When the infected tsetse fly bites humans, it transcutaneously injects the metacyclic tripomastigotes present in its salivary glands. Once in the host, the parasites enter the lymphatic system and the blood where they turn into circulating tripomastigotes and begin an incessant reproduction activity by binary scission (hemolymphatic stage).
Parasitemia is periodic: moments where it is elevated alternate with others in which it is abruptly reduced due to the production of specific antibodies by the infected organism. A group of parasites, however, by modifying the surface glycoproteins, is able to evade the antibody response. Therefore, the multiplication and lysis cycle is repeated.
Later in the course of the infection, trypanosomes invade the lymph nodes, spleen, liver and many other organs. The death can occur a few days after the involvement of the central nervous system ( meningo-encephalitic stage ).
The cycle is maintained when a tsetse fly ingests the blood of an infected man or animal. In the vector, the protozoan multiplies in the intestine, before migrating into the insect's salivary glands. The tse-tse fly can transmit the infection to another mammal or to humans with a new sting, if at least 20-30 days have passed since it became infected.
