Chapter 327. Enterobiasis (Pinworm)

Enterobiasis is caused by the pinworm Enterobius vermicularis, a strictly human parasite infecting the gastrointestinal tract. Infection occurs worldwide, and clustering of cases in families is common. Ingested eggs with first-stage larvae hatch in the duodenum, and the larvae develop into adults in the cecum, where they mate. The gravid female detaches from the cecal mucosa and migrates down the large bowel, usually passing out the anus onto the perianal and perineal skin, leaving a trail of eggs on the surface of the skin. Yellow-white female adult pinworms measuring 8 to 13 mm may be seen emerging from the rectum of infected children, most often around 10 or 11 pm. In approximately 5% of patients, eggs are deposited in the bowel and may be found in feces. Generally, the worm dies after ovipositing is completed, so repeated infections are the result of autoinfection or reinfection from other environmental sources. There is no good evidence that retrograde infection occurs.1

The eggs average 55 μm by 35 μm and appear flattened on one side and convex on the other. They are fully mature and infective 3 to 8 hours after being deposited, but at normal room temperature, less than 10% of eggs live for 48 hours.

In the United States, infection rates in young school children vary from 10% to 45%. Infection is unrelated to poor sanitary facilities or tropical climates. Young girls have pinworm more frequently than boys of the same age, and whites are more often infected than African Americans. Infection is most common between early fall and late spring, perhaps related to transmission in schools. For unknown reasons, some individuals seem to be predisposed or vulnerable to reinfection. Unlike soil-transmitted helminths such as Ascaris, enterobiasis is more common in urban settings.

Infection occurs by ingestion of embryonated eggs excreted in the stool of infected persons and may occur by hand-to-mouth transmission or by oral contact with infected fomites, such as toys, bedding, or clothing. The eggs average 55 μm by 35 μm and appear flattened on one side and convex on the other. They are fully mature and infective 3 to 8 hours after being deposited, but at normal room temperature, less than 10% of eggs live for 48 hours. Ingested eggs with first-stage larvae hatch in the duodenum, and the larvae develop into adults in the cecum, where they mate. The gravid female detaches from the cecal mucosa and migrates down the large bowel, usually passing out the anus onto the perianal and perineal skin, leaving a trail of eggs on the surface of the skin. Yellow-white female adult pinworms measuring 8 to 13 mm may be seen emerging from the rectum of infected children, most often around 10 or 11 pm. In approximately 5% of patients, eggs are deposited in the bowel and may be found in feces. Generally, the worm dies after ovipositing is completed, so repeated infections are the result of autoinfection or reinfection from other environmental sources. There is no good evidence that retrograde infection occurs.1

Pinworms rarely produce serious pathology, and many infections are asymptomatic. Perianal and perineal pruritus are the most common complaints.2 Although pruritus probably results from crawling worms, some patients with heavy pinworm infections and many worms in the rectum have little or no itching. Pruritus may provoke such severe scratching that local bleeding, secondary pyogenic infection, and lichenification can occur. Whether pinworms are a primary cause of appendicitis remains unsettled; most pathologists consider their presence in an acutely inflamed appendix to be incidental, although infections in the colon with associated gastrointestinal cramping and diarrhea have been reported. Vaginal infection in young girls is common and may be associated with vaginitis and discharge. Pinworms occasionally have been found in the fallopian tubes, resulting in intraabdominal ectopic migration and symptomatic granulomatous inflammation in the peritoneal cavity.

Nocturnal perianal pruritus strongly suggests pinworm infection, especially in children. Small, creamy-white worms are often found if the perianal region is examined when the child is awakened by itching (Fig. 327-1). Ova are not often seen in the stools, and the cellophane tape swab technique is the diagnostic method of choice (Fig. 327-2). A 6-cm piece of transparent (not translucent) cellophane tape is folded with its sticky side out over the end of a wooden tongue blade and then firmly applied against either side of the perianal region. Next, the tape is placed sticky side down on a microscope slide, which can be examined for pinworm ova. The swabs should be taken 2 to 3 hours after going to bed or in the morning immediately before the patient gets out of bed. Slides from specimens collected on consecutive days may be sealed and stored in the refrigerator until delivered. Neither eosinophilia nor serologic tests are useful for diagnosis.

As infection often is present in several members of a household, each family member should be examined, or the entire family should be treated simultaneously. Otherwise, reinfection may occur. Mebendazole (100 mg) or albendazole (400 mg) as a single dose given twice initially and 2 weeks later, are treatments of choice.3 However, experience with these anthelminthics in children younger than 2 years of age is limited. The later dose reduces the risk of repeated infection by autoinoculation. Pyrantel pamoate is an alternative, but is rarely necessary because the less toxic drugs are effective. Repeated infections are common and should be treated in the same fashion as initial infections. Vaginitis is self-limited and does not require separate treatment.

Parents and patients should be reassured that pinworms are ubiquitous and that the infection is not a reflection of poor hygiene or the result of an unclean home. Strict attention to hygiene, especially in the days immediately following treatment, is useful to prevent reinfection. Good hand-washing is the most effective means of prevention. Bedclothes, linens, and underclothes of infected children should be handled carefully and not shaken to avoid dispersing ova into the air, and they should be laundered promptly, especially following anthelminthic treatment. Infected persons should bathe well in the morning following treatment, as this frequently removes a large number of infective eggs. Control of infection in childcare centers and schools may be difficult because of high rates of reinfection, and in some cases mass and simultaneous treatment of children and adults in institutions may be necessary.