The life cycle of Giardia lamblia (also known as Giardia intestinalis) is comprised of two stages: cyst and trophozoite.
The cysts are approximately 7-10 µm in length and are oval in shape. The mature cyst contains four nuclei. They are environmentally resistant and responsible for disease transmission.1
Cysts may remain viable for several months in cool, moist conditions, and have been detected in natural surface waters.2 They are also able to survive standard concentrations of chlorine used in water purification systems.3
Infection occurs after cysts are ingested. This marks the beginning of the life cycle. After ingestion, mature cysts in the small intestine release trophozoites through a process called excystation.3 Cysts are able to survive exposure to gastric acid; gastric acid may actually trigger excystation.2
The trophozoite stage is responsible for producing clinical disease in humans.
Trophozoites have two distinct nuclei and four pairs of flagellae. They resemble teardrops when viewed from the top and they are spoon shaped from the side. They are 12-15 µm in length.
Trophozoites colonize the small intestine, attaching to the mucosa of the bowel using a ventral sucking disks. The trophozoites then multiply by longitudinal binary fission.4.
As the Giardia trophozoites move toward the colon, they retreat into the cyst stage (known as encystation) and the new cysts are excreted in the feces. Bile salts and intestinal mucus boost trophozoite multiplication and encystations.5
Source: CDC
- Wolfe MS. Giardiasis. Clin Microbiol Rev. 1992 Jan;5(1):93-100.
- Farthing MJ. Giardiasis. Gastroenterol Clin North Am. 1996 Sep;25(3):493-515.
- Jones JE. Giardiasis. 1988:872-882. In A. Balows, WJ Hausler, M. Ohashi, and A. Turano (ed.), Laboratory diagnosis of infectious diseases, vol. 1. Springer-Verlag, New York.
- Hill DR. Giardiasis. Issues in diagnosis and management. Infect Dis Clin North Am. 1993 Sep;7(3):503-25.
- Adrabbo K, Peura D. Giardiasis: A Review. Practical Gastroenterology. 2002 June; 15-29.
