The life cycle of giardia consists of two from that is the cyst form and trophozoite form. The most infectious form is cyst form and once if it gets entered into the host cells, it might gets transferred into the trophozoite form. As the trophozoites continue along the gastrointestinal tract they gets revert back to their cyst form which is excreted with the faecal matter. Ingestion of only a few of these cyst is needed to generate infection in another host cells.
Infection with Giardia results in decreased expression of brush border enzymes , the shape of the microvillus gets altered , increased intestinal permeability and the programmed cell death of small intestinal epithelial cells occurs. Both trophozoites and cyst are contained within the gastrointestinal tract and they do not invade beyond it. The attachement of trophozoites to the small intestine causes dysfunction of the microvilli and also distrupts the enzyme that break down dissacharides sugar in the intestinte. The group of microorganisms that grows in the intestine may get enlarge in size and due to this further symptoms arises. The alteration of the villi leads to an inability of nutrients and water absorption from the intestine, resulting in diarrhoea which is one of the main symptoms.
The species Giardia intestinalis uses enzymes that break down the protein to attack the villi of the brush border and appears to increase crypt cell proliferation and crypt length of crypt cells existing on the sides of the villi.
In the immunological level, the activated host T- lymphocytes attack those endothelial cells that have been injured in order to remove the cell. This occurs after the damage to the proteins that connects brush border endothelial cells to one another. The result is increase in intestinal permeability.
There arises programmed cell death by Giardia intestinalis which further damages the intestinal barrier and increases permeability. There is significant upregulation of programmed cell death cascade by the parasite and furthermore also simultaneously downregulation of the anti apoptotic proteins Bcl-2 and upregulation of the proapoptotic protein Bax. Giardia protects its own growth by reducing the formation of the nitric oxide gas by consuming all local arginine which is the amino acid necessary to make the nitric oxide. Arginine starvation is known to be cause of programmed cell death and the local removal is strong apoptotic agent.