Giardiasis is the second most common cause of parasitic human diarrhea in America. The most common cause is pinworms, which are not zoonotic. Giardia is the most common cause of water-borne disease. Giardiasis is found in all regions of the United States and has been called back-packer’s disease, beaver disease, and traveler’s diarrhea.
Giardiasis is caused by a microscopic, single-celled, protozoan parasite, G. intestinalis, also known as G. lamblia. There have been other Giardia species identified, but G. intestinalis is considered the primary cause of giardiasis in humans and warm-blooded animals.
G. intestinalis has a two-stage life cycle. In the first stage, the disease-causing trophozoite stage, G. intestinalis is found in the small intestine. In the second stage, the cyst stage, the organism is passed in the host’s feces. At this stage, it can remain viable in a moist environment for several months because of its thick protective capsule. The cysts can even withstand short-term freezing. Cysts present in the environment are swallowed when a potential host eats feces-contaminated food, drinks feces-contaminated water, or puts any feces-contaminated object in its mouth. The protective capsule is removed by the acid in the stomach, and two trophozoites are formed. The trophozoites are carried to the first part of the small intestine, where they attach to the intestinal wall and reproduce by binary fission. Some of the trophozoites will encyst and pass in the feces. Some unencysted trophozoites will also pass in the feces (Figure 17).
Giardiasis is found in humans and a wide variety of domestic and wild animals including dogs, cats, cattle, sheep, horses, pigs, guinea pigs, chinchillas, deer, mice, beavers, bears, and muskrats. Theoretically, any warm-blooded animal, domestic or wild, could be susceptible.
Giardiasis is spread by the fecal-oral route and can be very contagious. The host must ingest feces contaminated with G. intestinalis cysts via food, water, hands, or an inanimate object. When the cysts are passed in the feces of an infected host they are immediately infective, and it takes very few ingested cysts to cause infection.