Giardia duodenalis, also known as Giardia intestinalis and Giardia lamblia, is a flagellated parasitic protozoan microorganism of the genus Giardia that colonizes the small intestine, causing a diarrheal condition known as giardiasis. The parasite attaches to the intestinal epithelium by an adhesive disc or sucker, and reproduces via binary fission. Giardiasis does not spread to other parts of the gastrointestinal tract, but remains confined to the lumen of the small intestine. The microorganism has an outer membrane that makes it possible to survive even when outside of its host, and which can render it tolerant to certain disinfectants. Giardia trophozoites are anaerobic, and absorb their nutrients from the intestinal lumen. If the organism is stained, its characteristic pattern resembles the familiar "smiley face" symbol.
Giardia duodenalis
Multiple views of a G. lamblia cyst imaged by confocal microscopy: Bar = 10 micrometers (A) Cyst imaged by transmission (differential interference contrast) (B) Cyst wall selectively imaged through use of fluorescent-labelled antibody (C) Cyst imaged through use of carboxy fluorescein diacetate, a viability stain (D) Composite image of (B) and (C) (E) Composite image of (A), (B), and (C)
Drawings of a Giardia trophozoite and cyst by Charles E. Simon in 1921
Giardia trophozoites stained with Giemsa; 100x magnification
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson characterised parasites as "predators that eat prey in units of less than one". Parasites include single-celled protozoans such as the agents of malaria, sleeping sickness, and amoebic dysentery; animals such as hookworms, lice, mosquitoes, and vampire bats; fungi such as honey fungus and the agents of ringworm; and plants such as mistletoe, dodder, and the broomrapes.
A fish parasite, the isopod Cymothoa exigua, replacing the tongue of a Lithognathus
Head (scolex) of tapeworm Taenia solium, an intestinal parasite, has hooks and suckers to attach to its host
The parasitic castrator Sacculina carcini (highlighted) attached to its crab host
Human head-lice are directly transmitted obligate ectoparasites