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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 anus (back passage) or vagina, which can be worse at night and can sometimes disturb sleep.

Read more about the symptoms of threadworms.

You can treat threadworms yourself with medication available at pharmacies. However, treatment does not kill the eggs hatched by threadworms. Good hygiene is the only way to prevent the eggs from spreading and causing further infection.

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

  • You're pregnant or breastfeeding.
  • Your child has threadworms and they are under the age of two.

How are threadworms spread?

Threadworms are spread from person-to-person as a result of poor hygiene. If one member of a household is infected, there is a high risk that other members will also be infected.

It is therefore necessary to treat the entire household and to practise particularly thorough hygiene for six weeks (this is how long the worms live) to prevent re-infection.

Read more about treating threadworm infections.

Preventing threadworms

Threadworms can be prevented from occurring by always maintaining good hygiene.

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 help to avoid a skin infection.

As itching is worse at night, wearing cotton gloves while sleeping may help.

Threadworm life cycle

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.

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.

Read more about what causes threadworms.

My child has threadworms – should they be allowed to go to school?

A threadworm infection should be treated as soon as it's identified, but the Health Protection Agency (HPA) advises that 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
  • using dedicated laundry facilities
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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.

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

  • loss of appetite
  • weight loss
  • secondary skin infection if bacteria enter the scratches caused by itching
  • insomnia (difficulty getting to sleep, or staying asleep)

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

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.

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 and 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

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-infestation is often easy.

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

Animals and Pets

Threadworm 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.

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If you think that you or your child has threadworms, you could look for the worms and begin treatment yourself. You should always visit your GP if you think that you may have threadworms and: 

  • you are pregnant,
  • you are breastfeeding
  • Your child has threadworms and they are under two years old.

Looking for threadworms

Threadworms are difficult to see because of their size and colour.  The worms look like short threads of white cotton. Female worms are usually 8-13mm long, and the male worms are shorter at  2-5mm long. The male worm is rarely seen because it remains inside the intestine.

The best time to try to find 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 2-3 hours after they have fallen asleep. The worms may be visible on your child’s night clothes or bed sheets.

Sometimes, worms can be found in stools (faeces). Threadworm eggs are not visible to the naked eye.

Visiting your GP

Your GP will normally be able to diagnose threadworms from the symptoms of itching around the anus and itching at night. 

Your GP or nurse may also take a sample from around your anus using a moistened swab to pick up eggs. The sample will then be sent to a laboratory for testing.

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

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

To do this you can either:

  • follow a strict hygiene method (as outlined below) for six weeks, or
  • take medication and follow a strict hygiene method for two weeks.

Some treatments are 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 should see your GP if you have threadworms and:

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

See below for treatment advice for pregnant and breastfeeding women and babies.

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 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, 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 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.

Even after the infestation has cleared up, you should 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 also be used to treat threadworms. It should be taken by everyone in the household.

The risk of transmission  between household members is high (around 75%) which means that everyone in the household is likely to be infected, even if they do not have any symptoms.

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

  • mebendazole, and
  • piperazine.


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

Mebendazole is the preferred treatment for children over two years of age. 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.

As threadworm re-infestation is very common, a second dose of mebendazole may be prescribed to be taken after two weeks.  You should follow the dosage information that is provided on the label, or in the patient information leaflet that comes with the medicine.

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


Piperazine paralyses the threadworms until they are passed naturally out of the bowel. It is combined with a medication called senna, which has a laxative effect (helps you empty your bowels) to expel the worms more quickly.

Piperazine and senna usually come in a sachet of powder, which you mix with a small amount of milk or water before drinking.

Piperazine can be used to treat children who are between  three months to two years of age. As re-infection is very common, a second dose may be taken after two weeks.  

Dosage information will be provided on the label, or in the patient information leaflet that comes with the medicine.

Piperazine is not recommended if you have epilepsy (a condition that causes seizures) or problems with your liver or kidneys.

Mebendazole and piperazine are between 90-100% effective at killing the threadworms, but they do  not kill the eggs. Therefore the hygiene measures outlined above should be followed for two weeks after treatment.

You should see your GP if the infection persists after using medication. They may recommend that you begin a second course of medication.

Pregnancy and breastfeeding

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

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.

Babies under three months of age

Medication is not advised for babies who are under three months of age. Instead, you should follow the hygiene method.

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.

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Re-infection is common and often occurs if threadworms are not promptly treated.

Left untreated, threadworms can lead to more serious problems such as:

  • weight loss
  • insomnia: difficulty getting to sleep or staying asleep
  • bedwetting (enuresis)

Secondary skin infection

Threadworms can cause intense itching around the anus and vagina, and continual scratching can cause your skin to become red and inflamed. In rare cases, if the skin is broken, this may lead to a secondary skin infection if bacteria enter the wound. See your GP or call NHS Direct Wales on 0845 4647 for further advice if you think that you have a skin infection.

Infection outside the intestine

A threadworm infection outside of the intestine is very rare. It has been known to occur in:

  • the vagina,
  • the uterus (womb),
  • the pelvic peritoneum (lining on the inside of the abdomen),
  • the abdominal cavity (stomach area),
  • the liver, and
  • the lungs.
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