Enterobius vermicularis

Parasite Name: Enterobius vermicularis (formerly Oxyuris vermicularis ) Synonyms: pinworm, threadworm, seatworm Classification: Helminth (nematode) TaxonomyAnimalia Nematoda Secernetea Rhabditida Oxyuridae Enterobius vermicularis

Epidemiology

Pinworm is a cosmopolitan parasite with particularly high prevalence in countries with a temperate climate. It has the widest distribution of any parasitic helminth, and it is estimated that approximately 200 million people are infected internationally. It has become the most common intestinal parasite seen in a primary care setting, regardless of factors such as race, socioeconomic status, and culture. As such, pinworm serves as an exception to the general rule that intestinal parasites are uncommon in affluent societies. Prevalence in children in certain communities has been found to be as high as 61% in India, 50% in England, 39% in Thailand, 37% in Sweden, and 29% in Denmark.

Reservoir

Humans are the only natural host of pinworms, and there currently is no confirmed pinworm reservoir.

Transmission

As with many other GI nematodes, pinworms do not need to rely on a vector for transmission. Pinworm infection usually occurs via ingestion of infectious eggs by direct anus-to-mouth transfer by fingers. This is facilitated by the perianal itch (pruritis ani) induced by the presence of pinworm eggs in the perianal folds, and commonly occurs as a result of nail biting, poor hygiene, or inadequate hand-washing. Retroinfection is also possible, where some of the pinworm larvae which hatch on the anus return to the gastrointestinal tract of the original host, leading to a very high parasitic load as well as ensuring continued infestation. However, the transfer can also occur by touching contaminated surfaces, such as clothing, bed linen, and bathroom fixtures followed by ingestion, or even through inhalation or ingestion of aerosolized eggs from the aforementioned surfaces. As such, pinworm infections are easily spread among young children with the habits of nail biting and or poor hygiene, and infected children can easily spread the infection to other family members through the mechanisms listed above.
enterobius vermicularis transmission

enterobius vermicularis transmission

Life Cycle

Enterobius vermicularis Life Cycle

Enterobius vermicularis Life Cycle

 

Clinical Presentation

Pinworm infection is usually benign, and 1/3 of those infected are asymptomatic.

By far the most common clinical sign of pinworm infection is perianal or perineal itching, and the itching is usually the most severe at night. This is caused by the migration of female pinworms to the anus to lay eggs (specifically, insertion of the tail pin into the mucosa for ovideposition), and scratching leads to finger contamination and subsequently the spread of ovum to others. The scratching may also excoriate the skin and lead to secondary bacterial infections.

Secondary symptoms, which are due to disturbed sleep caused by pruritis, include anorexia and irritability.

However, pinworm infection is not always harmless, as it has been implicated in causing appendicitis (as high as 2.39% of cases in developing countries), intestinal obstruction, intestinal perforation, enterocolitis mimicking Crohn’s disease, and eosinophilic ileocolitis.

Extra-intestinal infections are also possible, and while most involve the female genital tract, infection of other sites has been documented, including the lungs, breast, liver, and spleen.

These extra-intestinal infections may lead to pruritis vulvae, urinary tract infections, postmenopausal bleeding, epididymitis, pelvic mass, chronic sialoadenitis, and unilateral salpingitis, among others. The dead parasites and eggs deposited in ectopic sites can also be responsible for the formation of granulomas and abscesses.

 

The causative agent :

The adult female has a sharply pointed posterior end, is 8 to 13 millimeters long, and 0.5 millimeter thick.

 The adult male is considerably smaller, measuring 2 to 5 millimeters long and 0.2 millimeter thick, and has a curved posterior end.
 The eggs are translucent and have a surface that adheres to environmental objects.The eggs measure 50 to 60 micrometers by 20 to 30 micrometers, and have a thick shell that is flattened on one side.
The small size and colourlessness of the eggs make them invisible to the naked eye, except in barely visible clumps of thousands of eggs.
Eggs may contain a developing 
embryo or a fully developed pinworm larva.The larvae grow to 140–150 micrometers in length.

 

enterobius vermicularis male and female

enterobius vermicularis male and female

 

An ova of Enterobius vermicularis  in stool samples

An ova of Enterobius vermicularis in stool samples

 

enterobius-vermicularis_eggs

enterobius-vermicularis_eggs

 

Diagnostic Tests :

The most common means of diagnosing pinworm infection is via the “Scotch tape” test, where a clear adhesive cellulose tape is applied to the anal area early in the morning before bathing or defecation.

This is then observed under a microscope for the presence of pinworm eggs. These eggs may also be stained blue with lactophenol cotton blue, which aids in detection and identification. The sensitivity of this test when performed for three consecutive mornings is 90%. Anal swabs (“Swube tubes:” paddles coated with adhesive material) may also be used.

Scotch tape test

Scotch tape test

 

Enterobius vermicularis ovas using scotch tape test

Enterobius vermicularis ovas using scotch tape test

Therapy

For typical pinworm infections, mebendazole can be used. This is given in single doses of 100 mg by mouth, repeated for two weeks to prevent reinfection, and is arguably the safest and most efficient therapy.
Albendazole may also be used in 100 mg doses for patients under two years old, or in doses of 400 mg for older patients. As in mebendazole, this should also be repeated for two weeks.

References :
http://www.stanford.edu

 

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