In addition, a third subspecies of the parasite known as Trypanosoma brucei brucei is responsible for affecting animals but not humans.Humans are the main reservoir for T. b. gambiense but this species can also be found in pigs and other animals. Wild game animals and cattle are the main reservoir of T. b. rhodesiense. These parasites primarily infect individuals in sub-Saharan Africa because that is where the vector (tsetse fly) is located. The two human forms of the disease also vary greatly in intensity. T. b. gambiense causes a chronic condition that can remain in a passive phase for months or years before symptoms emerge and the infection can last about three years before death occurs.T. b. rhodesiense is the acute form of the disease, and death can occur within months since the symptoms emerge within weeks and it is more virulent and faster developing than T. b. gambiense. Furthermore, trypanosomes are surrounded by a coat that is composed of variant surface glycoproteins (VSG). These proteins act to protect the parasite from any lytic factors that are present in human plasma. The hosts immune system recognizes the glycoproteins present on the coat of the parasite leading to the production of different antibodies (IgM and IgG).These antibodies will then act to destroy the parasites that circulate around the blood. However, from the several parasites present in the plasma, a small number of them will experience changes in their surface coats resulting in the formation of new VSGs.
Links to pictures of Sleeping Sickness (Hardin MD/ University of Iowa)
Hale Carpenter G (1920). A Naturalist on Lake Victoria, with an Account of Sleeping Sickness and the Tse-tse Fly. Unwin. OCLC 2649363.