Giardia

Trophozoites live in the intestine of a variety of mammals, and attach to the epithelium using their ventral sucking disks. Flagellar action reduces pressure under the disk, resulting in adhesion to the epithelium. They feed on secretions from the epithelium. Multiplication is by binary fission, and large numbers can build up within a week. Some trophozoites secrete a cyst as they pass down the intestinal tract. Within the cyst the trophozoite partially divides. If the cyst is deposited in fresh water it can remain infective for about 1 week. It can also survive a brief period on land, as long as it is soon washed into nearby water. When cysts are ingested in contaminated drinking water, they excyst in the intestine, complete their division, and begin feeding and multiplying.