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Page last reviewed: 13/07/2011

Threadworms (Enterobius vermicularis) are small worm parasites that infect the intestines of humans.

Threadworms, sometimes known as pinworms, are white and look like a small piece of thread. The adult female worm can be 8-13mm long, and the male worm 2-5mm long. Adult worms live for up to six weeks.

Threadworms do not always cause symptoms, although some people will experience itchiness around their anus and vagina (in females). The itchiness is particularly noticeable at night and can disturb sleep.

See Threadworms - symptoms for more information.

The life cycle of threadworms

Female threadworms lay tiny eggs around the anus and, in females, around the vagina. This usually happens at night, when the infected person is asleep.

When laying eggs, the female worm also secretes a mucus that causes the person to scratch the area. The eggs can then become stuck on the person's fingertips and under their fingernails. From here, they can be transferred to the mouth or to surfaces and clothes. It is then possible for other people to touch an infected surface and transfer the eggs to their mouth.

Threadworm eggs can survive for up to three weeks before hatching. If they hatch around the anus, they can re-enter the bowel. If the eggs have been swallowed, they will hatch in the intestine. After two weeks, the worms reach adult size and begin to reproduce, starting the cycle again.

See Threadworms - causes for more information.

How common are threadworms?

The threadworm is the most common worm parasite that infects children. It is estimated that up to 40% of children under 10 years of age may be infected with threadworms.


Threadworms are spread through poor hygiene. Usually, if one member of a household is infected, others will be too. It is therefore necessary to treat the entire household to prevent re-infection. Following strict hygiene measures for up to six weeks can help prevent infections returning.

Treatment for threadworms is available from your GP or from pharmacies. Treatment may not be suitable for everyone, so always follow the manufacturer's instructions.

See Threadworms - treatment for more information.

Treatment does not kill threadworm eggs. Good hygiene is the only way to prevent eggs from spreading and causing another infection.

Humans are thought to be the only host for threadworms. Animals cannot catch or pass on threadworms, unless the eggs are transported on the animal's fur after human contact.

The intestines are the part of the digestive system between the stomach and the anus that digests and absorbs food and liquid.
The anus is the opening at the end of the digestive system where solid waste leaves the body.
The bladder is a small organ near the pelvis that holds urine until it's ready to be passed from the body.

The threadworm, sometimes called a pinworm, is the most common worm parasite that infects children.

Should my child still go to school?

The Health Protection Surveillance Centre advises that children should still go to school if they have a threadworm infection. Schools and nurseries should follow good hygiene practices to limit the spread of infection. This will include:

  • cleaning toys and equipment
  • encouraging children to wash their hands regularly
  • using dedicated laundry facilities

Page last reviewed: 13/07/2011

Threadworms often go unnoticed, but symptoms can include:

  • intense itching around the anus, particularly at night when the female worms are laying eggs
  • itching around the vagina
  • disturbed sleep as a result of the itching, which can make you irritable

If you have a severe infection or persistent infections, threadworms can cause:

  • loss of appetite
  • weight loss insomnia
  • (difficulty getting to sleep or staying asleep)
  • severe irritability

As threadworms do not always cause symptoms, all members of your household should be treated, even if only one person notices symptoms.

The anus is the opening at the end of the digestive system where solid waste leaves the body.
Loss of appetite
Loss of appetite is when you do not feel hungry or want to eat.

Page last reviewed: 13/07/2011

Threadworms are usually passed on through poor hygiene. 

Female threadworms lay tiny eggs around the anus. When a person scratches this area, the threadworm eggs can be transferred on their fingers from their anus to their mouth or to another surface. If someone else touches that surface and then touches their mouth, they will also become infected.

The cycle of infection

If you have threadworms, the female worms lay eggs around your anus or vagina (in females).

One female threadworm can release thousands of eggs, which are too small to be seen by the naked eye.

Transferring eggs

These eggs can be transferred from your anus to:

  • underwear or clothes
  • bed sheets
  • towels
  • carpet

When laying eggs, the female threadworm also releases a mucus that causes itching. If you scratch the affected area, eggs may be transferred on to your hands. Once the eggs are on your hands, they can be transferred to anything you touch, including:

  • children's toys
  • kitchen utensils
  • toothbrushes
  • furniture
  • surfaces in the kitchen or bathroom

Swallowing the eggs

Threadworm eggs can survive on surfaces for up to three weeks. If you touch the eggs, they will be transferred onto your hands. If you then touch your mouth, you may swallow the eggs.

It is also possible to breathe in the eggs and then swallow them. The eggs are so small that they can become airborne, for example if you shake a towel or bed sheet that has eggs on it.

Once you have swallowed the eggs, they hatch in your intestines. After around two weeks, the threadworms can reproduce and the cycle of infection will start again. 

Poor hygiene

Threadworms are most common in small children because they are not fully aware of the importance of good hygiene and they often forget to wash their hands. Children can also prolong their infection by continually swallowing fresh eggs. As children regularly come into close contact with one another and share toys or hold hands while playing, re-infection is easy.

Threadworms are often found in families, particularly in crowded conditions. The risk of transmission between family members can be as high as 75%.

Threadworms only infect humans and cannot be caught from animals. However, there is a small risk that threadworms can be caught from household pets if the animal's fur becomes contaminated with eggs. This could happen during petting or stroking. The eggs may then be passed onto the next human who touches the animal's fur.

The anus is the opening at the end of the digestive system where solid waste leaves the body.

Page last reviewed: 13/07/2011

If you think your child may have threadworms, you could look for the worms and begin treatment yourself. Alternatively, you can see your GP. Always see your GP if you think you may have threadworms and:

  • you are pregnant
  • you are breastfeeding
  • you have a baby under three months old

Spotting the worms

Threadworms are difficult to see because of their size and colour. The worms are white and not much larger than a staple. Female worms are usually 8-13mm long, and the male worms are 2-5mm long. The male worm is rarely seen because it remains inside the intestine.

The best time to try to see the threadworms is at night, when the female worms come out to lay their eggs. If you are trying to see whether your child has threadworms, the best time to look is two to three hours after they have fallen asleep. The worms may be visible on your child's underwear, pyjamas or bed sheets.

Sometimes, worms can be found in stools (faeces). The worms look like small pieces of white cotton thread, which is where their name comes from.

Threadworm eggs are not visible to the naked eye.

Your GP

If you see your GP, they will normally be able to diagnose threadworms from the symptoms of itching around the anus and itching at night. Your GP or a nurse may also take a moistened swab from around your anus to pick up eggs. The sample can then be sent to a laboratory for testing.

The anus is the opening at the end of the digestive system where solid waste leaves the body.

Stool (also known as faeces) is the solid waste matter that is passed from the body as a bowel movement.

The intestines are the part of the digestive system between the stomach and the anus that digests and absorbs food and liquid.

Page last reviewed: 13/07/2011

To successfully treat threadworms, the entire household must be treated, even if not everyone has symptoms.

The aim of treatment is to get rid of the threadworms and prevent re-infection.

You can do either of the following:

  • follow a strict hygiene method (see below) for six weeks
  • take medication and follow a strict hygiene method for two weeks

Some medications may be available from your pharmacy without prescription. Always follow the manufacturer's instructions because these types of medications may not be suitable for everyone. See your GP if you have threadworms and:

  • you are pregnant
  • you are breastfeeding
  • you have a baby under three months old

Hygiene method

Strict hygiene measures can be used to clear up a threadworm infection and reduce the likelihood of re-infection. As the life span of the threadworms is approximately six weeks, it is important that these hygiene methods are followed for six weeks. Everyone in the household must follow this advice.

  • Wash all sleepwear, bed linen, towels and cuddly toys when you are first diagnosed (this can be done at normal temperatures but make sure that the washing is well rinsed).
  • Thoroughly vacuum and dust the whole house, paying particular attention to the bedrooms. Continue to vacuum regularly and thoroughly. 
  • Carefully clean the bathroom and kitchen by damp-dusting surfaces and washing the cloth frequently in hot water. Continue to clean bathroom and kitchen surfaces regularly and thoroughly. 
  • Avoid shaking any material that may have eggs on it, such as clothing or bed sheets, as this may transfer the eggs to other surfaces.
  • Do not eat food in the bedroom because you may end up swallowing eggs that have been shaken off the bedclothes.
  • Keep your fingernails short.
  • Discourage nail biting and finger sucking. In particular, make sure that children do not suck their thumb.
  • Wash your hands frequently and scrub under your fingernails, particularly before eating, after going to the toilet and before and after changing a nappy.
  • Wear close-fitting underwear at night and change your underwear every morning.
  • Wearing cotton gloves at night may help prevent scratching while you are asleep.
  • Bath or shower regularly, particularly first thing in the morning, and make sure you clean around your anus and vagina to remove any eggs. 
  • Ensure that everyone in your household has their own face flannel and towel. Avoid sharing towels.
  • Keep toothbrushes in a closed cupboard and rinse them thoroughly before use.

Even after the infestation has cleared up, continue with good general hygiene measures, such as washing your hands after going to the toilet. Children can easily pick up another threadworm infection from friends or at school, and good hygiene may help prevent another outbreak.


Medication can be used to treat threadworms. It should be taken by everyone in the household. The high risk of transmission (around 75% between family members) means that everyone is likely to be infected, even if they do not have any symptoms.

See the information below if you are pregnant, breastfeeding or you have a baby under three months old.

The most common medications that are used to treat threadworm infections are:

  • mebendazole
  • piperazine


Mebendazole prevents the threadworms from being able to absorb glucose, which means they will die within a few days.

Mebendazole is the preferred treatment option for children over two years of age. It can be bought over-the-counter or prescribed by your GP, and is available as a chewable tablet or in liquid form.

As re-infection is very common, a second dose of mebendazole may be prescribed to be taken after two weeks. Follow the dosage information that is provided on the label or in the patient information leaflet that comes with the medicine.  
In rare cases, mebendazole can cause abdominal pain or diarrhoea, particularly if the infection of threadworms is severe.

Pregnancy and breastfeeding

If you are pregnant or breastfeeding, the hygiene method is preferred for treating a threadworm infection. Medication for threadworm is not usually recommended.

See your GP if you are more than three months pregnant or if you are breastfeeding, and you have problems treating the threadworm infection using only the hygiene method. In certain circumstances, your GP may consider prescribing medication.

Babies under six months of age

Medication is not advised for babies under six months of age. Instead, follow the hygiene method. Make sure that you wash the baby's bottom gently but thoroughly every time you change their nappy. Also wash your hands before and after changing their nappy. 

Mebendazole isn't licensed for use in children under 2 years of age, but GPs may decide to prescribe it off-label for children over 6 months.

Page last reviewed: 13/07/2011

Threadworms can cause intense itching around the anus and vagina. Continual scratching can cause your skin to become inflamed. In rare cases, if the skin is broken, bacteria can enter the wound, leading to another infection. See your GP for more advice if you think you may have another infection.

A threadworm infection outside the intestine is very rare, but has been known to occur in the:

  • vagina 
  • womb
  • pelvic peritoneum (lining on the inside of the abdomen)
  • stomach area
  • liver
  • lungs


Re-infection is common if threadworms are not treated. Threadworms can become an ongoing problem. If this happens, it can lead to more serious problems, including:

  • insomnia (difficulty getting to sleep or staying asleep)
  • bed wetting
  • weight loss

The anus is the opening at the end of the digestive system where solid waste leaves the body.

Bacteria are tiny, single-celled organisms that live in the body. Some can cause illness and disease and some are good for you.

The liver is the largest organ in the body. Its main jobs are to secrete bile (to help digestion), detoxify the blood and change food into energy.

The uterus or womb is a hollow, pear-shaped organ in a woman where a baby grows during pregnancy.

The vagina is a tube of muscle that runs from the cervix (the opening of the womb) to the vulva (the external sexual organs).

Page last reviewed: 13/07/2011

Threadworms can be prevented by always maintaining high standards of hygiene (see Threadworms - treatment for more information).

Children should be taught to wash their hands regularly, particularly after going to the toilet and before meals. Also, clean your kitchen and bathroom surfaces regularly.

Encouraging children not to scratch the affected area around their anus will help prevent re-infection. It will also help avoid any secondary infections as a result of the scratching.

As itching frequently occurs at night, it may be possible to scratch the area while you are asleep without realising. Therefore, always wash your hands in the morning and ideally have a shower or bath.

The anus is the opening at the end of the digestive system where solid waste leaves the body.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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