The giant intestinal roundworm, Ascaris lum-bricoides, is a common parasite with worldwide distribution. Adult worms are 15-35 cm (6-14 inches) long and reside in the lumen of the small intestine. Sometimes, however, they are passed in the feces and, if vomited into the oral cavity, may exit from the host's mouth or nostrils; thus they have been known to medical observers for millennia. Female worms produce up to 200,000 fertilized eggs daily, which are passed in the feces. Eggs incubate in soil for at least 2-3 weeks to produce an infective larval stage within them. The eggs are resistant to chemicals, desiccation, and extreme temperatures but mature or "em-bryonate" most rapidly in warm, moist, shady conditions in clay soils. People become infected by eating embryonated eggs in contaminated food or water; or, in the case of toddlers, infection occurs by direct ingestion of eggs with dirt. Poor rural sanitation and the use of human feces for fertilizer obviously favor transmission. Mature eggs hatch in the small intestine, and the larvae then undergo a remarkable migration in the host. They penetrate the intestinal wall and are carried in blood or lymph vessels to the liver and heart, and then the lungs. Here they break out into the air sacs, develop, and molt for about 3 weeks, and then climb up the trachea to the throat, where they are subsequently swallowed to establish themselves as adults in the small intestine.

This nematode was known to ancient writers in China, India, Mesopotamia, and Europe and was present in pre-Columbian America. The World Health Organization has estimated that between 800 and 1,300 million people harbor an average of six worms each. The true figure may be even higher. Surveys have demonstrated infection in more than 50 percent of sampled populations in countries such as Bangladesh, Brazil, China, Colombia, India, Iran, Kenya, Mexico, Tanzania, and Vietnam, and the rate approaches 100 percent in many rural areas. In China, it was estimated that the 1947 Ascaris population produced 18,000 tons of eggs a year; they may be even more productive today. The worm is also common in developed countries, although improved sanitation has greatly reduced prevalence in recent decades.

Symptoms of ascariasis vary widely. As is often true of helminthic infections, low worm-loads may cause few or no symptoms. Large numbers of larvae in the lungs may produce ascaris pneumonitis, with symptoms resembling pneumonia. Allergic reactions can cause asthma attacks. Larvae can reach atypical (ectopic) sites such as the brain, eye, or kidney, where they may produce grave, life-threatening conditions, but such events are fortunately rare. Adult worms in the intestine can cause fever, abdominal discomfort, diarrhea, and allergic reactions to their proteins. Fever may induce worms to wander to the larynx, where they can cause suffocation, or to exit the mouth or nostrils. Heavy infection robs the host of nutrients, and tangled masses of worms can result in fatal intestinal obstruction if not treated promptly. Intestinal ascariasis is especially serious in young children. In the Third World, ascariasis may produce signs of protein-energy malnutrition in many children and often retard their growth. Even if severe effects occur in only a small percentage of cases, the ubiquity of the worm makes it an important cause of morbidity in many countries.

K.. David Patterson

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