Causes of threadworms 

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.


Page last reviewed: 28/08/2013

Next review due: 28/08/2015