi), the undulating membrane development (exhibiting regular waves in T. evansi and irregular ones in T. lewisi). Finally, an unusual feature can be observed on one specimen of T. lewisi with a very long posterior extremity, this morphotype use to be called “longicaudale” .