Wednesday, 29 March 2017

Pinworm – A Common Intestinal Worm

Have you ever had a pinworm infection? Probably. Infecting 400 million people, pinworm is the most common nematode parasite of humans. It owes its success largely to the simplicity of its life cycle.

Pinworm eggs can only be seen through a microscope.
Adapted from Susan A Secretariat Catalina Maya Rendón

Pinworm (sometimes called seatworm) infection is infestation with the tiny roundworm Enterobius vermicularis. The infection, called enterobiasis or oxyuriasis, is common in children because they tend to be careless about hand washing and often put fingers in their mouths. It is also more common in women than in men, probably because women spend more time caring for children and therefore come in contact with the worm more often.

The female pinworm is 8mm to 13mm long (five-sixteenths to half an inch), while the male is so tiny that he would be very difficult to see without a microscope.

The Life Cycle of Pinworm

Human infection with E. vermicularis usually begins when an infective egg is swallowed:
  • Eggs hatch in the duodenum (small intestine), releasing larvae.
  • Larvae mature to adult male and female worms, which are found in the greatest numbers in the cecum (at the beginning of the large intestine).
  • Male and female worms mate and each female produces five thousand to fifteen thousand microscopic eggs.
  • Females travel along the length of the large intestine, ultimately exiting the body via the anus.
  • The majority of eggs are deposited on the perianal skin, though eggs are also laid internally and are passed in stool.
  • Eggs are distributed to clothing, bedding, and towels, and spread via the hands to household objects such as doorknobs, handles and taps. They also become airborne and settle almost anywhere with dust.
  • At normal body temperature, a pinworm egg matures within about six hours and an active larva can be found within.
  • Infective eggs are swallowed or inhaled and the life cycle begins again.

Life cycle of Enterobius vermicularis CDC
Some human parasites, such as Ascaris lumbricoides, the large intestinal roundworm, are infective only after the eggs have spent time maturing in warm moist soil. Others, such as Taenia saginata, the beef tapeworm, must pass through another host before they can infect humans again. These requirements limit parasites to places where the climate is warm or the other host is available. Because E. vermicularis passes directly from person to person, however, it spreads unchecked through human populations all over the world.




Symptoms of Pinworm Infection

We hear anecdotal reports of pinworm infections that sound positively agonizing. But many infections (as many as a third) cause no symptoms or very mild discomfort. In rare cases enterobiasis has more serious consequences.

The most common and revealing symptom is itching in the perianal area, possibly a result of the movements of female worms. Sensitivity to the eggs is also thought to develop with repeated infection. Scratching in response to the itching leads to lesions, increased irritation, and possibly bacterial infection.

Many people with symptoms complain of abdominal pain, and small lesions may develop in the intestine causing inflammation and sometimes infection. Other general symptoms include insomnia, irritability, grinding the teeth during sleep, loss of appetite, nausea, and even vomiting. Female worms sometimes migrate away from the anus and invade the vulva or vagina. Very rarely, worms penetrate intestinal tissue with serious consequences.

Diagnosis and Treatment of Pinworm Infection

Enterobiasis is usually confirmed when adult female worms and/or eggs are recovered from the perianal skin. Stool samples for parasitology sometimes yield worms and eggs as well, but this is not the best specimen choice.

Antiparasitic drugs clear up enterobiasis. Consult a physician or pharmacist, however, before self-treating. Unfortunately, reinfection typically occurs because other family members are also infected and infective eggs remain in the house. Physicians generally recommend that all family members take the drug, with a repeat treatment two weeks later. Because eggs in the environment dry out fairly quickly and it takes about a month for a swallowed larvae to mature and produce eggs, the second treatment should prevent a recurrence.

Further reading about pinworm


Roberts, Larry S., and John Janovy Jr. 2009. Gerald D. Schmidt & Larry S. Roberts’ Foundations of Parasitology. Boston, McGraw Hill.

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