Giardia duodenalis

Giardia duodenalis
Giardia lamblia cell, SEM
Scientific classification
Domain:	Eukaryota
Phylum:	Metamonada
Order:	Diplomonadida
Family:	Hexamitidae
Genus:	Giardia
Species:	G. duodenalis
Binomial name
Giardia duodenalis Stiles, 1902
Synonyms
Cercomonas intestinalis Lambl, 1859 Lamblia intestinalis Blanchard, 1888 Giardia lamblia (Lambl, 1859) Kofoid & Christiansen, 1915 Giardia intestinalis Kulda & Nohýnková, 1995

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.^[1][2][3] The parasite attaches to the intestinal epithelium by a ventral disc (syn. adhesive disc or sucker), and reproduces via binary fission.^[4][5] G. duodenalis is a non-invasive parasite, that does not spread to other parts of the gastrointestinal tract, but remains confined to the lumen of the small intestine.^[6][7] The parasite exists in two forms; trophozoites and cysts. The microorganism can undergo encystation, transforming into a dormant cyst that enables it to survive outside of its host.^[8] 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.^[9]

Chief pathways of human infection include ingestion of untreated drinking water (which is the most common method of transmission for this parasite),^[3] food, soil contaminated with human feces, and sewage, a phenomenon particularly common in many developing countries.^[10][3] Contamination of natural waters also occurs in watersheds where intensive grazing occurs.

Giardia infections occur worldwide. It is the most commonly identified intestinal parasite among children in day-care centers, hikers and immunocompromised patients. About 20,000 cases per year in the United States are reported.^[11]

Almost half of those infected with giardiasis remain asymptomatic. For those who do experience symptoms, they usually appear 1 to 2 weeks after infection. Common symptoms include abdominal pain, nausea, and bloating, along with large, watery, foul-smelling, and greasy stools. Due to frequent loose stools, individuals with giardiasis often experience dehydration.^[12] It has also been shown that G. intestinalis damages the intestinal epithelium, which directly affects nutrient absorption.^[5] In severe cases, giardiasis can lead to chronic diarrhea, chronic fatigue syndrome and cognitive impairment in children.^[13]