You get threadworms by swallowing their microscopic eggs, almost always through your hands touching your mouth after contact with a contaminated surface. The eggs are invisible to the naked eye and can survive on bedding, clothing, toys, and household surfaces for two to three weeks, making them remarkably easy to pick up without realizing it.
The Main Route: Hand to Mouth
The most common way threadworm infection starts is straightforward. A person who already has threadworms scratches the itchy skin around their anus, picks up eggs under their fingernails, and then touches shared objects like door handles, towels, bedding, or food. When someone else touches those same surfaces and then puts their fingers near their mouth, they swallow the eggs. Children do this constantly without thinking, which is why up to 30% of children globally are affected, with the highest rates in kids aged 4 to 11.
Once swallowed, the eggs hatch in the small intestine. The larvae mature into adult worms and migrate to the large intestine, where they live and feed. After a few weeks, female worms crawl out at night to lay thousands of eggs on the skin around the anus. This causes intense itching, which triggers scratching, which picks up fresh eggs, and the cycle restarts. This self-reinfection loop is the reason threadworms are so persistent: the same person keeps re-swallowing their own eggs.
Less Obvious Ways Eggs Spread
Direct hand-to-mouth contact isn’t the only pathway. Eggs can also become airborne when you shake out bedsheets, towels, or pajamas. In rare cases, these airborne eggs are inhaled and then swallowed. This is why health guidelines specifically warn against shaking contaminated laundry.
Shared bathwater is another route. If an infected child bathes in a tub, eggs can float in the water and transfer to anyone bathing after them, or to a sibling sharing the same bath. Washcloths and towels that aren’t washed between uses carry the same risk.
There’s also a less common route called retroinfection, where newly hatched larvae on the skin around the anus migrate back into the rectum on their own, restarting the infection without any eggs being swallowed at all. How often this actually happens isn’t well understood, but it may explain why some infections seem stubbornly hard to clear.
Why Children Are Most Affected
Threadworms aren’t a sign of poor hygiene or a dirty household. They thrive wherever young children gather. In Europe, infection rates among school-age children range from 4% to 30%, and in some lower-income countries that figure reaches 50%. Classrooms, playgrounds, and shared toys create ideal conditions for eggs to pass between children who frequently touch their faces. Once one child in a household is infected, the rest of the family often follows because everyone shares the same bathroom, towels, and living spaces.
Pets Don’t Spread Threadworms
Threadworms (Enterobius vermicularis) are exclusively a human parasite. Dogs, cats, and other household pets cannot carry or transmit them. If your child has threadworms, the source is always another person or a contaminated surface, never the family pet. Different species of worms can affect animals, but they aren’t the same organism and don’t cross over.
How to Confirm an Infection
The classic sign is intense itching around the anus, especially at night when female worms are active. You might also spot tiny white thread-like worms (about 1 cm long) in your child’s stool or around their bottom.
If you’re unsure, the standard test is simple. First thing in the morning, before washing or using the toilet, press a piece of clear sticky tape against the skin near the anus. The tape picks up any eggs the worms deposited overnight. Seal it in a bag or container and take it to your doctor, who can examine it under a microscope. Doing this on three consecutive mornings gives the most reliable result, since worms don’t necessarily lay eggs every single night.
Breaking the Reinfection Cycle
Treatment involves two doses of medication, with the second dose given two weeks after the first. The medication kills the adult worms but cannot kill eggs. That two-week gap is deliberate: it allows time for any surviving eggs to hatch into new worms, which the second dose then eliminates. Without the second dose, those newly hatched worms simply restart the infection.
Medication alone won’t solve the problem if eggs are still all over your home. For at least two weeks after the final dose, the whole household needs to follow strict hygiene measures to mop up remaining eggs in the environment:
- Handwashing: Wash hands thoroughly with soap and warm water after every toilet visit, after changing nappies, and before handling food. This is the single most effective measure.
- Fingernails: Keep them trimmed short and scrub underneath them. Long nails trap eggs that survive ordinary handwashing.
- Morning showers: Bathe every morning and change underwear immediately. This removes eggs deposited overnight before they can spread to clothing and furniture.
- Laundry: Wash pajamas, underwear, towels, and bedding in hot water (at least 55°C / 130°F) and dry on a high heat setting. Don’t shake these items before putting them in the machine, as that flings eggs into the air.
- Separate bathing: Have children shower rather than bathe in a tub. If baths are necessary, don’t let siblings share the same water, and don’t reuse washcloths.
Everyone in the household should be treated at the same time, even if only one person has symptoms. Threadworm eggs spread so easily that other family members are likely carrying the infection without knowing it yet. Treating just the symptomatic person while others continue shedding eggs is the most common reason threadworms keep coming back.