Trypanosoma gambiense and Trypanosoma rhodesiense
These two species cause African sleeping sickness in man. T. rhodesiense is thought to occur also in antelopes (Hoare, 1955) and was isolated once from a bushbuck (Heisch, McMahon and Manson-Bahr, 1958). T. gambiense does not occur in wild game. Neither occurs in domestic animals. They are morphologically indisguishable from each other and from T. brucei, and for this reason some people prefer to consider all three as subspecies of T. brucei. However, the biological and epidemiologic differences between them make it more convenient to retain separate names. Whatever the names used, it is clear that these species arose from strains of T. brucei which became adapted to man.
Human trypanosomosis occurs in tropical Africa, roughly between 15° N and 15° S latitude. T. rhodesiense, which causes an acute form of the disease, occurs in Rhodesia, Tanganyika, Nyasaland, Bechuanaland and Portuguese East Africa, while T. gambiense, which causes a chronic form of the disease, occurs in a large part of the remainder of the area. Kunert (1953) prepared a map of the distribution of human sleeping sickness in Africa together with climatologic and other information. Ashcroft (1959a) and Morris (1960) reviewed its epidemiology.
In general, T. gambiense causes a "domesticated" type of disease, transmitted by tsetse flies from man to man in regions of human habitation, while T. rhodesiense causes more of a woodland disease and people become infected with it away from their village areas.
Altho it is certain that some wild animals must serve as reservoirs of Trypanosoma rhodesiense, it has been isolated from them only once. Heisch, McMahon and Manson-Bahr (1958) isolated it from a bushbuck (Tragelaphus scriptus) in Kenya by inoculation of a human volunteer.
Epidemiologic evidence for a wild animal reservoir is exemplified by the observation that every year fishermen and honey hunters become infected with T. rhodesiense near the Ugalla River in the Western Province of Tanganyika, yet this is an uninhabited region, and no people are there at all during the 6-month rainy season. The Ugalla River is part of the Malagarasi river system of the Western Province. It runs thru a sparsely populated, woodland region inhabited by many wild animals and infested with Glossina morsitans. Jackson (1955) described 25 cases of sleeping sickness in fly-boys stationed in remote outposts in this area between 1935 and 1939, and concluded
that there was strong evidence that game was acting as a reservoir. Over half the cases of T. rhodesiense infection diagnosed in Africa in 1953, 1954 and 1955 were contracted in this region (Ashcroft, 1958); 2069 cases were reported in the Western Province in these years (Apted, 1955).
The only way to be positive that a brucei-like strain of trypanosome isolated from wild animals is actually T. rhodesiense is to inoculate human volunteers with it, and very few such attempts have been made. In one of the latest of these, Ashcroft (1958) inoculated a strain which he had isolated from a Coke’s hartebeest (Alcelaphus cokei) in Tanganyika into 2 African volunteers, but no infection resulted and he concluded that the organism was T. brucei.
The life cycles of the human trypanosomes are the same as that of T. brucei. The vectors are species of tsetse flies of the genus Glossina. The chief vectors of T. gambiense are the riverine tsetse flies, G. palpalis and G. tachinoides, while those of T. rhodesiense are the game tsetse flies, G. morsitans, G. swynnertoni and G. pallidipes.
Human trypanosomosis is similar to nagana in its manifestations. For further information, any human parasitology text may be consulted.