Ascariasis is the most common helminthic infection, with an estimated worldwide prevalence of 25% (0.8-1.4 billion people). Usually asymptomatic, infections are most prevalent in children of tropical and developing countries, where they are perpetuated by contamination of soil by human feces or use of untreated feces as fertilizer. Symptomatic disease may be manifested by growth retardation, pneumonitis, intestinal obstruction, or hepatobiliary and pancreatic injury. In developing countries, ascariasis may exist as a zoonotic infection associated with exposure to pigs or pig manure.

Pathophysiology
Ascaris lumbricoides is the largest of the common nematode (roundworm) infections of man. The white or yellow adult is 15-35 cm long and lives 10-24 months in the jejunum and middle ileum of the intestine. Each female produces 240,000 eggs per day, which are fertilized by nearby male worms. A recent study from China showed that 45% of infected persons shed only fertilized eggs, 40% shed fertilized and unfertilized eggs, and 20% shed only unfertilized eggs. Unfertilized eggs accounted for only 6-9% of eggs shed. Fertilized eggs released into favorable soil may become infectious within 5-10 days. Eggs may remain viable in soil for up to 17 months. Infection occurs through soil contamination of hands or food, ingestion, and the subsequent hatching of eggs in the small intestine.

Second-stage larvae pass through the intestinal wall and migrate through the portal system to the liver (4 d) and then the lungs (14 d). A large exposure may produce subsequent pneumonia and eosinophilia. Symptoms of pneumonitis include wheezing, dyspnea, nonproductive cough, hemoptysis, and fever. Larvae are expectorated and swallowed, eventually reaching the jejunum, where 65 days are needed for maturation to adults.

Adult worms feed on digestion products of the host. Children with marginal diet may be susceptible to protein, caloric, or vitamin A deficiency, resulting in retarded growth and increased susceptibility to infectious diseases such as malaria. Intestinal (usually ileal), common duct, pancreatic, or appendiceal obstruction may occur from the large and tangled worms. Mean worm burden varies from more than 16 to 4 and appears related to host factors, particularly age, geophagy, and immunity. Worms do not multiply in the host. For infection to be maintained beyond the 2-year maximum lifespan, re-exposure must occur.

Ascaris lumbricoides suum, a swine nematode, may be responsible for zoonotic infection. Distinguishing this worm from A lumbricoides is difficult. A suum appears to be the predominant infection in well-developed countries with excellent sanitation. In this setting, such as Denmark or the United States, children have low worm burden and may present with only acute eosinophilia or eosinophilic liver lesions that are seen on CT scan