Parasites can infect many parts of the human body with the intestine easily proving the most common site of infection. Many intestinal parasites affect hundreds of millions of people at any one time. Even in the United States where sanitation practices are high, certain intestinal parasites prove still quite common. With few exceptions, people acquire intestinal parasites through drinking contaminated water, eating undercooked meats and through ingestion of the parasite's egg through the soil.
Giardia lamblia
Giardiasis is a very common intestinal protozoan parasite usually acquired through drinking contaminated water. Giardia is one of the common causes of traveler's diarrhea. According to L.K. Pickering, et al, by the late 1980's, Giardia lamblia had become the number one intestinal parasitic disease in the United States. Giardia causes acute diarrhea with abdominal pain. In cases of chronic diarrhea, the malabsorption of fats is a frequent occurrence. Waterborne outbreaks occur particularly among campers and hikers. Children in day care centers also prove a high-risk group for giardiasis.
Ascaris lumbricoides
This is the most common intestinal parasite with nearly one billion people infected at any one time worldwide. The giant intestinal roundworm is found predominately in developing areas of the world where people live in poverty, malnutrition and in lower standards of sanitation. People typically become infected by ingesting the Ascaris egg from soil, food or water. The eggs have become resistant to the environment and can survive for years in the soil. The adult worm resembles an earthworm in shape but has a smooth appearance and lacks striations. The adult worm can grow to a length of 35 mm. Serious disease from Ascaris lumbricoides includes bowel obstruction and if the worm wanders from the intestine, the bile ducts and the pancreatic ducts can become obstructed.
Enterobius vermicularis
The pinworm is the most common intestinal roundworm found in the United States. This very common childhood infection spreads through hand to mouth transmission. The adult female pinworm, which is about the size of a staple, leaves the anus at night and deposits her eggs in the perianal folds causing intense itching. This parasitic roundworm infection proves most common in children, however whole households can easily become infected.
Hookworm
According to the Global Network for Neglected Tropical Diseases, over one-half a billion people become infected with this intestinal roundworm. Unlike the intestinal parasites already mentioned, hookworm infects the person by the larva penetrating the skin, frequently when people walk barefoot. There are two species of hookworm that prove generally indistinguishable, Necator americanus and Ancylostoma duodenale which both attach to the intestinal wall and suck blood. Very heavy hookworm infections can result in severe anemia.
