Life Cycle of Giardia

Cysts are swallowed with water or food. In the duodenum excystation occurs, which releases the trophozoite. This measures 12-18 m. It attaches itself to the duodenal and jejunal intestinal villi by means of a kind of ventral sucking disk. The parasite reproduces only by asexual division. The trophozoites may multiply until the whole surface of the intestine is coated with parasites. Possibly this mechanical screening off of the intestine contributes to malabsorption.

As trophozoites are carried to the more distal parts of the intestine, the parasite encapsulates. The cyst is resistant in the outside world, but trophozoites perish. Cysts remain viable in a wet, cool outside environment. They are not very resistant to drying out, however. The cysts measure 10 x 7 m. Transmission is via direct faeco-oral contact, food or via water. There is an animal reservoir and this is sometimes involved in human infection (giardiasis is known in Canada as "beaver fever"). In industrial countries dogs and cats are frequently found to be infected, but almost always without symptoms. Research into host specificity has led to very complex studies, produces very variable results, but is of great importance in providing better knowledge of the epidemiology. The importance of animal reservoirs should be studied further.

Category: Medicine Notes