It is ingested as a cyst. These move to the large intestine and may invade cells. The host gives rise to numerous trophozoites and cysts. Only the cysts can survive in the environment. Infection causes ulceration and abscesses which can spread to other organs as the liver.
Chronic infection gives rise to recurrent diarrhea alternating with constipation.
The flagellate Giardia lambia is similarly spread and causes serious gastrointestinal disorders. It is also carried by dogs, deer, and other mammals. Cryptosporidium parvum is still a problem in the US and current methods may not remove this parasite from water supplies. The nematode Trichurus trichura or whipworm has a simple life cycle. The eggs must mature in the environment for about 20 days before they can reinfect.