Describe Enterobius vermicularis with its transmission, diagnosis and treatment.

The Parasite and its Transmission

• Enterobius vermicularis (pinworm, threadworm) is a small white nematode — the female about 10 mm and the male 3 mm — living in the caecum, appendix and adjacent colon
• It is the commonest helminth infection in temperate countries and is very common in Indian children, particularly in crowded households, schools and institutions. It is not related to poor sanitation in the way the soil-transmitted helminths are, and occurs in every social class
• The gravid female migrates out of the anus at night and deposits thousands of eggs on the perianal skin, then dies. The eggs become infective within about 6 hours — far faster than any other helminth egg
• Modes of transmission — autoinfection by scratching the perianal area and transferring eggs to the mouth on fingers and under nails, which is why infection persists indefinitely in one child; direct person-to-person spread; and inhalation or ingestion of eggs from bedding, clothing and household dust, where they remain viable for up to 2 to 3 weeks. retroinfection, in which larvae hatch on the perianal skin and migrate back into the anus, also occurs
• The egg is characteristic — oval and PLANO-convex (flattened on one side), thin-shelled and containing a coiled larva

Clinical Features and Diagnosis

• Perianal pruritus, worst at night, is the cardinal symptom, caused by the migrating female and the local reaction to eggs. It leads to scratching, excoriation, secondary bacterial infection, disturbed sleep, irritability and restlessness in a child
• Vulvovaginitis and dysuria in girls, from worms migrating forward into the genital tract; enuresis
• Uncommon complications — appendicitis (worms are frequently found in appendicectomy specimens, though a causal role is debated), and ectopic granulomas in the peritoneum, fallopian tube or elsewhere
• Most infections are asymptomatic, and eosinophilia is absent, since the worm does not invade tissue
• Diagnosis is by the cellophane tape (Graham) test — clear adhesive tape is pressed against the perianal skin first thing IN the morning, before washing or defaecation, then mounted on a slide and examined for eggs. Three consecutive morning specimens detect about 90% of infections
• Stool examination is useless — eggs are deposited on the perianal skin, not passed in faeces, and are found in fewer than 5% of stool specimens. This is the single most important practical point, and a request for "stool for ova and parasites" will miss the diagnosis
• Adult worms may be seen on the perianal skin at night or on the surface of stool, and are what parents commonly describe

Treatment and Applied Aspects

• Albendazole, mebendazole or pyrantel pamoate as a single dose, repeated after 2 weeks to kill worms that have matured from eggs ingested since — because the drugs do not act on the eggs
• Treat the whole household at the same time, symptomatic or not; this is what determines success
• Hygiene measures on the day of treatment — wash all bedclothes, nightwear and towels; damp-dust rather than sweeping, which raises eggs into the air; cut the child's nails short; discourage nail-biting and thumb-sucking; morning bathing or showering to remove eggs deposited overnight; and handwashing before meals and after using the toilet
• Close-fitting nightwear reduces contamination of the hands and bedding during sleep
• Reassure the family; pinworm is harmless, extremely common, and carries no implication of neglect — parents frequently feel blamed, which matters when their cooperation with household measures is what determines the outcome
• Consider it in an irritable child sleeping badly, and in a girl with recurrent vulvovaginitis; both presentations are commonly investigated at length before anyone applies a piece of tape
• Do not send a stool sample; it will be negative in over 95% of infections and its negativity is then wrongly taken to exclude the diagnosis
• Repeat the dose at 2 weeks in every case; the drugs are not ovicidal, and a single dose leaves the eggs already swallowed to mature
• Damp-dust rather than sweep on the day of treatment, since sweeping raises eggs into the air where they can be inhaled
• Pinworm is harmless but genuinely distressing, and the sleeplessness and irritability it causes in a child are reason enough to treat it properly rather than dismissively