Giardia lamblia

Giardia lamblia, single-celled parasite of the order Diplomonadida, the cause of the diarrheal illness giardiasis. Similar to other diplomonads, the cells of Giardia lamblia have two nuclei and eight flagella. G. lamblia cells are further distinguished by the presence of minute organelles known as mitosomes. The parasite can survive outside of host organisms—which include humans and other mammals—for prolonged periods of time, because of a cyst stage in its life cycle, during which it is enveloped in a tough outer shell.

G. lamblia has two life stages: a motile, replicative trophozoite stage, in which the parasite survives in the small intestines of the host, and a nonreplicative cyst stage, in which the parasite survives in the environment. Upon ingestion by a host species, trophozoites adhere to the epithelium of the small intestine, where they then divide by binary fission. Fission may result in the production of additional trophozoites or in the generation of cysts. Cysts pass through the intestines, ultimately being shed in host feces. Once in the environment, under moist conditions, dormant G. lamblia cysts can survive for weeks or even months. Cysts may divide to produce new trophozoites.

In humans, acute giardiasis is a common disease among hikers, campers, and travelers to undeveloped countries who drink untreated water, and it is also quite common among children in day care centres and people who use crowded public swimming areas. The clinical symptoms include large, foul-smelling, fatty stools, stomach cramps, and bloating. The parasite is passed directly from the stools of infected people or animals to persons who ingest infected water, food, or other material. Symptoms appear after a 10-day incubation period and may persist for weeks. The disease is generally mild and self-limiting, though infected children can develop chronic malabsorption of nutrients from the intestines. It is treated with antimicrobial drugs and with fluids to prevent dehydration.