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Ascariasis

Ascariasis is infection caused by Ascaris lumbricoides, an intestinal roundworm. The infection often causes no symptoms but may cause fever, coughing, and wheezing, followed by abdominal cramps. In severe infections, children may not grow normally, or worms can block the intestine, resulting in severe pain and vomiting.

Ascariasis is the most common roundworm infection in people, occurring in over 1.4 billion people worldwide. The infection is common in areas with poor sanitation and often occurs in tropical or subtropical areas.

Infection begins when a person swallows Ascaris eggs, often in contaminated food. Food is contaminated through contact with soil that has been contaminated by human stool (feces) containing the eggs. Ascaris eggs are hardy and can survive in the soil for years.

Once swallowed, Ascaris eggs hatch and release larvae in the intestine. Each larva migrates through the wall of the small intestine and is carried through the lymphatic vessels and bloodstream to the lungs. Once inside the lungs, the larva passes into the air sacs (alveoli), moves up the respiratory tract and into the throat, and is swallowed. The larva matures in the small intestine, where it remains as an adult worm. This process takes 2 to 3 months. Adult worms range from 6 to 20 inches (15 to 50 cm) in length and from 1/10 to 2/10 inch (0.25  0.51 cm) in diameter. They live 1 to 2 years. Eggs laid by the adult worms are excreted in stool, develop in the soil, and begin the cycle of infection again when they are ingested.

Symptoms and Diagnosis

Many people who have ascariasis do not develop symptoms. However, the migration of larvae through the lungs can cause fever, coughing, wheezing, and sometimes blood in phlegm (sputum). A large number of worms in the intestine can cause abdominal cramps and, occasionally, a blockage of the intestine, most commonly in children living in areas with poor sanitation. A blockage can cause nausea, vomiting, abdominal swelling (distention), and abdominal pain. Sometimes adult worms migrate to the mouth or nose, are vomited up, or passed in the stool—situations that can be psychologically distressing. Adult worms occasionally block the opening into the appendix, biliary tract, or pancreatic duct, producing severe abdominal pain. Infected children may not grow or gain weight normally.

Ascariasis is diagnosed by identifying eggs or adult worms in a stool sample or, rarely, by seeing adult worms that have migrated to the throat or nose. If computed tomography (CT) or ultrasonography is done for other reasons, adult worms may be seen. Rarely, the effects of larvae migrating through the lungs can be seen on a chest x-ray.

Prevention and Treatment

The best strategies for preventing ascariasis include using adequate sanitation and avoiding uncooked and unwashed foods, particularly in areas where human feces is used as fertilizer.
To treat a person with ascariasis, a doctor prescribes albendazole or pyrantel pamoate. However, because these drugs may harm the fetus, they should not be taken by pregnant women.





http://www.merck.com/mmhe/sec17/ch196/ch196d.html

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