The flagellated unicellular parasite Giardia duodenalis (also sometimes known as G. intestinalis or G. lamblia) resides in the intestines of humans and a range of other vertebrates. Giardia lack common eukaryotic subcellular compartments such as mitochondria, peroxisomes, and apparently also a traditional Golgi apparatus. Six Giardia species are currently recognized. Five of these are represented by isolates from amphibians (G. agilis), birds (G. ardeae, G. psittaci), mice (G. muris), and voles (G. microti). Thje sixth species (G. duodenalis, sometimes treated as a species complex rather than a single species) includes Giardia strains isolated from a large range of other mammalian hosts (including humans, domestic dogs, cats, and livestock). Plutzer et al. (2010) reviewed the biology, epidemiology, detection, and control of Giardia. (Plutzer et al. 2010 and references therein; Tangtrongsup and Scorza 2010).
Giardia cysts are resistant forms and are responsible for transmission of giardiasis. Both cysts (the dormant, environmentally resistant stage, which measure around 12 microns long and 7 microns wide) and trophozoites (the active, motile stage, which measure around 15 microns long and 8 microns wide) can be found in the feces. The cysts are hardy and can survive several months in cold water. Infection occurs by the ingestion of cysts in contaminated water or food, or by the fecal-oral route (hands or fomites, i.e., inanimate objects or substances capable of transferring pathogens). In the small intestine, excystation releases trophozoites (each cyst produces two trophozoites). Trophozoites multiply by longitudinal binary fission, remaining in the lumen of the proximal small bowel, where they can be either free or attached to the mucosa by a ventral sucking disk. Encystation occurs as the parasites move toward the colon. The cyst is the stage found most commonly in nondiarrheal feces. Because the cysts are infectious when passed in the stool or shortly afterward, person-to-person transmission is possible. Non-human animals are infected with Giardia, but their importance as a reservoir for human infection is unclear. Giardia infects humans worldwide, but is more prevalent in warm climates and in children. (Tangtrongsup and Scorza 2010; Centers for Disease Control Parasites and Health Website)