Giardiasis is a common parasitic infection caused by the protozoan Giardia lamblia. Transmission usually occurs via the fecal-oral route (e.g., from contaminated drinking water) when traveling or living in an endemic region. Giardia live in two states: as active trophozoites in the human body and as infectious cysts surviving in various environments. Following the ingestion of the cyst, individuals may experience abdominal cramps and frothy, greasy diarrhea. Diagnosis of giardiasis involves analyzing stool for microscopic confirmation of cysts or trophozoites, and possibly immunoassays to detect antigens. Treatment consists of antibiotic administration with metronidazole.
- Giardia lamblia is widespread throughout the world and affects ∼ 200 million people per year worldwide.
- Incidence: estimated 5–8/100,000 per year in the US
- In the US, giardiasis is the most common intestinal disease caused by parasites.
Epidemiological data refers to the US, unless otherwise specified.
- Pathogen: Giardia lamblia; (also known as Giardia intestinalis or duodenalis), a protozoan
- Waterborne: from drinking recreational water (e.g., lakes, rivers, ponds, swimming pools)
- Fecal-oral; (e.g., through food handlers, people in daycare and nurseries, oral-anal sexual contact): Giardia cysts are passed into the environment from the feces of infected people and animals. 
- Infection is more likely to occur after traveling to endemic regions such as the tropics, subtropics, and North-American mountain regions.
- Giardia have 2 stages in the life cycle.
- Ingestion of cysts → excystation and conversion to trophozoite form → rapid multiplication, adhesion to intestinal walls → encystation in large bowel → excretion of cysts → possible reinfection
- Although several theories exist, it is commonly suspected that infection with Giardia leads to impaired function and structure of intestinal tissue , resulting in malabsorption and diarrhea.
- IgA deficiencies (e.g., selective IgA deficiency, X-linked agammaglobulinemia, common variable immunodeficiency) increases susceptibility to giardiasis because of the disruption of gastrointestinal protective barrier. 
Diarrhea: foul-smelling, voluminous, frothy, and fatty stools (stools tend to float and do not appear bloody)
- May lead to dehydration
- Excessive gas (flatulence, bloating), abdominal pain, and cramps
- Fatigue, nausea/vomiting, anorexia
- Can be asymptomatic
- Stool analysis: microscopic confirmation of cysts or multinucleated trophozoites 
- Immunoassay: detection of Giardia lamblia antigens in stool
- Gastroduodenoscopy: confirms trophozoites in duodenal fluids, but generally not indicated
- Metronidazole (commonly used in the US)
- Tinidazole (single oral dose)
- Alternatives: nitazoxanide, albendazole, or mebendazole