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Fig. 17.2 Life cycle of Trichuris trichiura.

Epidemiology and Pathology

The epidemiology of Trichuris (Fig. 17.2) is similar to that of Ascaris and the two parasites often cohabit in the same host, although Ascaris eggs tolerate a less moist environment. The whipworm is especially prevalent in areas of high rainfall, high humidity and dense shade.

One female Trichuris can produce 3,000-10,000 barrel-shaped ova daily (Fig. 17.3). These are passed in the stool and measure 50 x 25 micra. They gain a brownish color from impregnation of the outer portion of the thick shell by bile as they pass through the human intestine. Under optimal conditions in warm damp soil, the eggs require 2 to 4 weeks to develop into infective first stage larvae. Some eggs remain latent in the soil for 5 years and probably remain infective for at least 1 to 2 years.

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Fig.17.3. (A) Trichuris trichiura unembryonated egg in stool, iodine stained. (B) Trichuris egg with typical barrel shape and bipolar prominences suggesting plugs. X1210. AFIP 56-3352 (Courtesy of Dr. Herman Zaiman).

Man becomes infected by ingesting contaminated soil, food or water containing infective Trichuris eggs previously passed in feces. Small children aged 3 to 9 years are more often infected than adults and are more likely to develop heavy worm burdens, since they play with contaminated soil and thus transfer embryonated eggs to their mouths by their fingers. In the United States and Europe, mentally handicapped children in institutions may have high rates of infection.

After ingestion, the shell of the egg is weakened by intestinal juices in the duodenum. The larva is freed and attaches itself to the villi of the proximal small bowel for about a week to gain nourishment. Subsequently, it continues into the cecum (its normal habitat) or elsewhere in the large intestine and, after molting four times, grows into the adult form attached to the mucosa. The adult worm takes 2 to 3 months to develop, after which female worms may produce 2,000-10,000 eggs per day for up to 5 or more years. In severe infections, Trichuris may infect the entire colon, including the rectum and appendix; it is rarely found in the terminal ileum. Once established, Trichuris may live in the bowel for several years, but because of its firm attachment to the intestinal mucosa the infection may be totally unsuspected and no investigation may be made unless the patient also harbors other parasites, such as the freely moving Ascaris. If reinfection does not occur, most of the worms will die within 3 years.

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