Intestinal infection caused by the protozoan parasite, Giardia lamblia. Infection results from ingestion of the cysts that excyst into trophozoites which colonize the small intestine and cause the symptoms. The cycle is continued when the trophozoites encyst in the small intestine and water, food, or hands are contaminated by feces of the infected person. Most infections result from fecal-oral transmission or ingestion of contaminated water, less commonly from contaminated food.

• System(s) affected: Gastrointestinal
• Genetics: N/A
• Incidence/Prevalence in USA: 5% of patients with stools submitted for ova and parasite exams. Overall prevalence is lower and variable.
• Predominant age: All ages, but most common in early childhood
• Predominant sex: Slightly more common in males

• Approximately 25-50% of infected persons are symptomatic
• Chronic diarrhea (lasting more than 5-7 days and frequently weeks)
• Abdominal bloating
• Flatulence
• Loose, greasy, foul-smelling stools
• Weight loss
• Nausea
• Lactose intolerance

Protozoan parasite (Giardia lamblia) infection acquired through fecal-oral transmission or ingestion of contaminated water, less commonly from contaminated food

• Day care centers
• Male homosexuality
• Wilderness camping