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

Threadworms are a common type of worm infection in the UK, particularly in children under the age of 10.

The worms are white and look like small pieces of thread. You may notice them around your child's bottom or in their poo.

They don't always cause symptoms, but people often experience itchiness around their bottom or vagina. It can be worse at night and 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 a prescription.

You only need to see your GP if you think you have threadworms and you're pregnant or breastfeeding, or if you think your child has threadworms and they're under two years old. In these circumstances, the recommended treatment is usually different.

How are threadworms spread?

Threadworms lay their eggs around an infected person's anus (bottom), usually at night. Along with the eggs, the worm also secretes a mucus that causes itching.

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

Threadworm eggs can survive for up to two weeks before hatching. If the eggs hatch around the anus, the newborn worms can re-enter the bowel. Eggs that have been swallowed will hatch inside 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 as there's a high risk of the infection spreading. This includes those who don't have any symptoms of an infection.

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

During treatment and for a few weeks afterwards, it's also important to follow strict hygiene measures to avoid spreading the threadworm eggs. This includes regularly vacuuming your house and thoroughly washing your bathroom and kitchen.

If you're 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's 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.

If your child is infected, encouraging them not to scratch the affected area around their anus or vagina will help prevent reinfection and reduce the risk of the infection spreading to others.

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Threadworms often go unnoticed by people who have them.

However, they can cause intense itching around the anus (and the vagina in girls), particularly at night when the female worms are laying eggs. This can disturb sleep.

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 and are about about one centimetre long.

Severe or persistent threadworm infections can cause:

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

In very rare cases, threadworms can spread outside the intestine to the urinary tract or liver, or the vagina or womb in girls or women.

Seeking medical help

Speak to your pharmacist about treatment if you suspect you or your child may have threadworms.

See your GP if:

  • you think you have threadworms and you're pregnant or breastfeeding
  • your child has threadworms and they're under two years old

Read more about treating a threadworm infection.

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A threadworm infection is passed from person to person by swallowing threadworm eggs.

A female threadworm can lay thousands of tiny eggs around the anus or vagina. The female threadworm also releases mucus, which can cause an itchy bottom.

Scratching the anus or vagina, or wiping them after going to the toilet, can cause the eggs to stick to your fingertips or under your fingernails.

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

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

Transferring eggs

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

bed sheets and bed clothes
flannels and towels
children's toys
kitchen utensils
kitchen or bathroom surfaces

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

As well as being swallowed by a person who touches a contaminated object or surface, threadworm eggs can also be swallowed after being breathed in. This can happen if the eggs become airborne – for example, after shaking a contaminated towel or bed sheet.

Animals and pets

Threadworms only infect humans and aren't spread in animal faeces. However, there's a small risk that threadworms can be caught from pets if the animal's fur becomes contaminated with eggs after an infected person strokes it. If another person then touches the animal's fur, the eggs could be passed on to them.

Who's at risk?

Threadworm infections most commonly affect young children because they often forget to wash their hands and they share toys with other children.

People who are in close contact with someone with a threadworm infection also have a high risk of infection. This is why all members of a household need to be treated when someone has a threadworm infection.

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To treat threadworms successfully, all household members must be treated, even if they don't have any symptoms. This is because the risk of the infection spreading is very high.

The aim of treatment is to get rid of the threadworms and prevent reinfection. 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 a prescription. However, it's important to follow the manufacturer's instructions as it isn't suitable for everyone.

You only need to see your GP if you think:

  • you have threadworms and you're pregnant or breastfeeding
  • your child has threadworms and they're under two years old

This is because the recommended treatment in these circumstances usually isn't the same as that recommended for most other people.


Mebendazole is the main medication used to treat threadworm infections. It can be bought over the counter from your local pharmacy or prescribed by your GP. It's available as a chewable tablet or a liquid.

Mebendazole works by preventing the threadworms absorbing sugar, 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 severe.

Hygiene measures

Strict hygiene measures can help clear up a threadworm infection and reduce the likelihood of reinfection.

The lifespan of threadworms is approximately six weeks, so it's important that hygiene measures are followed for at least this length of time. Everyone in the household must follow the advice outlined below.

  • wash all night clothes, bed linen, towels and soft toys when you're first diagnosed – this can be done at normal temperatures, but make sure the washing is well rinsed
  • thoroughly vacuum and dust the whole house, paying particular attention to the bedrooms – this should be repeated regularly
  • carefully clean the bathroom and kitchen by damp-dusting surfaces and washing the cloth frequently in hot water – this should be repeated regularly
  • avoid shaking any material that may be contaminated with eggs, such as clothing or bed sheets – this will prevent eggs being transferred to other surfaces
  • don't eat food in the bedroom – you may end up swallowing eggs that have been shaken off the bedclothes
  • keep your fingernails short – encourage other members of your household to do the same
  • discourage nail-biting and sucking fingers – in particular, make sure children don't suck their thumb
  • wash your hands frequently and scrub under your fingernails – it's particularly important to do this 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
  • bathe or shower regularly – it's particularly important to bathe or shower first thing in the morning: make sure you clean around your anus and vagina to remove any eggs
  • ensure everyone in your household has their own face flannel and towel – don't 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, so maintaining good hygiene may help prevent reinfection.

Pregnancy and breastfeeding

Medication isn't usually recommended for pregnant or breastfeeding women. Instead, you should follow the hygiene measures above.

See your GP if you're more than three months pregnant, or if you're breastfeeding and you continue to experience problems after only taking hygiene measures. In certain circumstances, your GP may consider prescribing medication.

If a child under two years old has threadworms

Make sure you wash your baby's bottom gently but thoroughly every time you change their nappy. Also wash your hands thoroughly before and after changing their nappy.

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

If medication isn't used, the hygiene measures outlined above are recommended instead.

Do I need to avoid work or school?

It isn't necessary to stay off work or school if you or your child has a threadworm infection.

However, you should inform staff at your child's school or nursery so they can take steps to prevent the spread of infection, such as cleaning toys and equipment and encouraging children to wash their hands regularly.

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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: 09/09/2015 15:25:07

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