Giardia intestinalis has been recognized as an intestinal protozoan since its discovery in the 1600s by van Leeuwenhoek. Giardiasis occurs worldwide and is particularly common in warm climates.
Giardiasis life cycles has only two stages: the trophozoite, which exists freely within the small bowel, and the cyst, which is passed into the environment. The infection is acquired by ingesting cysts, which are excreted in the feces. As few as 10-25 cysts are sufficient to establish an infection in some humans. Some livestock may be infected by as few as 1–10 cysts.
The cyst is ingested orally and excysts in the stomach and duodenum releasing the trophozoite. Trophozoites firmly attach to the intestinal microvillus surface with their sucking disks or move about free in the lumen. Trophozoites multiply in the small bowel by longitudinal binary fission.
Giardia cysts can be transmitted directly between hosts, or on various fomites including contaminated water and food. It can also be spread by contact with fecal material from infected people (hand to-hand or hand-to-mouth). This may be a more common route for infants and children.
Transmission of giardiasis
