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Threadworms, also known as pinworms, are tiny parasitic  worms that hatch eggs in and infect the large intestine of humans.

Threadworms are the most common type of worm infection in the UK, and they are particularly common in young children, infecting up to half of all children under the age of 10.

Threadworms are white and look like a small piece of thread. You may notice them around your child's bottom or in your or your child's stools (poo). Threadworms do not always cause symptoms. Some people notice itchiness around their bottom (back passage) or vagina, which can be worse at night and can sometimes disturb sleep.

Read more about the symptoms of threadworms.

When to see your GP

If you think you or your child may have threadworms, you can usually treat the infection yourself with medication available at pharmacies without prescription.

You only usually need to see your GP if you think you have threadworms and you are pregnant or breastfeeding, or if you think your child has threadworms and they are under two years old. This is because the treatment recommended in these circumstances is usually different to what is recommended for most people.

How are threadworms spread?

Threadworms lay their eggs around an infected person's anus (back passage), usually at night. When laying the eggs, the female worm also secretes a mucus, which causes itching.

If the eggs become stuck on the person's fingertips when they scratch they can be transferred to their mouth, or onto surfaces and clothes. Other people who touch an infected surface can then transfer the eggs to their mouth.

Threadworm eggs can survive for up to three weeks before hatching. If the eggs hatch around the anus, the newly born worms 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.

Read more about what causes threadworms.

Treating threadworms

If you or your child has threadworms, everyone in your household will need to be treated because the risk of the infection spreading is high. This includes people without any symptoms of a threadworm infection.

For most people, treatment will involve a single dose of a medication called mebendazole to kill the worms. Another dose can be taken after two weeks, if necessary.

During treatment and for a few weeks afterwards, it is also important to follow strict hygiene measures to avoid spreading the threadworm eggs.

This involves things such as regularly vacuuming your house and thoroughly washing your bathroom and kitchen.

If you are pregnant or breastfeeding, hygiene measures are usually recommended without medication. This is also often the case for young children.

Read more about treating threadworm infections.

Preventing threadworms

It is not always possible to prevent a threadworm infection, but you can significantly reduce your risk by always maintaining good hygiene and encouraging children to do the same.

Children should wash their hands regularly, particularly after going to the toilet and before mealtimes. Kitchen and bathroom surfaces should be kept clean.

Encouraging your children not to scratch the affected area around their anus or vagina (in girls) will help prevent re-infection and reduce the risk of the infection spreading to other people.

Should work or school be avoided?

A threadworm infection should be treated as soon as it's identified, but it is not necessary to stay off work or school.

However, it's important to inform the school or nursery so that they can follow good hygiene practices to limit the spread of infection. These will include:

  • cleaning toys and equipment
  • encouraging children to wash their hands regularly
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Threadworms often go unnoticed by people who have them, but symptoms can include:

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

In some cases, you may spot threadworms on your bed clothes or sheets at night, or you may notice them in your stools. The worms look like threads of white cotton about one centimetre long.

Severe or persistent threadworm  infections can cause:

  • loss of appetite
  • weight loss
  • skin infection around the anus, if bacteria enter the scratches caused by itching (wearing cotton gloves while sleeping may help prevent this)
  • insomnia (difficulty getting to sleep, or staying asleep)
  • bedwetting

In extremely rare cases, threadworms can spread outside the intestine to places such as the vagina or womb (in girls or women), the urinary tract and the liver.

Seeking medical help

If you suspect that you or your child may have threadworms, speak to your pharmacist about treatment.

See your GP if you think you have threadworms and you:

  • are pregnant or breastfeeding
  • your child has threadworms and they are under two

Read more about treating a threadworm infection.

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A threadworm infection is usually passed from person to person as a result of poor personal hygiene.

A female threadworm can lay thousands of tiny eggs around the anus or vagina. While laying eggs, the female threadworm also releases a mucus that causes itching.

Scratching the anus or vagina, or wiping them after going to the toilet, can result in the eggs becoming stuck on your fingertips or under your fingernails.

If you don't wash your hands, the eggs can then be transferred to your mouth or onto food or objects, such as toys and kitchen utensils. If someone else eats the contaminated food or touches the contaminated object and then touches their mouth, they will become infected.

After the eggs have been swallowed they will pass into a person's intestine, where they will hatch. After about two weeks, the threadworms will have grown into adults, will reproduce and the cycle of infection will start again.

Transferring eggs

Threadworm eggs can be transferred from your anus (or vagina) to anything that you touch including:

  • bed sheets and bed clothes
  • flannels and towels
  • children's toys
  • kitchen utensils
  • toothbrushes
  • furniture
  • kitchen and bathroom surfaces

Threadworm eggs can survive on surfaces for up to three weeks.

They can be swallowed after touching contaminated surfaces and they can also be breathed in and then swallowed. This can happen if the eggs become airborne, if, for example, you shake a contaminated towel or bed sheet.

Poor hygiene

Threadworm infections are most common in small children because they they often forget to wash their hands after going to the toilet and they often share things like toys with other children.

People who are in close contact with someone who has a threadworm infection, for instance living in the same house, are also at a high risk of infection. This is why all members of a household where someone has a threadworm infection need to be treated.

Animals and Pets

Threadworm only infect humans and cannot be caught from animal faeces. 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.

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

This is because the risk of the infection spreading to other people in the same household is very high.

The aim of treating threadworms is to get rid of the parasites and prevent re-infection. This will usually involve a combination of medication to kill the worms and strict hygiene measures to stop the spread of the eggs.

The main medication used to treat threadworms is available from your local pharmacy without prescription. You should always follow the manufacturer’s instructions because these types of medications may not be suitable for everyone.

You only usually need to see your GP if you think that you have threadworms and:

  • you are pregnant
  • you are breastfeeding
  • your child has threadworms and they are under two years old

This is because the treatment recommended in these circumstances is not usually the same as for most other people. See below for treatment advice for pregnant and breastfeeding women and babies.


Mebendazole is the main medication used to treat threadworm infections. It can be bought over-the-counter (OTC) from your local pharmacy, or prescribed by your GP, and is available as a chewable tablet or as a liquid.

Mebendazole works by preventing the threadworms from absorbing sugar (glucose), which means they should die within a few days.

This medication is 90%-100% effective at killing the threadworms, but it doesn't kill the eggs. This is why the hygiene measures outlined below should also be followed for six weeks.

Visit your GP if the infection continues two weeks after treatment. They may recommend a second dose of medication.

In rare cases, mebendazole can cause abdominal pain or diarrhoea, particularly if the infection of threadworms is very severe.

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 at least six weeks. Everyone in the household must follow the advice outlined below.

  • Wash all sleepwear, bed linen, towels and soft 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. This will help prevent eggs being transferred to other surfaces.
  • Do not eat food in the bedroom because you may end up swallowing eggs that have beenshaken off the bedclothes
  • Keep your finger nails short. Encourage other members of your household to do the same.
  • Discourage nail biting and finger sucking - in particular, make sure that children do not suck their thumb.
  • Wash your hands frequently, and scrub under finger nails, particularly before eating, after going to the toilet, and before and after changing your baby's nappy.
  • Wear close-fitting underwear at night and change your underwear every morning.
  • Bath, or shower, regularly, particularly first thing in the morning, and make sure that you clean around your anus and vagina to remove any eggs.
  • Ensure everyone in your household has their own face flannel and towel. Do not share towels.
  • Keep toothbrushes in a closed cupboard and rinse them thoroughly before use.

Children can easily pick up another threadworm infection from friends or at school, and good hygiene may help prevent another outbreak.

Pregnancy and breastfeeding

If you are pregnant or breastfeeding, medication for threadworm is not usually recommended. To treat threadworms, use the hygiene method described above.

You should 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.

If a child under two years old has threadworms

Mebendazole is not licensed for use in children under two years of age, but GPs may choose to prescribe it ‘off-label’ (outside its licence) for children who are over six months of age.

If medication is not used, the hygiene measures outlined above are recommended instead.

Make sure that you wash the baby’s bottom gently, but thoroughly, every time that you change their nappy. Also, ensure that you wash your hands thoroughly before and after changing their nappy.

Do I need to avoid work or school?

If you or your child has a threadworm infection, it is not necessary to stay off work or school.

However, it's important to inform your child's school or nursery so they can take steps to limit the spread of infection.

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Selected links

NHS Direct Wales links


Tapeworm infections

External links

Patient UK - Threadworms 

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The information on this page has been adapted by NHS Wales from original content supplied by NHS Choices.
Last Updated: 06/11/2014 10:29:53

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