Centers for Disease Control/Division of Parasitic Diseases and Malaria
EOL staff
Life cycle of Trichomonas vaginalis, the cause of trichomoniasis in humansTrichomonas vaginalis resides in the female lower genital tract and the male urethra and prostate (1), where it replicates by binary fission (2). The parasite does not appear to have a cyst form, and does not survive well in the external environment. Trichomonas vaginalis is transmitted among humans, its only known host, primarily by sexual intercourse (3).From Centers for Disease Control Parasites and Health website
Centers for Disease Control/Division of Parasitic Diseases and Malaria
EOL staff
Life cycle of Chilomastix mesnili The resistant cyst stage in the life cycle of Chilomastix is responsible for transmission. Both cysts and trophozoites can be found in the feces (diagnostic stages) (1). Infection occurs by the ingestion of cysts in contaminated water or food or by the fecal-oral route (via hands or fomites, i.e., inanimate objects such as towels that transmit infectious organisms to a host) (2). In the large (and possibly small) intestine, excystation releases trophozoites.From Centers for Disease Control Parasites and Health website.
Centers for Disease Control/Division of Parasitic Diseases and Malaria
EOL staff
Life cycle of Enteromonas hominisBoth cysts (dormant stage) and trophozoites (active stage) of Enteromonas hominis are shed in feces. Infection occurs after the ingestion of cysts in fecal-contaminated food or water, or on fomites (inanimate objects or substances capable of transferring pathogens). In the large (and possibly small) intestine, excystation releases trophozoites. Enteromonas hominis resides in the large intestine, where it is regarded as a commensal (benefiting from its host but doing no harm) and is not known to cause disease.FromCenters for Disease Control Parasites and Health website.
Centers for Disease Control/Division of Parasitic Diseases and Malaria
EOL staff
Life cycle of the flagellate Pentatrichomonas hominisPentatrichomonas hominis is a trichomonad flagellate with a worldwide distribution. Only trophozoites are shed in feces (1) as there is no known cyst stage for this species. Infection occurs after the ingestion of trophozoites in fecally-contaminated food or water or on fomites (i.e., other non-living objects or substances that can transmit them) (2). These organisms reside in the large intestine, where they are regarded as commensals (i.e., benefiting from but not harming their host) and are not known to cause disease in humans.From Centers for Disease Control Parasites and Health website
Centers for Disease Control/Division of Parasitic Diseases and Malaria
EOL staff
Life cycle of Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense, cause of African Sleeping SicknessDuring a blood meal on the mammalian host, an infected tsetse fly (genus Glossina) injects metacyclic trypomastigotes into skin tissue (trypomastigotes are the characteristic developmental stage that infects humans. The parasites enter the lymphatic system and pass into the bloodstream (1). Inside the host, they transform into bloodstream trypomastigotes (2), are carried to other sites throughout the body, reach other blood fluids (e.g., lymph, spinal fluid), and continue to replicate by binary fission (3). The entire life cycle of this parasite is represented by extracellular stages. The tsetse fly becomes infected with bloodstream trypomastigotes when taking a blood meal on an infected mammalian host (4,5). In the fly’s midgut, the parasites transform into procyclic trypomastigotes, multiply by binary fission (6), leave the midgut, and transform into epimastigotes (7). The epimastigotes reach the fly’s salivary glands and continue multiplication by binary fission (8). The cycle in the fly takes approximately 3 weeks. Humans are the main reservoir for Trypanosoma brucei gambiense, but this species can also be found in animals. Wild game animals are the main reservoir of T. b. rhodesiense. The subspecies Trypanosoma brucei brucei infects domestic and wild animals but usually not humans (but see the phylogeographic analysis by Balmer et al. 2011, which concludes that the three "subspecies"of T. brucei are not actually genetically or historically distinct lineages).From Centers for Disease Control Parasites and Health website.
Centers for Disease Control/Division of Parasitic Diseases and Malaria
EOL staff
Life cycle of Leishmania protozoans, the cause of leishmaniasis in humansProtozoans in the family Leishmania are well known as the cause of leishmaniasis in humans. Leishmaniasis is transmitted by the bite of infected female phlebotomine sandflies (Psychodidae:Phlebotominae). The sandflies inject the infective stage (i.e., promastigotes) from their proboscis during blood meals (1). Promastigotes that reach the puncture wound are phagocytized by macrophages (2) and other types of mononuclear phagocytic cells. Progmastigotes transform in these cells into the tissue stage of the parasite (i.e., amastigotes) (3), which multiply by simple division and proceed to infect other mononuclear phagocytic cells (4). Parasite, host, and other factors affect whether the infection becomes symptomatic and whether cutaneous or visceral leishmaniasis results. Sandflies become infected by ingesting infected cells during blood meals (5,6). In sandflies, amastigotes transform into promastigotes, develop in the gut (7) (in the hindgut for leishmanial organisms in the Viannia subgenus; in the midgut for organisms in the Leishmania subgenus), and migrate to the proboscis (8).From Centers for Disease Control Parasites and Health website.