G. lamblia is a cosmopolitan parasite of worldwide distribution and is an important cause of traveler’s diarrhea. Giardia is the most common intestinal parasite found in the United States. Its prevalence may be increasing. In 1979, Giardia was found in an average of 4% of stool specimens submitted to state diagnostic laboratories. This prevalence increased to 6% to 7% in the late 1980s and 1990s. The states in which it appears most frequently are located in the Midwest and the Northwest. Giardia also is prevalent in the mountainous western United States, where infection can be contracted by drinking water from mountain streams that have been contaminated by feces from humans, dogs, and other species susceptible to G. lamblia. The beaver acts as a reservoir for the organism during the summer months by becoming infected (presumably from humans) and then defecating directly into streams. Water can be disinfected by adding 13 mL of a saturated solution of iodine to 1 L of clear water, or 26 mL to 1 L of cloudy water. All organisms are killed after 15 minutes of incubation at 20°C; at 3°C, however, this method is not totally effective. Boiling water for 10 minutes kills all organisms.

Giardia also can be spread by close person-to-person contact in which fecal contamination may occur, such as in day-care centers and residential institutions. In addition, contaminated food may act as a vector for this parasite. Human milk may contain secretory anti-Giardia antibodies that can prevent symptoms of diarrhea but not Giardia infection in breast-fed infants.

Three species of Giardia have been described. G. lamblia is the species that is specific to humans, but it can be cross-transmitted to other animals, such as dogs, cats, rats, gerbils, guinea pigs, beavers, raccoons, bighorn sheep, and pronghorn antelope. G. muris infects rodents and birds; G. agilis is specific to amphibians. G. lamblia is the name used for the species infecting humans in North America. This same organism is called G. intestinalis in Europe and Lamblia intestinalis in Russia and Eastern Europe. It has also been called G. duodenalis and G. enterica.

G. lamblia is a flagellate protozoan belonging to the family Hexamitidae. The trophozoite or motile form is characterized by its symmetry: two oval, dorsally situated nuclei and four pairs of flagella. In addition, it has two median bodies and a ventral adhesive disc by which the parasite adheres to the intestinal mucosa and other surfaces. Attachment seems to be mediated through the cytoskeleton by contractile filaments and
microtubules to lectin-binding sites on the mucosa. Interference in such binding can occur through the ingestion of lectins such as wheat-germ agglutinin normally consumed in the diet. The organism also exists in a cyst form, which results when the trophozoite rounds up and elaborates a cyst wall. These cysts allow the organism to survive passage out of the host. Giardia cysts are resistant to destruction in hypotonic solutions such as water and can survive for more than 2 months in water at 8°C but for only 4 days in water at 37°C. When cysts are ingested, the excystation process is induced by gastric acid and is completed in the duodenum with the emergence of trophozoites. Infection is established if the trophozoite can survive, attach to the intestinal mucosa, and multiply. This process may require nutrients within the intestinal fluid.

The pathogenesis of diarrhea and steatorrhea in giardiasis is not understood completely. Initially, the organisms were believed to damage the intestinal mucosa either through direct invasion or the elaboration of some toxin. Careful histologic studies with light and electron microscopy have shown some reduction in villous height and mild crypt hyperplasia, but this effect appears to be related to specific Giardia strains.

Mechanical blockage of nutrient absorption resulting from the mass of Giardia organisms adhering to the intestinal mucosa also has been postulated. Histologic examination of the intestinal mucosa of diseased individuals, however, usually does not reveal enough organisms to support this hypothesis. Giardia appears to have the capability to alter intestinal motility, which may play a role in the development of symptoms.

Finally, careful electron microscopy has revealed that Giardia seems to stimulate excessive mucus production by the intestinal mucosa. This excessive mucus causes thickening of the unstirred layer or glycocalyx adherent to the intestinal brush border and may result in a diffusion barrier for nutrients, ultimately causing diarrhea and malabsorption.