Causal Agents

“African trypanosomes” or “Old World trypanosomes” are protozoan hemoflagellates of the genus Trypanosoma, in the subgenus Trypanozoon. Two subspecies that are morphologically indistinguishable cause distinct disease patterns in humans: T. b. gambiense, causing chronic African trypanosomiasis (“West African sleeping sickness”) and T. b. rhodesiense, causing acute African trypanosomiasis (“East African sleeping sickness”). The third subspecies T. b. brucei is a parasite primarily of cattle and occasionally other animals, and under normal conditions does not infect humans.

Life Cycle

During a blood meal on the mammalian host, an infected tsetse fly (genus Glossina) injects metacyclic trypomastigotes into skin tissue. The parasites enter the lymphatic system and pass into the bloodstream image . Inside the host, they transform into bloodstream trypomastigotes image , are carried to other sites throughout the body, reach other body fluids (e.g., lymph, spinal fluid), and continue the replication by binary fission image . The entire life cycle of African trypanosomes is represented by extracellular stages. The tsetse fly becomes infected with bloodstream trypomastigotes when taking a blood meal on an infected mammalian host image , image . In the fly’s midgut, the parasites transform into procyclic trypomastigotes, multiply by binary fission image , leave the midgut, and transform into epimastigotes image . The epimastigotes reach the fly’s salivary glands and continue multiplication by binary fission image . The cycle in the fly takes approximately 3 weeks. Rarely, T. b. gambiense may be acquired congenitally if the mother is infected during pregnancy.


Life cycle image and information courtesy of DPDx.

Page last reviewed: April 29, 2019