Which lifecycle stage of Giardia lamblia is typically infective for humans?

Giardia lamblia, a protozoan parasite, has two lifecycle stages: the trophozoite and the cyst. Among these, the cyst stage is the one that is infective to humans. The cysts are resilient, environmentally stable forms of the organism that can survive outside the host for extended periods, making them effective at transmitting the infection through contaminated water or food.

When humans ingest these cysts, they can enter the digestive tract and subsequently excyst in the intestines, releasing trophozoites that can then attach to the intestinal lining and cause giardiasis. The trophozoite stage, while important in the lifecycle and pathogenicity, is not the form that is transmitted from one host to another; it is the cyst that is shed in feces and is capable of causing infection when ingested.

The other lifecycle stages mentioned, such as oocysts and eggs, are not relevant to Giardia lamblia, as they pertain to different protozoan or helminthic parasites. This highlights the importance of recognizing the specific lifecycle characteristics of Giardia lamblia for understanding its transmission and infection processes.