Classification: Taxonomic ranks under review (cf. Encyclopedic Reference of Parasitology, 2001, Springer-Verlag)

Metazoa (Animalia) (multicellular eukaryotes, animals)
Nemathelminthes (nematodes)
Secernentea (Phasmidea) (with chemoreceptors known as phasmids)
Oxyurida (pinworms; pointed tails)
Oxyuroidea (eggs attached around anus of host)

Family: Oxyuridae
Oxyurid worms are commonly called “pin-worms” because of their characteristic tapering shape and pointed tails. They have simple direct life-cycles involving faecal-oral transmission of eggs containing infective larvae. The eggs, however, are oviposited around the anus (perineum) where they are subsequently dislodged and ingested by their hosts. Pinworms are common in many animal species, and infections in humans may cause intense pruritis (itching), irritability, insomnia and sometimes diarrhoea.

Enterobius vermicularis [this species causes perianal pruritis (enterobiasis) in humans]

Parasite morphology: These worms form three developmental stages: eggs, larvae and adults. The eggs are elongate-oval in shape, measure 50-60µm in length by 20-30µm in width, and are characteristically asymmetric about the long axis being distinctly flattened on one side. Infective larvae develop rapidly within the eggs. Adult worms appear as elongate whitish tubes with pointed tails. They have three lips surrounding the anterior mouth, a large oesophageal bulb, and a conspicuous anterior cuticular inflation (swollen head). Male worms are 1-4 x 0.2-0.4mm in size, have a single spicule 100-140µm long, and their posterior ends are strongly curved ventrally. Female worms are 8-13 x 0.3-0.6mm in size and have pronounced slender pointed tails.

Host range: The species E. vermicularis is the most common worm found in humans worldwide, particularly in temperate regions. They are commonly found as group infections in children, in families and in institutions (where contact between individuals is high and hygiene may be low). They are estimated to infect some 400 million people, but few countries consider them to be of public health significance due to their low pathogenicity. Infections are more irritating than debilitating, causing embarrassment, low morbidity and rarely mortality. However, individual families often spend considerable time and money trying to rid themselves of infections. Numerous pin-worm species have been described from a range of mammals, birds, reptiles, amphibians, insects and millipedes, but they appear to be highly host-specific. Curiously, dogs and cats do not become infected with pin-worms so companion animals should never be considered as sources of human infection.

Site of infection: Adult worms tend to congregate in the ileocaecal region of the gut where they attach to the mucosa, but they may wander throughout the intestines from the stomach to the rectum. Fertilized female worms migrate out through the anus and deposit eggs of the perianal skin.

Pathogenesis: While many infections remain asymptomatic, worm burdens may increase with time resulting in damage to the intestines by adult worms and/or damage to the perineum resulting from egg deposition. Adult worms attach to the mucosa and feed on intestinal content, bacteria and possibly epithelial cells, causing minute ulcerations which may lead to mild catarrhal inflammation with diarrhoea, eosinophilia and bacterial infection. More commonly, however, infections are characterized by intense perianal itching (pruritis ani) caused by host sensations and reactions to female worms depositing sticky eggs on the skin. Patients vigorously scratch themselves attempting to relieve the itching, but in doing so, often cause skin damage, bleeding, bacterial infection and intensified itching. Heavy infections in children may cause restlessness, irritability, anorexia, insomnia, nightmares, bed-wetting, nausea and vomiting. Occasionally, wandering worms have been associated with appendicitis, vaginitis, and rarely, extra-intestinal granulomas in ectopic sites.

Mode of transmission: Pinworms have direct life-cycles involving the oral ingestion of eggs containing infective larvae. The eggs, however, are not excreted with faecal material, but are attached to the perianal skin. Such transmission is therefore not strictly faecal-oral, but rather contaminative, involving the transfer of eggs to the mouth via host behaviours or inanimate objects. Gravid females migrate out through the anus onto the perineum, particularly during the night, and leave trails of eggs (up to 10,000) as they crawl about. After oviposition, the females die whereas the males die soon after copulation. Larvae develop within the eggs within six hours and become infective. The eggs are dislodged by host scratching and contaminate hands, bedding, clothing, toys, and furniture. They are very light and easily disseminated with house dust by the slightest of air currents. They remain viable in cool moist conditions for up to one week. Following ingestion, the eggs hatch in the small intestine and the larvae migrate to the large intestine and mature over 2-6 weeks. Alternatively, eggs trapped in perianal folds may hatch and the larvae may enter the intestines directly via the anus (process called retro-infection). Occasionally, larvae may enter the vulva and infect the vagina of women. The parasite may complete its whole life-cycle in 2-13 weeks, and infections may become progressively heavier due to continual parasite uptake (through auto-infection, re-infection and retro-infection).

Differential diagnosis: Worm eggs are rarely found in faeces so conventional coprological examination techniques are not used. Instead, infections are best diagnosed by the macroscopic detection of adult worms or the microscopic detection of eggs on the perineum. Motile worms may be seen on perianal skin glistening under bright light when close visual examinations are conducted during the night or early in the morning. Adult worms may sometimes be observed on the surface of fresh stool samples. Alternatively, sticky-tape may be quickly applied to the perianal skin first thing in the morning and then stuck onto a glass slide for microscopic examination of adherent eggs (aptly-named perianal sticky-tape test). Parents of infected children should be trained to collect appropriate samples to respect patient rights and privacy (especially involving minors) and alleviate any shame or embarrassment.

Treatment and control: Anthelmintic treatment for pin-worm infections is readily available from most pharmacies. The drug of choice is mebendazole, although albendazole, levamisole and pyrantel pamoate are also effective. Piperazine has been used for many years but requires a longer course of treatment. Treatment should be repeated after about 10 days to kill any newly-acquired worms. It is advisable to institute whole group treatment where appropriate, so that other group, cohort or family members do not continue to act as sources of infection. To avoid constant re-infection, it is imperative that strict personal hygienic precautions are introduced, particularly frequent hand-washing. Household decontamination is difficult as infective eggs can survive for many days in cool moist house dust and for a few days on toys or furniture. Nonetheless, clothes, bed linen and towels should be laundered in hot water, dusty areas should be well vacuumed and potentially contaminated surfaces should be cleaned. While the eggs are very resistant to many disinfectants, they are susceptible to desiccation in dry conditions.