Causative agent of enterobiosis (oxyuriosis)
Occurrence. The pinworm occurs in all parts of the world and is also a frequent parasite in temperate climate zones and developed countries. The age groups most frequently infected are five- to nine-year-old children and adults between 30 and 50 years of age.
Parasite, life cycle, and epidemiology. Enterobius vermicularis which belongs to the Oxyurida has a conspicuous white color. The males are 25 mm long, the females 8-13 mm. The long, pointed tail of the female gives the pinworm its name.
Sexually mature pinworms live on the mucosa of the large intestine and lower small intestine. Following copulation, the males soon die off. The females migrate to the anus, usually passing through the sphincter at night, then move about on the perianal skin, whereby each female lays about 10 000 eggs covered with a sticky proteinaceous layer enabling them to adhere to the skin. In severe infections, numerous living pinworms are often shed in stool and are easily recognizable as motile worms on the surface of the feces.
The eggs (about 50 x 30 im in size) are slightly asymmetrical, ellipsoidal with thin shells (Fig. 10.1, p. 544). With their sticky surface they adhere to skin and other objects. Freshly laid eggs contain an embryo that develops into an infective first-stage larva at skin temperature in about two days. Eggs that become detached from the skin remain viable for two to three weeks in a moist environment.
Infection occurs mainly by peroral uptake of eggs (each containing an infective larva) that are transmitted to the mouth with the fingers from the anal region or from various objects. The sticky eggs adhere to toys and items of
everyday use or are disseminated with dust. In the intestinal tract, larvae hatch from the ingested eggs, molt repeatedly, and develop into sexually mature pinworms in five to six weeks. "Retroinfection" is also conceivable, whereby infective larvae would be released at the anus to migrate back into the intestine.
Pathogenesis and clinical manifestations. The pinworms living on the large intestine mucosa are fairly harmless. Occasionally, different stages of the pin-worm penetrate into the wall of the large intestine and the appendix or migrate into the vagina, uterus, fallopian tubes, and the abdominal cavity, where they cause inflammatory reactions.
The females of Enterobius produce in particular a very strong pruritus that may result in nervous disorders, developmental retardation, loss of weight and appetite, and other nonspecific symptoms. Scratch lesions and eczema-tous changes are produced in the anal area and can even spread to cover the entire skin.
Diagnosis. A tentative diagnosis based on clinical symptoms can be confirmed by detection of pinworms spontaneously excreted with feces and eggs adhering to the perianal skin (Fig. 10.1). Standard stool examination techniques are not sufficient to find the eggs. Egg detection by the "adhesive tape method" has proved most efficient (p. 622).
Therapy and prevention. The following drugs are effective: albendazole, me-bendazole, and pyrantel. Reinfections are frequent, so that treatment usually should be repeated once or more times, extended to include all potential parasite carriers (e.g., family members, kindergarten members), and combined with measures, the purpose of which is to prevent egg dissemination: washing the perianal skin (especially in the morning), covering it with ointments, washing the hands, hot laundering of underwear, and cleaning contaminated objects with hot water.
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