Causative agent of ascariosis
Occurrence. The human large roundworm occurs worldwide. The number of infected persons is estimated at 1.38 billion (WHO, 1998). The main endemic regions, with prevalence rates of approx. 10-90%, include countries in Southeast Asia, Africa, and Latin America. Autochthonous infections are rare in central Europe.
Parasite and life cycle. The adult ascarids living in the small intestine (ascaris: worm) are 15-40 cm in length, about as thick as a pencil and of a yellowish pink color (Fig. 10.12). The sexually mature females produce as many as 200 000 eggs per day, which are shed with feces in the unembryonated state. The round-to-oval eggs are about 60 x 45 im in size, have a thick, brownish shell and an uneven surface (Fig. 10.13). At optimum temperatures of20-25 °C with sufficient moisture and oxygen, an infective larva in the egg develops within about three to six weeks.
Human infections result from peroral ingestion of eggs containing larvae, which hatch in the upper small intestine and penetrate into the veins of the intestinal wall. They first migrate hematogenously into the liver and then, four to seven days p.i., into the lungs, where they leave the capillary network and migrate into the alveoli. Via tracheopharyngeal migration they finally reach the digestive tract, where they further differentiate into adults in the small intestine. The prepatent period lasts for seven to nine weeks. The lifespan of these parasites is 12-18 months.
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