Causes of threadworms 

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
  • toothbrushes
  • furniture
  • 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.

Read more about treating threadworms.

Page last reviewed: 24/08/2015

Next review due: 24/08/2017