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Epidemiology and Pathology

The adult C. philippinensis is a small slender nematode or roundworm (Fig. 14.1). The male worms average 2.6 mm in length; the female worms are somewhat larger, averaging 3.6 mm in length. The eggs in utero are ovoid or peanut-shaped and operculated with flattened bipolar plugs. Superficially they resemble the eggs of the whipworm, Trichuris trichiura, but have broader shoulders and a striated shell.

Fig. 14.1. Capillaria philippinensis, adult female, showing narrow anterior and wider posterior portions. Note stichosome (thick arrow) and eggs in the uterus (thin arrow) (From J. Cross, in D.H. Connor and F.W. Chandler, eds. Pathology of Infectious Diseases. Appleton & Lange, Stamford, Conn., 1997)

The life cycle of C. philippinensis includes small fresh- or brackish-water fish as the intermediate hosts and fish-eating birds as the usual definitive hosts. In the provinces of northwestern Luzon along the South China Sea, where the disease is endemic, raw fresh-water fish, known as "birut," "bagsang", and "bagsic," constitute a staple of the family diet. Experimental infection of these fish with the parasite has been accomplished, with the fish being subsequently ingested by monkeys who acquired the infection and passed eggs. Both oviparous and larviparous female parasites have been found at autopsy in the issues of humans dying of this disease, which indicates that autoinfection occurs within the human host. Thus the parasite can carry out its life cycle entirely within the human intestine.

Autopsy on the first victim, who died of extreme emaciation after 3 weeks of intractable diarrhea, revealed innumerable parasites in the lumen of the small and large intestine. The adult worms of Capillaria philippinensis are more usually found partially embedded in the mucosa of the small intestine, primarily the jejunum, having developed there from larvae within one to two months after ingestion. They may be present in enormous numbers, with 40,000 worms found at autopsy in one patient. They do not penetrate beyond the muscularis mucosae and are probably intraepithelial, like the closely related Trichuris and Trichinella. The intestinal villi become flattened and the crypts of Lieberkühn atrophy. The parasites cause a mild inflammatory response with edema of the basement membrane, distention of mucous glands, and infiltration of the lamina propria with plasma cells, lymphocytes, and occasional eosinophils (Figs. 14.2 and 14.3). In severe infections the small bowel wall is thickened and indurated and the bowel is distended with gray watery fluid. Occasionally parasites have been noted in the lumen of the larynx, esophagus, stomach and colon, and rarely near the portal triads of the liver.

The mechanism by which this nematode causes protein-losing enteropathy and malabsorption remains unknown. Pathological changes may occur in distant parts of the body, primarily due to the effects of malnutrition and electrolyte depletion. Lobar pneumonia, lung abscesses, congested spleen, and damaged kidneys have been reported.

Fig. 14.2. Capillaria philippinensis. Multiple tangential sections of adult worms in the human small bowel mucosa. The worms live in the lumen of the glands or crypts and sometimes the anterior ends of the worms penetrate into the lamina propria, causing infiltration with lymphocytes and plasma cells (Courtesy of Dr. Herman Zaiman).

Fig. 14.3. Autopsy specimen of human intestine showing numerous sections of C. philippinensis (arrows) and debris.

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Copyright: Palmer and Reeder