The causative agent Enterobius vermicularis (pinworm, threadworm), is a less than 13 mm long, just under 1 mm wide white nematode worm that lives in the colon. It is exclusively a human parasite and transmits directly from human to human.
The infestation is obtained by ingestion of mature eggs. The female worms come out from the colon through the anus, often during sleep, to lay eggs onto the perianal skin.
The eggs remain viable for weeks e.g. in the bedclothes.
The time from infestation to symptoms is approximately 1-2 months.
The infections are mostly found in children at the age of 3 to 10, but they are not rare in older children or even in adults.
Clinical picture
The most usual symptom is perianal pruritus, particularly at night. This may cause awakenings and enuresis during the night.
Scratching may lead to bacterial infections in the perianal region.
Anorexia or irritability may occur.
Pinworm infection has sometimes been suggested to be associated with appendicitis but the causal association is questionable.
As rare complications, girls may develop vulvovaginitis, urethritis or even salpingitis. These are caused by pinworms that migrate in wrong direction on the perineal skin. Symptomatic urethritis and salpingitis are probably caused by intestinal bacteria transported by the worms.
Diagnosis
The samples to detect pinworm are taken in the morning before morning wash or toilet visit.
The primary method to obtain a sample is to use a cotton swab soaked in saline solution to rub the margins of the anus. The swab stick is also inserted 0.5 cm into the anal orifice.
Sensitivity of the perianal sampling is increased by taking several samples in the mornings after nights when symptoms have occurred.
The eggs should not be searched for by faecal samples.
If a clearly moving 8-13 mm long pinworm is seen in the perianal area the diagnosis is certain without laboratory samples. An immobile structure resembling a worm seen in the perianal region is not sufficient for the diagnosis. However, if such a structure is suspected to be a worm it can be sent for microscopic investigation if necessary.
Treatment
Pyrvine 7.5-10 mg/kg as a single dose or mebendazole 100 mg as a single dose (for over 12 year olds).
Also albendazole (400 mg as a single dose) is effective. Use of albendazole may require a special license in some countries.
During pregnancy, the first-line drug is pyrvine. Caution has been excercised in relation to using mebendazole and albendazole during pregnancy because their teratogenicity has been noted in experiments in rats and rabbits, albeit their teratogenicity has not been reported in humans. The safety of pyrvine during pregnancy has not been systematically researched but it is considered safe both during pregnangy and during breast-feeding.
The treatment should always be repeated after 2 weeks.
The bed is usually vacuum-cleaned or aired and the bedclothes and towels changed on the day after the treatment. Other cleaning measures are probably not useful.
The whole family should be treated at the same time, including asymptomatic family members who are potential carriers of the worm.
If at least one third of children in a day care facility are infected it is advisable to treat the whole group. Pinworms are not a cause to refuse admittance to the day care.
Pinworm infection readily recurs, mainly due to the high pinworm prevalence in the children under school-age. Clear drug resistance has not been encountered so far.
Prevention
Hand washing and toilet hygiene are emphasized; children are reminded of the importance of hand washing after using the toilet and before meals.
Children's fingernails are kept short.
Sleeping in the same bedclothes as a pinworm carrier should be avoided.
Because pinworm infections, even if recurrent, are not associated with inadequate housecleaning and because the infection is not dangerous, excessive cleaning sometimes associated with the pinworm problem should be avoided.
Deworming is not always succesful with the first treatment courses given 2 weeks apart. The most common reasons include reinfections and that the drug is non-absorbable, which makes it is effective only inside the intestines and, consequently, it is possible that some worms survive e.g. in the appendix.
Before relying on repeated treatments, it is essential to ensure that the relapse or reinfection is real. Observing clearly moving worms is certainly a sure sign of infection, but the parents' description is not alway fully reliable.
In an unclear situation or in a clinically problematic recurring cycle of infection, it is worth verifying the relapse by a laboratory investigation of an anal sample.
References
Sodergren MH, Jethwa P, Wilkinson S et al. Presenting features of Enterobius vermicularis in the vermiform appendix. Scand J Gastroenterol 2009;44(4):457-61. [PubMed]
World Health Organization (anonymous). Breastfeeding and maternal medication - Recommendations for Drugs in the Eleventh WHO Model List of Essential Drugs, 2003. WHO and UNICEF http://apps.who.int/iris/handle/10665/62435.