Life Cycle and Transmission

The life cycle of G. duodenalis is relatively simple and alternates between the actively metabolizing motile stage, the trophozoite, and the dormant, environmentally resistant cyst. 

The trophozoite resembles a pear cut in half longitudinally, is binucleate, ranges in size from 10 to 15mm in length and 6-10mm in width and 2-4mm thick, and swims in the intestinal fluid and tethers itself to the mucus strands by means of 4 pairs of flagella.

 The swimming movements appear erratic and twisting resembling the motion of a falling leaf. In addition to its flagella Giardia has other distinctive cytoskeletal elements consisting of the funis, a distinctive pair of median bodies (the “mouth” in the giardial face), and a concave surface on the anterior two-thirds of the ventral surface, the adhesive disc. The cytoplasm contains ribosomes and endoplasmic reticulum,glycogen granules. There is controversy whether there is a Golgi apparatus but the cells clearly lack mitochondria, glycosomes, and hydrogenosomes.   The nuclei are spherical, contain a central karyosome, and lack a nucleolus. The chromatin is localized around the periphery of the nucelus. They divide by binary fission. A double-stranded RNA virus, giardiavirus, specifically infects some strains of Giardia and is associated with decreased adherence and slower growth rates in vitro. Trophozoites are sometimes found in the feces in cases of acute diarrhea. 

The cyst form is 5-8mm and is surrounded by a wall that is 0.3mm thick. The outer wall consists of many filaments, whereas the inner wall contains outer and inner cyst membranes separated by a thin layer of cytoplasm. The major sugar of the outer cyst wall is N-acetylgalactosamine (GalNAc) plus galactose, and the cyst appears not to contain chitin. In addition, there are two cyst wall proteins that have been characterized and whose genes have been sequenced.

The trophozoites live in the small intestine (mid-jejunum) attached to the surface of the enterocytes by the ventral adhesive disc, dividing every 9-12hr. In 10 days there could be 1 million parasites and in two weeks a billion Giardia.  Trophozoites may detach intermittently,  but this is of minimal duration since it places the parasite in the position of being swept downstream by peristalsis.

 Giardia and its “footprints”

In the mid-jejunum  they are exposed to a cholesterol-rich environment (from both food and bile). Cholesterol is absorbed primarily in the jejunum whereas bile acid absorption occurs in the ileum. Bacterial populations also decrease the amount of bile and chemically alter the cholesterol thereby decreasing its availability for the trophozoites. Thus, as trophozoites pass to the posterior part of the small intestine they are exposed to increasedlevels of bile and higher pH, and this triggers encystation. Within 5 hr  encystation-specific antigens can be found in the endoplasmic reticulum, and are packaged into encystation-specific vacuoles, which appear in 6-18 hr, and these are transported to the forming cyst wall. Cyst formation begins on the dorsal surface, there is a change in shape--flattened pear-shaped to spherical--the flagella curl up inside cyst wall and the trophozoite becomes enclosed in the wall. Nuclei divide during encystment, the disc disorganizes, and  a quadrinucleate cyst is discharged with the feces. It is not clear whether the cyst is infective immediately after passage, however,  there is some evidence to show that it may take 7 days before full infectivity can occur.

 Infection begins by ingestion of the cyst. The ingested cyst passes unharmed through the stomach because it is resistant to gastric juice (whereas the trophozoite is not). Excystation occurs after it leaves the stomach and enters the small intestine. The low pH of the stomach appears to be the predisposing factor for excystation, however, excystation may also occur at a higher pH in response to pancreatic secretions (trypsin, chymotrypsin) and/or bicarbonate. Within 5-10min after being placed in an “excystation medium” flagellar motion begins and the trophozoite exits through a break in the cyst wall. The break in the wall is due to the action of three cysteine proteases (cathepsin B-like) secreted from surface vacuoles in the trophozoite. Cytokinesis begins within 30 minutes and two binucleate individuals emerge from the cyst. Excystation leads to rapid colonization of the gut, and cyst production may begin within 4-15 days.

Summary of Giardia Life Cycle: