“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.
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
. Inside the host, they transform into bloodstream trypomastigotes
, are carried to other sites throughout the body, reach other body fluids (e.g., lymph, spinal fluid), and continue the replication by binary fission
. 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
. In the fly’s midgut, the parasites transform into procyclic trypomastigotes, multiply by binary fission
, leave the midgut, and transform into epimastigotes
. The epimastigotes reach the fly’s salivary glands and continue multiplication by binary fission
. 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.