A threadworm infection is passed from person to person as a result of swallowing threadworm eggs.

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, at which point they 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 or 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.

Animals and pets

Threadworms only infect humans and and are not spread in animal faeces. However, there’s a small risk that threadworms can be caught from household pets if the animal’s fur becomes contaminated with eggs during petting or stroking from an infected person. The eggs could then be passed to another human if they touch the animal’s fur.

Who is at risk?

Threadworm infections are most common in young children because they often forget to wash their hands regularly 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.

Read more about treating threadworms.

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 under the age of 10.  

Threadworms are white and look like small pieces of thread. You may notice them around your child’s bottom or in your or your child’s stools.

They don’t always cause symptoms, but people often notice itchiness around their bottom or vagina. This 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 (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 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. 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 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.  

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

Threadworms often go unnoticed by people who have them, but symptoms can include:

• intense itching around the anus (or the vagina in girls), particularly at night when the female worms are laying eggs
• disturbed sleep as a result of the itching, which can lead to irritability

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 any scratches caused by itching (wearing cotton gloves while sleeping may help prevent this)
• difficulty getting to sleep or staying asleep (insomnia)
• 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.

To successfully treat threadworms, all household members must be treated, even if they have no symptoms.

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

The aims of treatment are to get rid of the threadworms 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, but make sure you follow the manufacturer’s instructions because it is not suitable for everyone.

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 not usually the same as for most other people. See below for treatment advice in these circumstances.

Medication

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 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 threadworm infection is severe.

Hygiene measures

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

The life span of threadworms is approximately six weeks, so it’s important that the hygiene methods are followed for at least this long. Everyone in the household must follow the advice outlined below

• Wash all night clothes, 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. 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 help prevent eggs being transferred to other surfaces.
• Don’t eat food in the bedroom, because 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 that children don’t 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 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. Make sure that you clean around your anus and vagina to remove any eggs.
• Ensure that 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 re-infection.

If you are pregnant or breastfeeding

If you are pregnant or breastfeeding, medication is usually not recommended. Instead, you are advised to follow the hygiene measures above.

See your GP if you are more than three months pregnant or if you are breastfeeding and you continue to experience problems using only the hygiene measures. 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 your baby’s bottom gently but thoroughly every time you change their nappy. Also wash your hands thoroughly before and after changing their nappy.