Which stage is shed in feces by Giardia and is highly infectious?

Which stage is shed in feces by Giardia and is highly infectious?

• A. Giardia Intestinalis Trophozoite
• B. Plasmodium species
• C. Giardia Intestinalis Cyst
• D. Trichomonas vaginalis trophozoite

Answer: (C)

The key idea is Giardia’s transmission form. In Giardia, the stage that is shed in feces and is highly infectious is the cyst. Cysts are the dormant, environmentally hardy form with a tough wall that lets them survive outside the host in water or on surfaces for long periods. When someone ingests these cysts, they pass into the small intestine and undergo excystation to release trophozoites, which then cause infection as they colonize the intestinal lining. The trophozoite is the active, feeding stage inside the host and is not as resistant to environmental conditions, so it’s not the form typically shed in feces in an infectious state. Other organisms listed don’t fit the fecal-oral, highly infectious transmission pattern: malaria-causing Plasmodium species have a different life cycle and transmission route, and Trichomonas vaginalis trophozoites are transmitted primarily through sexual contact.

The key idea is Giardia’s transmission form. In Giardia, the stage that is shed in feces and is highly infectious is the cyst. Cysts are the dormant, environmentally hardy form with a tough wall that lets them survive outside the host in water or on surfaces for long periods. When someone ingests these cysts, they pass into the small intestine and undergo excystation to release trophozoites, which then cause infection as they colonize the intestinal lining. The trophozoite is the active, feeding stage inside the host and is not as resistant to environmental conditions, so it’s not the form typically shed in feces in an infectious state. Other organisms listed don’t fit the fecal-oral, highly infectious transmission pattern: malaria-causing Plasmodium species have a different life cycle and transmission route, and Trichomonas vaginalis trophozoites are transmitted primarily through sexual contact.