Giardia duodenalis
Flagellated protozoan causing giardiasis worldwide.
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. It is a non-invasive parasite that attaches to the intestinal epithelium via a ventral disc and reproduces by binary fission. The parasite exists in two forms: trophozoites and cysts, and its characteristic stained pattern resembles a 'smiley face' symbol.
- field
- Parasitology, Microbiology
- known_for
- Causing giardiasis, a diarrheal disease; most commonly identified intestinal parasite in the United States and Canada
- infectious_dose
- As low as 10 cysts
- annual_global_infections
- Over 280 million
- annual_global_deaths
- Over 500,000
Lore & Background
Giardia duodenalis takes on two morphologically distinct forms during its lifecycle. Trophozoites are the replicative stage, characterized by a pear-shaped, motile, flagellated cell that survives only in the small intestine of the host. The trophozoites do not penetrate host cells but attach to the intestinal epithelium using a ventral adhesive disc. Cysts are the environmentally stable stage that facilitates transmission between hosts. The infection process begins with ingestion of cysts, which pass through the stomach into the duodenum, where exposure to digestive enzymes triggers excystation, releasing trophozoites. Trophozoites then adhere, feed, divide by binary fission, and cause disease. Some trophozoites differentiate back into cysts under specific conditions, such as high organism density, and are shed in feces.
Reader's Guide
Giardia duodenalis is the most widespread intestinal parasite affecting humans, found in both developing and industrialized nations. It is the most frequently identified intestinal parasite in the United States and Canada, with about 20,000 cases reported annually in the U.S. The parasite is extremely infectious, with an infectious dose as low as 10 cysts, and an infected individual can excrete between 1 million and 1 billion cysts daily. It is estimated to infect over 280 million people worldwide each year, resulting in over 500,000 deaths, with the most affected demographic being children 0 to 4 years of age. In developing countries, prevalence rates reach 15% to 20% in children under 10 years old. The cyst can survive for weeks to months in cold water and is resistant to conventional water-treatment methods such as chlorination and ozonolysis, making waterborne transmission the most common pathway. Giardia infections are a concern for hikers, children in day-care centers, and immunocompromised patients. Almost half of those infected remain asymptomatic, but symptomatic cases can include abdominal pain, nausea, bloating, large watery foul-smelling greasy stools, dehydration, and in severe cases, chronic diarrhea, chronic fatigue syndrome, and cognitive impairment in children.
Did You Know?
- The parasite's stained pattern resembles the familiar 'smiley face' symbol.
- An infected individual can excrete between 1 million and 1 billion cysts daily.
- The infectious dose can be as low as 10 cysts.
- The cyst can survive for weeks to months in cold water and is resistant to chlorination and ozonolysis.
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