what is the most common protistan intestinal parasite in the united states
G. lamblia. G. lamblia is a pear-shaped, flagellated protozoan (Figure 2) that causes a wide variety of gastrointestinal complaints. Giardia is arguably the most common parasite infection of humans worldwide, and the second most common in the United States after pin-worm.8, 9 Between 1992 and 1997, the Centers for Disease Control and Prevention (CDC) estimated that more than 2.5 million cases of giardiasis occur annually. It is the most prevalent nematode in the United States. Humans are the only known host, and about 209 million persons worldwide are infected. More than 30 percent of children worldwide are infected. 2. Adult worms are quite small; the males measure 2 to 5 mm, and the females measure 8 to 13 mm.