Giardia intestinalis (lamblia)

Giardia intestinalis is an intestinal flagellate that is probably the most common intestinal parasites worldwide. Developing countries will have an incidence of as high as 70% in preschool and school age children.

Disease / Pathogenesis

Giardia intestinalis will inhabit the lumen of the small intestine. There they attach to the enterocytes of the mucosal surface. The organisms produce disease by epithelial transport and barrier dysfunction at the enterocyte brush border. This will result in diarrhea that is characteristically fatty and foul smelling. Symptoms can be acute or chronic. When chronic in nature the host will also have symptoms of intestinal malabsorption that result in steatorrhea, vitamin deficiencies, and lactase deficiency. Many times patients with chronic giardiasis will present with profound weight loss and weakness.

Location in the Host

Giardia intestinalis is located in the small intestine.

Geographic Distribution

Giardia intestinalis is found worldwide.

Life Cycle

Transmission is by ingestion of the cyst. The cyst can found to contaminate food and water. Contamination of food can occur through mechanical vectors such as insects. Animals such as beavers are can act as reservoirs of this parasite and can thus contaminate water sources with the cysts. The cyst has excystation triggered by the sequential affects of gastric acid, pepsin and last but not least the alkaline environment of the small intestine. The motile trophozoite forms released are pear shaped at 9-19um by 5-14um in size. They will have two nuclei, linear axonemes can be seen in the interior, and eight flagella can sometimes be seen as well. There is a rarely visible ventral disc that the organism uses to attach to the enterocyte brush border.

Image courtesy of the CDC

Morphology & Diagnosis

Detection of the cyst or trophozoite forms in preserved stool. Though the development of EIA tests and multiplex PCR tests have made the detection of these infections much easier. It is important to note that EIA tests may have poor sensitivity. It is this author’s expereince that PCR is the better choice, thus cost may be a limiting factor in its acceptance into most laboratories. In either case these tests are usually bundled with other common parasitic organisms for even better diagnostic results. These include Cryptosporidium spp., Cyclospora spp., and Entamoeba histolytica.

When detecting in preserved stool Giardia intestinalis cyst measure 8-19um. When they are at the smaller size range they are many times difficult to identify and can be misidentified as yeast forms. Sometimes supporting structures (axostyles) can be seen in the interior along with four small nuclei.

The trophozoites are piriform in shape and bilaterally symmetrical with two anterior nuclei, two slender axostyles and four flagella can be visible. They measure about 9-19um by 5-14um wide. The anteroventral side of the organism has a sucking disc used for attachment, but this is not readily visible.


Giardia intestinalis cyst (8-19um) in Trichrome stain. Their smaller size can sometimes be mistaken for yeast however the presence of internal structures is the differentiating observation
– Photo by W. Vientos
Giardia intestinalis cyst (8-19um) in concentrate stained with Lugol’s Iodine stain 100X oil
Giardia intestinalis trophozoite (9-19um by 5-14um) in trichrome stained preparation 100x oil
Giardia intestinalis cyst (8-19um) in trichrome stained preparation 100x oil