Giardiasis is a major diarrheal disease found throughout the world. Giardiasis is caused by ingestion of Giardia cysts. After ingestion of cysts, excystation, and trophozoite multiplication, colonization of the upper small bowel occurs. The exact pathophysiology of giardiasis is unclear. Postulated mechanisms include damage to the endothelial brush border, enterotoxins, and altered gut motility and fluid hypersecretion via increased adenylate cyclase activity.
Most infections result from fecal-oral transmission or ingestion of contaminated water.
Contaminated food is a less common etiology. Endemic infection occurs most commonly from July through October, among children younger than 5 years and adults aged 25-39 years. Morbidity is moderate and involves primary GI symptoms. It can have high carrier rates within Native American populations residing on reservations. The majority of patients experience a more insidious onset of symptoms, which are recurrent or resistant.