Giardiasis: The Parasite Giardia duodenalis, Its Life Cycle and Diagnosis

Giardiasis is caused by Giardia duodenalis (also called G. lamblia or G. intestinalis), a flagellated protozoan of the order Diplomonadida. It occurs worldwide, more often in warm climates and in children.

Life cycle

• Cysts are the hardy form that spreads the infection; they can survive several months in cold water. Both cysts and the active form, trophozoites, can be found in faeces, where they are used for diagnosis.
• People are infected by swallowing cysts — in contaminated water or food, or from hands or objects contaminated with faeces.
• In the small intestine, each cyst releases two trophozoites.
• The trophozoites multiply by splitting lengthwise, staying in the upper small bowel, either free or attached to the lining by a sucking disk on their underside.
• As the parasites move toward the colon they form cysts again. Cysts are what is usually found in stools that are not diarrhoeal.
• Because cysts are infectious when passed or soon after, the infection can pass from person to person. Animals are infected with Giardia too, but how much they act as a reservoir is unclear.

The life cycle of Giardia duodenalis. CDC DPDx.

Illness

Infection ranges from carrying the parasite without symptoms to severe diarrhoea and malabsorption. Acute giardiasis begins 1 to 14 days after infection (7 on average) and usually lasts 1 to 3 weeks, with diarrhoea, abdominal pain, bloating, nausea and vomiting. In chronic giardiasis symptoms keep coming back, and malabsorption and debilitation can follow.

Under the microscope

• Cysts are oval to ellipsoid, 8–19 µm long (usually 10–14 µm). Mature cysts have four nuclei, immature ones two. Nuclei and fibrils can be seen in iodine-stained wet mounts and trichrome-stained smears; in some stained cysts the cytoplasm pulls away from the cell wall.
• Trophozoites are pear-shaped and 10–20 µm long. Stained specimens usually show two large nuclei, and the sucking disk (used to attach to the lining of the host's intestine), median bodies and eight flagella may also be visible.

A G. duodenalis cyst in a wet mount stained with iodine. CDC DPDx.

A G. duodenalis cyst stained with trichrome. CDC DPDx.

A G. duodenalis trophozoite stained with trichrome. CDC DPDx.

Sources

• Centers for Disease Control and Prevention, DPDx — Laboratory Identification of Parasites of Public Health Concern, "Giardiasis." https://www.cdc.gov/dpdx/giardiasis/index.html