Colonization of the rectum of Triatoma infestans by Trypanosoma cruzi studied by scanning electron microscopy: influence of blood uptake by the bug.

The colonization of the different regions of the rectum of Triatoma infestans by Trypanosoma cruzi was studied in unfed larvae, during and after feeding of the bug by scanning electron microscopy. The rectal pads always possessed the highest population densities, but in some bugs the main rectal sac and the region around the anus were also covered by a "carpet" of flagellates. Such high densities were never observed at the midgut/rectal junction. A slight decrease in this region might be caused by blood ingestion and the resulting excretion of urine. However, the flagellates in this region cannot be responsible for a phenomenon described by other authors, namely that the percentage of metacyclics is low in the first drop of faeces and increases in the following drops of deposited urine. Our observations indicate that metacyclics lying on the "carpet" of flagellates in bugs before and right after the start of feeding might later become loosened. In all dissections, numerous metacyclics were attached to the rectal wall. Further studies are necessary to clarify the exact origin of metacyclics in the urine.