What is the life cycle of Giardia during transmission of giardiasis?

Updated: Oct 01, 2018
  • Author: Hisham Nazer, MBBCh, FRCP, DTM&H; Chief Editor: Burt Cagir, MD, FACS  more...
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Answer

Giardia has one of the simplest life cycles of all human parasites. The life cycle is composed of 2 stages: (1) the trophozoite (see the first image below), which exists freely in the human small intestine; and (2) the cyst, which is passed into the environment. No intermediate hosts are required.

Upon ingestion of the cyst (see the second image below), contained in contaminated water or food, excystation occurs in the stomach and the duodenum in the presence of acid and pancreatic enzymes. The trophozoites pass into the small bowel where they multiply rapidly, with a doubling time of 9-12 hours. As trophozoites pass into the large bowel, encystation occurs in the presence of neutral pH and secondary bile salts. Cysts are passed into the environment, and the cycle is repeated.

Giardia lamblia, cyst form. Giardia lamblia, cyst form.
Giardia lamblia trophozoites in culture. Giardia lamblia trophozoites in culture.

The trophozoite form of G lamblia is teardrop-shaped and measures 9-21 micrometers long by 5-15 micrometers wide. The trophozoite has a convex dorsal surface and a flat ventral surface that contains the ventral disk, a rigid cytoskeleton composed of microtubules and microribbons. The trophozoite also contains 4 pairs of flagella, directed posteriorly, that aid the parasite in moving. Two symmetric nuclei with prominent karyosomes produce the characteristic facelike image that appears on stained preparations.

The ventral disk, which is often referred to as the sucking or adhesive disk, provides the parasite with powerful adhesion, catching, and holding abilities. In the murine model of giardiasis, the ventral disk adhesion imprints are marked but less impressive than in the human small intestine. However, this direct injury is an unlikely cause of the more extensive reduction in microvillus surface area, the reduction in disaccharidase activities, and the more pronounced abnormalities of villous architecture that are seen in giardiasis. [19, 20]

The cyst form of the protozoan is smooth-walled and oval in shape, measuring 8-12 micrometers long by 7-10 micrometers wide. As the cyst matures, nuclear division occurs and readies the cyst to release 2 trophozoites upon excystation. Once the host is infected, trophozoites may appear in the duodenum within minutes. [21] Excystation occurs within 5 minutes of exposure of the cysts to an environment with a pH between 1.3 and 2.7.

After infection, the trophozoites attach to the enterocytes via the ventral adhesive disk. This may occur through the presence of lectin on the surface of the trophozoite or through other mechanical means. Encystation is a continuous process during infection.

As the trophozoites encounter neutral pH and/or secondary bile salts, encystation-specific secretory vesicles (ESVs) appear. After 15 hours, cyst wall proteins are visible. Within 24 hours after the appearance of ESVs, the trophozoite is covered with these cyst wall proteins, the form of the cyst has emerged, and new antigenic differences are present.


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