Pathogenic factors include inhibition of some host intestinal enzymes active in lipid and carbohydrate digestion. This can result from both mechanical damage to the microvillar glycocalyx and substances released by the parasite. Rapid sloughing of intestinal epithelial cells leads to failure of new epithelial cells arising in the crypts, which would differentiate into columnar cells with microvilli. Blunting of intestinal villi and microvilli results in decreased absorptive surface area. Malabsorption of nutrients, including fat, disaccharides, some vitamins, and iron can occur.
Diagnosis is by recovering and microscopically identification of either stage in feces.
A non-clinical carrier state appears to be common. Treatment in dogs is with quinacrine, although side effects are common. Treatment in caged birds is with dimetridazole. Giardia is the most common intestinal parasite of people in North America. Infants and children in day care facilities appear to be the group most at risk.