ENTEROBIUS VERMICULARIS (Pinworm)

GEOGRAPHICAL DISTRIBUTION

The parasite has a world wide distribution with a high prevalence in cold climates. This is not a soil transmitted infections.

MORPHOLOGY

The adult worms are small (1cm) with both ends pointed. Males have curved tail ends and are rarely seen. The ‘cervical alae’ extend right down the sides of the body so that in cut sections they are seen as two projections on either side of the body.

LOCATION IN HOST

The worm is found attached to the mucosa of the large intestine particularly the caecal area. They are not blood suckers.

LIFE CYCLE

A gravid female carries approximately 10,000 eggs in the uterus. These females migrate down the large intestine to reach the anus. This occurs during night. The female then lays the eggs on the anal verge. Following egg deposition the female dies.

The eggs are plano-convex in shape and have double walls (50*25 µm). The outer wall is albuminous and sticky. The embryos develop into infective larvae inside the eggs in 6 hours. When these eggs are ingested via contaminated fingers or via fomites they hatch in the lower part of the small intestine. The larvae move down to the large intestine to mature into adult males and females. There is no pulmonary migration of the larvae as in trichuriasis. Some infective eggs may hatch on the anal area and the larvae can move into the large intestine through the anus (retro-infection, a type of auto infection).

PATHOGENESIS AND CLINICAL FEATURES

The damage cause by the worm is not extensive. Some times they produce inflammation of the colonic mucosa leading to granulomatous condition. Complications include perforation of the gut (rarely) and migration to ectopic sites, commonest being into the female vagina. The moving females and the deposited eggs cause severe pruritus ani (itching of the anus). This may lead to loss of sleep. The infected children become irritable and may interfere with schoolwork.

EPIDEMIOLOGY

Since the lightness of the eggs, they are carried by wind and air currents. They are deposited on object found in the environment such as tabletops, doorknobs, chairs and seats. The infection makes no exception to any social class. It is a household infection and is highly prevalent where there is overcrowding such as in refugee camps, armed forces camps, prisons, hostels, orphanages etc. Generally one member of the family gets infected soon the all in the household acquire the infection.

MODES OF TRANSMISSION

1. Direct infection from the anal and perianal region by fingernail contamination ( a type of autoinfection).

2. Exposure and ingestion of viable eggs on soiled night clothes and other contaminated objects in the environment.

3. By way of contaminated dust from bed clothes, toys and furniture.

4. Retro-infection (see above)

DIAGNOSIS

Old methods include NIH (National Institute of Health, USA) swab and Graham’s Scotch Tape method. A simple ‘cello-tape’ strip can be used. The procedure should be carried out in the morning before washing the anal area or bathing. Precautions against contamination of hands of the examiner should be taken.

PREVENTION AND CONTROL

1. cut finger nails short

2. wash hands with soap and water regularly

3. avoid the scratching the anal region

4. treat every one in the household including boarders and servants ( in institutions all inmates including staff have to be treated at the same time)

5. Following treatment all bed linen and personal clothes should be washed and dried in the hot sun.

6. Mats and mattresses should be exposed to hot sun

7. Wet mopping of floors and surfaces should be done

TREATMENT

All affected individuals should be given a dose of mebandazole 100 mg,

Albendazole 400 mg, or pyrantel pamoate 11 mg/kg or maximum, 1 g, with the same treatment repeated after 10 to 14 days. Treatment of household members is also advocated to eliminate asymptomatic reservoirs of potential re-infection. But still it is very difficult to cure because it may be impossible to avoid re-infection.