excyst

Excyst is the process by which a cyst, a dormant or enclosed stage of a microorganism, ruptures or sheds its surrounding envelope to release an active form such as a trophozoite, sporozoite, or other motile stage. It is the reverse of encystment and is a critical transition in the life cycles of many protozoa, helminths, and some algae. In many parasitic life cycles, excystation occurs after the cyst is ingested or otherwise exposed to conditions within a host.

Mechanism and triggers for excystation vary by organism but are commonly driven by environmental cues. Changes in pH, exposure to bile salts and digestive enzymes, temperature, and ionic conditions often trigger the breakdown of the cyst wall and release of the interior stage. The exact sequence of events differs; some organisms release a single trophozoite that then replicates, while others release multiple forms or sporozoites ready to invade host tissues.

Examples and relevance include Giardia lamblia, where ingestion of cysts leads to excystation in the small intestine and release of trophozoites, which colonize the host. Entamoeba histolytica cysts excyst in the small intestine to produce trophozoites. Coccidian parasites such as Cryptosporidium and Eimeria also undergo excystation in the gut to release invasive stages. Clinically, excystation is a key step in infection and influences diagnostic approaches, as environmental cysts indicate exposure whereas the presence of trophozoites reflects active infection.

In laboratory studies, excystation can be simulated to study parasite development or to culture organisms under controlled conditions.