Pinworm Infection

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What is a pinworm infection?

A pinworm infection is a parasitic intestinal infestation. The medical name for it is enterobiasis. It’s very common worldwide and is the most common intestinal worm infection in the United States. Estimates suggest about 42 million Americans—or 13% of the population—have pinworms. In other areas of the world, the infection rate is as high as 61%.

Pinworms are small roundworms—or nematodes—measuring about 0.25 to 0.5 inches long. They are white, thin and tapered, like a pin. Their formal name is Enterobius vermicularis. Other common names for them include seatworm and threadworm.

Humans are the only hosts for these worms. Pinworm eggs are microscopic and enter the body when people breathe them in or ingest them. They hatch in the small intestine and move to the large intestine where they attach their heads to the inside intestinal wall. After about 1 to 2 months, females move to the anus where they lay eggs in the outside skinfolds. Once the female lays eggs around the opening of the anus, a pinworm infection causes itchiness in the area. This usually occurs at night and can cause restlessness, which interrupts sleep.

Pinworms spread via the oral-fecal route—the eggs transfer from the anus to the mouth. This can happen directly, by scratching or touching the area and then your mouth, or indirectly, by touching contaminated bedding, clothing, food, or other objects and then your mouth.

School-aged children are most likely to have a pinworm infection. Pinworm is highly contagious, so household members and caretakers are at high risk of becoming infected. Kids often develop a constant pinworm infestation by continually transferring eggs into their mouths. This process is called autoinfection.

Fortunately, pinworm infection treatment is straightforward and effective. Over-the-counter and prescription antiworm medicines can clear the infection. Doctors often recommend treating the whole family to prevent the spread. Frequent laundering of clothing, towels and bedsheets can help prevent reinfection.

A pinworm infection doesn’t cause serious harm. Heavy infections may trigger bedwetting due to urethral irritation and can lead to infection of the female genitals in rare cases. See a doctor if your child complains of anal itching, especially at night or if you notice your child frequently scratching their bottom or genitals.

What are the symptoms of a pinworm infection?

Pinworm infection symptoms are usually mild and sometimes there are no symptoms at all. The main one is itching in the anal area, especially at night. During sleep, pinworms travel to lay eggs in the skinfolds outside the anus. This can lead to restless and interrupted sleep.

You may be able to see the worms in the anal area if you look a few hours after your child falls asleep. Sometimes, worms may also be visible in your child’s underwear or in the toilet. However, they aren’t usually present in stool samples. To diagnose them, your doctor may ask you to apply sticky tape to the perianal skin first thing in the morning to collect the eggs.

If the intestines are heavily infected, abdominal pain and nausea can occur. In girls, a pinworm infection can spread to the vagina and cause itching and discharge.

While a pinworm infection isn’t harmful, it can lead to secondary bacterial infections from scratching. Also, autoinfection and spreading occurs very easily. See a doctor promptly if you suspect a pinworm infection. Treatment usually clears the problem in a short time.

What causes a pinworm infection?

The roundworm, Enterobius vermicularis, causes a pinworm infection. These worms only live in humans and spread easily from person to person via oral-fecal transmission. Infection happens when you transfer the microscopic eggs to your mouth. When an infected person scratches, the eggs deposit on their fingers and under their fingernails. Contaminated fingers can now transfer the eggs to surfaces, clothing, personal items, food, drinks, and other people. The infected person can keep reinfecting themselves by touching their mouths. Pinworm eggs can survive on surfaces for 2 to 3 weeks.

Once you swallow the eggs, they hatch in the small intestine and move to the large intestine. An adult pinworm lives with its head attached to the inside of the intestine wall. Females move to the anus to lay eggs and the cycle repeats.

What are the risk factors for a pinworm infection?

Pinworm infections affect people of all ages and socioeconomic levels. School-aged children between 5 and 10 years of age are most likely to have a pinworm infection. The microscopic eggs spread easily to other people, putting family members, caretakers, friends and schoolmates at risk of becoming infected. This also makes pinworm infections more common in crowded or institutional living.

Reducing your risk of a pinworm infection

Handwashing is the number one way to reduce the risk of getting a pinworm infection. Teach your children to wash their hands after using the toilet or touching their bottom and before eating. Caretakers should also wash their hands after changing or handling dirty diapers.

Other habits to lower your risk of a pinworm infection and prevent the spread of an active infection include:

  • Avoiding scratching the anal area
  • Changing underwear, pajamas, towels, washcloths and bedding daily, laundering them in hot water, and drying them with high heat
  • Discouraging finger or thumb sucking and nail biting
  • Keeping fingernails short and clean
  • Showering, not bathing, in the morning to wash away eggs laid during the night

Friends and siblings of an infected child should not co-bathe or share personal items, such as washcloths and towels until treatment is complete.

How is a pinworm infection treated?

There are over-the-counter and prescription medicines that can treat a pinworm infestation. Treatment involves two doses you take two weeks apart. The medicine paralyzes the worms and causes them to detach from the intestinal wall. The body eliminates them with a bowel movement. The medicine is only effective for live adult worms. It will not work on eggs or larvae (immature worms after hatching). The second dose is necessary to eliminate worms that hatch after the first dose.

Because spreading and reinfection happen very easily, all household members and caretakers should receive treatment at the same time as the infected child. Strict hygiene is necessary for everyone being treated until both doses are complete. Follow the strategies for preventing an infection as outlined above, with an emphasis on handwashing.

Anal itching may continue for about a week after the pinworm infection clears. Your doctor may recommend a cream to relieve the itching and prevent scratching of the area.

What are the potential complications of a pinworm infection?

A pinworm infection usually isn’t harmful. However, it is possible to get a secondary bacterial infection from scratching. Although rare, it is also possible for the infection to affect the vagina in girls and cause itching and discharge. Should the worms enter the vagina, it is possible for them to move up into the uterus, fallopian tubes, and other pelvic structures. Other rare complications include urinary tract infection and weight loss.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Jul 14
  1. Pinworm Infection. Mayo Foundation for Medical Education and Research. https://www.mayoclinic.org/diseases-conditions/pinworm/symptoms-causes/syc-20376382
  2. Pinworm Infection: Biology. Centers for Disease Control and Infection. https://www.cdc.gov/parasites/pinworm/biology.html
  3. Pinworm Infection: Epidemiology and Risk Factors. Centers for Disease Control and Prevention. https://www.cdc.gov/parasites/pinworm/epi.html
  4. Pinworm Infection FAQs. Centers for Disease Control and Prevention. https://www.cdc.gov/parasites/pinworm/gen_info/faqs.html
  5. Pinworm Infections. Nemours Foundation. https://kidshealth.org/en/parents/pinworm.html
  6. Pray WS. Pinworm: a common human parasite. US Pharm. 2006;12:10-15.

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