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Because of its size and availability, Ascaris (Gr. askaris, intestinal worm) is usually selected as a type for study in zoology, as well as in experimental work. Thus it is probable that parasitologists know more about structure, physiology, and biochemistry of Ascaris than of any other nematode. This genus includes several species. One of the most common, A. megalocephala, is found in the intestine of horses. Ascaris lumbricoides is one of the most common nematode parasites found in humans; recent surveys have shown a prevalence of up to 25% in some areas of the southeastern United States, and more than 1.27 billion people are infected worldwide. The large roundworm of pigs, A. suum, is morphologically close to A. lumbricoides, and they were long considered the same species.
A female Ascaris may lay 200,000 eggs a day, carried by the host’s feces. Given suitable soil conditions, embryos develop into infective juveniles within 2 weeks. Direct sunlight and high temperatures are rapidly lethal, but the eggs have an amazing tolerance to other adverse conditions, such as desiccation or lack of oxygen. Shelled juveniles can remain viable for many months or even years in soil. Infection usually occurs when eggs are ingested with uncooked vegetables or when children put soiled fingers or toys in their mouths. Unsanitary defecation habits “seed” the soil or drinking water, and viable eggs remain long after all signs of the fecal matter have disappeared. Thus infection rates tend to be highest in areas where waste treatment practices do not control these factors.
When a host swallows embryonated eggs, the tiny juveniles hatch. They burrow through the intestinal wall into veins or lymph vessels and are carried through the heart to the lungs. There they break out into alveoli and are carried up to the trachea. If the infection is large, they may cause a serious pneumonia at this stage. On reaching the pharynx, juveniles are swallowed, passed through the stomach, and finally mature about 2 months after the eggs were ingested.
In the intestine, where they feed on intestinal contents, worms cause abdominal symptoms and allergic reactions, and in large numbers they may cause intestinal blockage. Parasitism by Ascaris is rarely fatal, but death can occur if the intestine is blocked by a heavy infestation. Perforation of the intestine with resultant peritonitis is not uncommon, and wandering worms may occasionally emerge from the anus or throat or may enter the trachea or eustachian tubes and middle ears. Infection rates tend to be highest in children, and males tend to be more heavily infected than females, presumably because boys are more likely to ingest dirt.
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