Giardia

Giardia is a genus of single-celled, flagellated protozoa that live in the intestines of vertebrates. The species that infects people, Giardia duodenalis, also called G. lamblia or G. intestinalis, is one of the most common gut parasites worldwide. It has two forms. The active trophozoite is pear-shaped, with two nuclei that give it a face-like look under the microscope, four pairs of flagella and a sucking disk on its underside for clinging to the inner surface of the small intestine. The cyst is a tough resting form that can survive for months in cold water and resists the chlorine levels normally used in drinking water.

Infection begins when a person swallows cysts in contaminated water or food, or from hands and surfaces, and only a few are needed. In the small intestine each cyst releases two trophozoites, which multiply by binary fission and cover the gut lining, interfering with absorption, especially of fats. Some people have no symptoms; others develop greasy, foul-smelling diarrhea, cramps, bloating and weight loss one to two weeks later. Trophozoites turn back into cysts as they move toward the colon, and cysts in feces carry the infection on.

Diagnosis is by stool tests for the parasite, its antigens or its DNA, and drugs such as metronidazole or tinidazole treat it.