Threadworms: why they keep coming back

Threadworms are common, harmless and very easy to be reinfected with especially in young children, which is why so many households treat them repeatedly and some struggle to fully get rid of them.

Threadworms, also called pinworms, are small white worms that live in the large intestine. An adult female is about a centimetre long and looks like a short length of white cotton thread. They infect people and nothing else, so you don't need to worry about a pet causing the household to be infected/reinfected.

The itch comes from the female at night migrating out through the anus and laying her eggs in the skin folds around it. This is why the itch is much worse at night in bed than during the day.

Eggs become infective within hours of being laid. The eggs are coated in a sticky material that makes them cling to skin, so the shortest route from one person's perianal skin to their own mouth, or someone else's, is a fingernail. Scratching in the sleep is the main way they enter the mouth, and it happens whether or not anyone is aware of it.

A swallowed egg takes two to four weeks to produce a worm that lays eggs of its own. There is a window in every infection where worms are present, growing, and producing no symptoms and nothing to find.

An adult worm lives roughly six weeks. If nothing new is swallowed, the infection ends on its own when the current generation dies.

The cycle is quite short and it only continues if eggs keep reaching mouths. Almost every threadworm problem that drags on for months is a problem with reinfection rather than a problem with the treatment.

What points towards it

Itching around the anus that is clearly worse at night, and worse in bed than on the sofa is usually how people notice they or their children may have a threadworm infection. In girls, itching or soreness around the vulva as well, which can happen because worms migrate forwards. If you notice any of these symptoms, don't assume it is definitely threadworms, instead take a look as adult worms are easily visible without any equipment.

Looking, properly

An hour or two after the child is asleep, take a torch, part the buttocks and look directly at the skin around the anus. If worms are there you will see them: small, white, thread-like, and moving slowly. Do it two or three nights running rather than once, because they are not out every night.

Checking the stool is the version most people try and it is no longer recommended. Eggs turn up in only around 5% of stool samples in people who definitely have the infection, because the eggs are laid on the skin outside the bowel rather than passed through it. A clear nappy or a normal-looking stool doesn't tell you that threadworms aren't present.

No symptoms is normal

Somewhere between 30 and 40% of infected people have none at all. In one European study of school-age children, 18% tested positive, rising to around a third of six to eleven year olds, and only two of the 72 positive children were known to have ever had them. This is why it is important to treat the household, as people may have threadworms without knowing.

When it isn't threadworms

In adults with an itchy bottom and no worms seen, threadworms are one cause among several. Haemorrhoids, anal fissures, eczema, psoriasis, thrush and simple irritation from over-washing all produce the same complaint, and some of them get worse if you treat them as worms and start scrubbing. If nobody in the house has actually seen a worm and a course of treatment has made no difference, we would recommend you talk to a healthcare professional instead of buying further worm treatments.

The treatment is mebendazole and it is available over the counter as Ovex, in chewable tablets and as a banana-flavoured suspension. It is a pharmacy medicine, so it is sold under a pharmacist's supervision rather than off the shelf, and it is licensed for adults and children from the age of two.

The dose is the same regardless of age or size: one 100mg tablet, or 5ml of suspension. Children are not given less and adults are not given more.

How it works

Mebendazole blocks the worm's ability to take up glucose. Without it the worm has no energy supply, and it dies over the following few days and is passed out in the stool. You may see them, but not always so not seeing them doesn't tell you whether treatment has worked or not. It starts working immediately but it is not instant, so a child still itching two nights later is not evidence of failure. Mebendazole cure rates for threadworm are around 95%.

Repeat dose in 14 days

It does not however kill eggs and there is curretnly nothing on the UK market that kills threadworm eggs. Any eggs already under fingernails, on nightwear or on the perianal skin when the dose is taken will survive it, and anything swallowed in the days that follow will hatch and mature exactly as if no treatment had happened.

By day fourteen, the worms that hatched after the first dose are mature enough for the drug to kill them, and they will not have started laying yet.

The pack of Ovex only recommends a second dose if reinfection is suspected. That does leave the judgement to you, and most people decide it doesn't apply because nobody has obviously caught it again. However since there is no harm in taking a second dose, it highly recommended to do so especially in a house with young children. Reinfection in the fortnight after the first dose is not an unusual event. Some countries practice is to repeat routinely at fourteen days, and again at twenty-eight for households that keep relapsing.

Set a notification on your phone or put it in a calendar 14 days after you have all taken the first dose.

Side effects and interactions

Most people have none. Abdominal discomfort, wind and loose stools are the common ones and they settle without anything being done.

Allergic reactions are very rare.

The is one interaction to look out for, you must avoid taking mebendazole and metronidazole together because the combination can cause a severe, life-threatening skin reaction called Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). If you are taking metronidazole tablets or using a product containing metronidazole such as roscea contact a pharmacist or doctor before taking Ovex.

Who cannot take it

Children under two, and anyone pregnant or breastfeeding. Children under two can be prescribed it by a doctor.

Hygiene measures

Make sure you do these everyday

These interrupt the route from skin to mouth, which is where nearly all reinfection happens.

Shower every morning, including the perianal area. Eggs are laid overnight, so washing them off before breakfast removes most of that night's load.

Wear pants or close-fitting nightwear in bed, changed every morning. It puts a barrier between fingers and skin during the night, which is when the scratching happens.

Keep nails short and scrub underneath them especially after the toilet and before food. Try to avoid nail biting.

Do this at the start of treatment

Strip and hot wash bedding, nightwear and towels. Sixty degrees or above kills the eggs. Do not shake anything out first, as this can cause eggs to be released into the air and around the room.

Vacuum the bedrooms, including the mattress.

Two more worth knowing

Do not share towels, and do not bath children together during the two weeks.

If a child scratches in their sleep despite pants, a onesie makes it genuinely harder to reach.

If this is the third or fourth round, the instinct is that the treatment is not working. Almost always, it is working fine.

Cure rates for threadworm are around 95%, and guidance is explicit that recurrence is usually reinfection rather than the drug failing.

Work through these first

Did everyone take a dose, including every adult? One untreated person keeps the whole house going.

Was it the same day, or spread across a week?

Did the second dose actually happen at fourteen days, for everyone?

Did you buy enough doses to cover both rounds?

Is there an outbreak at nursery or school? If eggs are arriving from outside every week, the household course clears what is there and the supply continues. That is not treatment failure either, but it does change what you do next.

Most repeat infections come apart at one of those five points.

The pulse approach. For households that keep relapsing despite doing everything correctly, a longer schedule is used: a single dose to everyone in the house every fourteen days, continued for around sixteen weeks. It works by covering several complete life cycles rather than one, so any egg swallowed at any point runs into a dose before it can lay. It is worth having a conversation with a GP or a pharmacist if you have been round this four or five times.

Recurrence in girls with nobody else affected. Threadworms migrate forwards, and in girls they can end up in the vulva or vagina, which causes itching or soreness there as well as around the anus. That matters because the vagina is a site the tablets reach poorly, so it can act as a reservoir: one child keeps relapsing while the rest of the household stays clear. There is a published case of a seven-year-old with repeated vaginal infection, never found in her gut, that only settled after three doses at two-week intervals.

If one girl in the house is the only person who keeps getting them, and particularly if the itching is at the front, we suggest you book in to see a GP rather than buy another pack.

Worth a GP appointment

Nobody has ever actually seen a worm and treatment has made no difference. Itching in the vulval area, or pain passing urine, in a girl with repeated infections. Tummy pain that is getting worse rather than better. Anyone under two, pregnant or breastfeeding.

Mebendazole cannot be bought over the counter for use in children under two, or in pregnancy, or while breastfeeding.

Hygiene alone can clear worms

An adult threadworm lives about six weeks. It cannot reproduce inside you: the eggs have to leave the body, get onto the skin, and be swallowed again to start a new worm. If you can break this cycle no worms can grow and replace the current generation so six weeks later they will have all died. You will have to perform the strict hygiene measures for the full six weeks for this to work effectively.

What that means in practice

Make sure you do every practice of the previous section, done properly, every day, for six weeks, including the morning wash, the pants at night, the nails, the hands.

For babies under six months, add gentle perianal washing or wet wiping through the day, roughly every three hours, since they cannot be asked not to scratch.

If you are pregnant and the rest of the house is being treated

Make sure you keep on performing the hygeine measures, everyone else in the household will clear up in a fortnight but you will need to keep going with the hygeine mesaures for the full six weeks to avoid reinfection. The safety data isn't known with mebendazole in pregnancy and you may want to talk on with your GP or midwife, who can weigh it against how bad your symptoms are and how far along you are.

Under two, and when to see someone

For a child between six months and two years, treatment is sometimes used off-label on a doctor's decision.

See a GP if the child is under two and symptoms are disrupting sleep or feeding, if hygiene measures have run six weeks without improvement, or if you are pregnant and the itching is severe.