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Pinworm infection is the most common type of roundworm infection in the United States. Roundworms are parasites that use your body as a host to stay alive and reproduce. Inside your body, the pinworm's microscopic eggs hatch and grow into adults, measuring up to 13 millimeters, or about half an inch. The worms mature in your intestine and then travel through your digestive system to lay eggs in the anal area.

Many pinworm infections cause no symptoms or cause only mild digestive problems. But if you're infected with hundreds of worms, more serious symptoms and complications can occur.

The good news is that pinworm infections can be treated effectively with medications. There are simple steps you can take to prevent infection with pinworms, mostly involving good hygiene.

Mild pinworm infections — with only a small number of adult worms in your body — may cause no symptoms. Symptoms occur with moderate or heavy infections.

A few weeks after you ingest pinworm eggs, the mature female worms migrate from your intestine to the area around your anus, where they lay their eggs. This typically happens at night. The maturing and reproduction stages may cause:

• Itching of the anal or vaginal area
• Insomnia, irritability and restlessness
• Vague gastrointestinal symptoms, such as intermittent abdominal pain and nausea

When female pinworms lay eggs in the area around your anus, the anal area usually begins to itch — sometimes severely. When you scratch the itchy area, the microscopic eggs cling to your fingers and then transfer to other surfaces, such as food, liquids or other people. Pinworm eggs can survive for several weeks on these surfaces. A new infection starts when the eggs reach another person's mouth by contaminated drink, food or hands. You can also reinfect yourself by unknowingly swallowing the eggs again.

Risk factors for pinworm infection include:

• Poor hygiene. Scratching the anal area directly and poor hand washing are to blame for the spread of pinworm infection.
• Warm climate. Pinworms thrive in milder climates, and higher numbers of infections have been reported in the southern United States and in other countries with warm temperatures year-round.
• Living with children. Pinworm infection is more common in children and is easily spread to others in the family.

If you have a mild case of pinworm infection, you may not experience any symptoms and may never need medical attention. Many children simply outgrow mild pinworm infections, when all female worms have traveled out of the anus so that reinfection doesn't occur.

If you have mild abdominal pain along with insomnia or irritability, or you're experiencing consistent or severe anal or vaginal itching, consult your doctor.

• Tape test. The most accurate test for pinworms is a cellophane tape test. Your doctor will press a piece of clear plastic tape against the skin around your anus and then examine the tape under a microscope for pinworm eggs. Because the parasite typically lays eggs at night, you may be able to get a good sample by testing yourself first thing in the morning before defecating or bathing. Press a piece of clear plastic tape to your anus, and then take the sample to your doctor. Several tape samples may be necessary to establish the diagnosis.
• Direct sighting of a worm. Sometimes the presence of pinworms is more obvious, as is the case when an adult worm is seen in the area around your anus.

Typical pinworm infections don't cause serious problems. In rare circumstances, complications can develop from a heavy infection, including:

• Urinary tract infection. More common in females, urinary tract infections sometimes are caused by heavy pinworm infestation. The parasite can also migrate to the bladder, causing cystitis.
• Infection of the peritoneal cavity. In women or girls, the parasite may travel from the anal area up the vagina to the uterus, fallopian tubes and the area around the pelvic organs. This can cause vaginitis, endometritis or other problems.
• Weight loss. When a large number of adult pinworms are living in your intestine, they can cause enough abdominal pain and take enough nutrients that you will lose weight.

Mild pinworm infection, or those with no symptoms, may never need treatment. For symptomatic infections, medication is almost always effective in eliminating the parasite. Because children so easily spread pinworms to their families, your doctor may prescribe medication to all members of your household to prevent infection and reinfection.

Anti-parasite (antihelminthic) medications are the first line of treatment against a pinworm infection. The most common anti-parasite medications for pinworms are mebendazole (Vermox) and pyrantel (Antiminth, Pin-Rid). They work by killing the adult worms as well as the larvae and eggs to prevent reinfection. You may have mild gastrointestinal side effects during the course of treatment, and you may need to take multiple doses to get rid of the pinworms completely. If anal or vaginal itching is severe or interfering with your sleep, your doctor may prescribe a soothing ointment or cream to use until the pinworms are eliminated.

Pinworm reinfection is common, so your doctor may remind you to avoid scratching the anal area, take special care to keep your fingernails clean, and thoroughly launder any potentially infected clothing or sheets.

The best defense against pinworms is common sense and good hygiene. Follow these tips to avoid infection:

• Practice good hygiene. Pinworm infections are most commonly spread by fingers contaminated with parasite eggs. To reduce your risk of getting or spreading an infection, be sure to wash your hands thoroughly after a bowel movement, avoid scratching your anal area and keep your fingernails clean.
• Keep your household clean. Because pinworm eggs can remain infective for several weeks on a surface, wash bedding and clean bathrooms regularly, and change your undergarments daily.