Table 1.

Comparison of major gastrointestinal protozoa and helminths.

Organism		Reservoir	Mode of transmission	Clinical manifestations
Protozoa
Entamoeba histolytica		Infected humans	Fecal–oral transmission by ingestion of feces containing infectious cysts	Bloody diarrhea (dysentery), intestinal and/or liver abscesses, asymptomatic intestinal infection
Giardia intestinalis		Infected human and other mammals	Fecal–oral transmission by ingestion of feces containing infectious cysts	Watery diarrhea, steatorrhea and malabsorption
Cryptosporidium parvum		Infected human and a variety of animal hosts (zoonosis)	Fecal–oral transmission by ingestion of feces containing infectious cysts	Watery diarrhea; intractable diarrhea in patients with AIDS
Cyclospora cayetanensis		Unknown	Foodborne and waterborne	Watery diarrhea
Helminths
Nematodes	Ascaris lumbricoides	Infected humans	Fecal–oral transmission by ingestion of eggs	Intestinal or biliary obstruction
	Hookworms (Ancylostoma uodenale and Necator americanus)	Infected humans	Fecal–oral transmission by ingestion (A. duodenale only) of larvae or passage through skin	Iron deficiency anemia from chronic gastrointestinal blood loss
	Trichuris trichiura	Infected humans	Fecal-oral transmission by ingestion of eggs	Damage to intestinal mucosa; malnutrition and/or anemia
Cestodes		Pigs, cattle and fish	Ingestion of raw or undercooked meat	Asymptomatic taeniasis to medium to heavy infection causing malabsorption and related mineral and vitamin imbalances. Cysticercosis (Taenia solium). Hydatid cysts (Echinococcus granulosus/multilocularis)
Trematodes		Aquatic plants and animals	Contaminated water or ingestion	Asymptomatic light infection to medium to heavy infections that can cause intestinal obstruction, ulceration and hemorrhage