The parasite is known to invade the colonic epithelium, but the significance of invasion is not known. The most important result of invasion by the parasite is subsequent activation of host immune processes. Steatorrhea, vitamin B12, vitamin A, protein, and D-Xylose malabsorption have all been documented in chronic infection. Symptoms of chronic infection include chronic diarrhea, malaise, nausea, and anorexia. Chronic, sporadic diarrhea may continue for months.
Extraintestinal manifestations are rare.
Symptoms such as urticaria and reactive arthritis have been reported. There may be some evidence of weight loss, but there are no known, unique physical findings attributable to giardiasis. The infectious load required to produce disease may be as low as 10-cysts. The protozoan is known to have multiple strains with varying abilities to cause disease, and several different strains may be found in one host during infection. Aspiration of duodenal contents and demonstration of trophozoites has also been used for diagnosis but is more invasive than stool examination and, in direct comparison studies to stool microscopy, may have a lower diagnostic yield.