The figure shows an Enterobius vermicularis (pinworm) egg found in a feces sample. Generally the ovum measures 50 to 60 μm long by 20 to 40 μm wide. The figure is of an embryonated ovum, flattened on one side, with a thin colorless shell.
Diagnosis of pinworm infection is usually based on the recovery of typical eggs by means of the transparent cellophane (“Scotch”) tape preparation. With tape held between the thumb and forefingers of both hands, the outward-facing adhesive surface is pressed against the skin across the anal opening with even thorough pressure. The tape is then transferred to a glass microscope slide with the adhesive surface downward in such a way as to firmly adhere to the slide. Because adult female pinworms live in the lower portion of the intestinal tract, females often lay their eggs on the surface of the fecal mass or on the anal skin folds. As a result, the eggs are not well mixed within the feces and usually will not be detected in routine stool examinations. We visualized the E. vermicularis ovum shown here, however, in a fecal sample prepared using the iodine-stained direct smear method. After the preparation was analyzed under light microscopy, the egg was photographed with a digital camera (Leica Microsystems) using an oil immersion objective (1,000x magnification).
E. vermicularis is a parasite of the large intestine of humans and is found primarily in young children. The ability of the organism to persist for extended periods on fomites and in the external environment leads to rapid dissemination of the infection from child to child and to adults. Infections are especially common in institutional settings. A gravid worm of this nematode migrates to the anus at night and dies after laying eggs. This deposition of eggs is irritating and causes itching around the anal area, known as pruritus ani. Adult females migrating out of the anus may enter the vagina and the uterus or fallopian tubes, where they die. Disintegration of the dead worms and liberation of the eggs contained in the uterus result in an inflammatory response and granuloma formation about the eggs in these sites.
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