Which organism lacks a cyst stage?

Which organism lacks a cyst stage?

• A. Entamoeba histolytica
• B. Dientamoeba fragilis
• C. Giardia lamblia
• D. Entamoeba coli

Answer: (B)

Many intestinal protozoa rely on a cyst form to survive outside the host and facilitate transmission. The organisms that form cysts include Giardia lamblia, Entamoeba histolytica, and Entamoeba coli, which have environmentally resistant cysts as their infectious stage. Dientamoeba fragilis is different in that it does not produce a cyst in its life cycle. It exists in the stool as trophozoites, and transmission is thought to occur via co-ingestion with Enterobius vermicularis eggs rather than cysts. Because there is no cyst stage, diagnosis focuses on detecting trophozoites in fresh or well-preserved stool samples.

Many intestinal protozoa rely on a cyst form to survive outside the host and facilitate transmission. The organisms that form cysts include Giardia lamblia, Entamoeba histolytica, and Entamoeba coli, which have environmentally resistant cysts as their infectious stage. Dientamoeba fragilis is different in that it does not produce a cyst in its life cycle. It exists in the stool as trophozoites, and transmission is thought to occur via co-ingestion with Enterobius vermicularis eggs rather than cysts. Because there is no cyst stage, diagnosis focuses on detecting trophozoites in fresh or well-preserved stool samples.