Contamination through the Feces

Contamination through the bugs’ feces is the major mechanism by which T. cruzi passes from vinchucas to humans. T. cruzi transforms from a trypomastigote to an epimastigote to a metacyclic trypomastigote inside vinchucas (see Figure 17). When infected vinchucas take a blood meal, they usually defecate and deposit a metacyclic trypomastigote, which is found in the insect’s intestines, onto the victim’s skin. Metacyclic trypanosomes (tryps) travel from the feces and enter through the bite site, the eye, or other abraded skin. People often scratch the wound and rub parasites into the skin. Metacyclic tryps circulate for a short while in the blood, then enter neuron cells and become encysted as amastigotes. Amastigotes reproduce tryps that enter the blood, where they can be picked up by a vinchuca during its blood meal. (Because T. cruzi is such an infectious parasite, some laboratories in the United States do not experiment with it).

Not all species defecate at the same rate: Rhodniusprolixus and Panstrongylus megistus defecate at a great rate, while Triatoma infestans defecates at only 30 percent of that rate, and Triatoma sordida at only 10 percent (Calvo et al. 1978:70‑81). Those species that defecate closest to the injection wound are the most infectious; consequently, T. infestans is most dangerous to humans and most adaptive for T. cruzi, being a very effective vector in transmitting this parasite to hosts. The longer T. cruzi is outside the vector and host, the more vulnerable it is to being lysed, or disintegrating. Children are easily contaminated with the feces of vinchucas. Babies and infants spend considerable time within and around the house, sometimes with little clothing, and they are defenseless victims for vinchucas to feed upon.