Microbiology

Parasitic diseases of the digestive system usually involve worms, also known as helminths. In most cases, the worms multiply in the system, and when the worm burden becomes high, the symptoms of disease ensue. Poor sanitation contributes to the occurrence of parasitic (helminthic) infections.

Pinworm disease. Pinworm disease is caused by the small round‐worm Enterobius vermicularis. Infections occur after ingestion of pinworm eggs. The eggs hatch, and adult females lay their eggs near the body surface, particularly near the anus. Young children are usually those infected. Several drugs are available for treating pinworm infection.

Roundworm disease. Roundworm disease is due to Ascaris lumbricoides. The infection begins with the ingestion of roundworm eggs, which yield roundworms that burrow through the intestinal wall to the bloodstream, ultimately reaching the lungs. The worms reenter the digestive system when they are coughed up from the lungs and swallowed. A large number of eggs cause respiratory distress, and intestinal obstruction may also develop due to heavy worm burdens.

Hookworm disease. Hookworm disease may be caused by either of two species of roundworms: Ancylostoma duodenale (the Old‐World hookworm) or Necator americanus (the New‐World hookworm). The larvae of the hookworm penetrate the human skin, usually through the foot, and the hookworms pass through the bloodstream to the lungs, from where they are coughed up and swallowed to the digestive system. The worms use their hooks to hold fast to the intestinal lining. Then they suck the blood and multiply. Infestations lead to anemia, with much fatigue and weakness. Hookworm disease is common where people go barefoot.

Strongyloidiasis. Strongyloidiasis is caused by the roundworm Strongyloides stercoralis. The worms penetrate the human skin and pass from the blood to the lungs, and eventually to the digestive system. Infestations result in high worm burdens and intestinal blockages. Invasion of the intestinal wall may accompany the disease, especially in immunocompromised individuals.

Whipworm disease. Whipworm disease is caused by the round‐worm Trichuris trichiura, called the whipworm because its body resembles a whip. Eggs are ingested in food and water, and they hatch in the digestive tract to become adults. The adults lay their eggs, which are passed in the feces. Heavy worm burdens in the intestine cause irritation, inflammation, and other symptoms of obstruction.

Trichinosis. Trichinosis is due to the roundworm Trichinella spiralis. This parasite infects the muscle tissues of pigs and is usually passed to humans by improperly cooked pork products. The worms enter the human bloodstream from the intestine and form cysts in the muscles. Heavy worm burdens in the rib muscles cause severe pain. The worms also migrate to the heart muscle, diaphragm, and lungs. Proper cooking of pork products is paramount in preventing infection.

Taenisias. Taenisias is caused by a tapeworm, which is a type of flatworm. Two tapeworms are important in humans: Taenia solium, the pork tapeworm, and Taenia saginata, the beef tapeworm. Humans are infected when they eat contaminated pork or beef, respectively. Adult worms attach to the intestinal lining using their sucker devices and hooks. As the tapeworm lengthens, it adds segments called proglottids. Eventually the worm may be several feet long. Proglottids break free and are released in the feces to infect pigs or cattle that feed in the soil. Heavy worm burdens may cause intestinal blockage and abdominal pain.

Hydatid disease. Hydatid disease is caused by a type of small flat‐worm called a tapeworm. The tapeworm involved is Echinococcus granulosus. Humans are infected by contact with animal feces (especially, that of canines), and the worms form hydatid cysts in the tissues. The large cysts cause damage to organs such as the liver or lung.

Liver fluke disease. The liver can be infected by a leaflike flatworm known as a fluke. Liver fluke disease is due to the sheep liver fluke known as Fasciola hepatica, or it may be caused by the Chinese liver fluke referred to as Clonorchis sinensis. The flukes are ingested with water plants such as watercress. The worm larvae migrate to the liver where they develop into adults. Liver damage and jaundice accompany the disease. Outside the body, the flukes live in snails, the intermediary hosts.