Pinworm Life Cycle: How Long They Live

The pinworm life cycle happens almost entirely in one person: swallowed eggs hatch in the small intestine, the worms grow into adults in the large intestine, and the females crawl out at night to lay new eggs on the skin around the anus. According to the CDC, it takes about one month from swallowing the eggs to the first new eggs being laid, and an adult pinworm lives for about two months.

Because the eggs can get from the skin to the fingers and back to the mouth, the cycle can restart in the same person or spread to the people around them. This page walks through each stage, how long pinworms and their eggs live, and what the life cycle means for testing and care. It is general education, not medical advice.

The pinworm life cycle, step by step

The CDC's parasite biology reference describes the pinworm (Enterobius vermicularis) life cycle in these stages:

1. Eggs are swallowed. People swallow the microscopic eggs, usually from fingers that have scratched the skin around the anus, or from contaminated surfaces, clothes and bed linens. Rarely, eggs float in the air and are breathed in and swallowed.
2. The eggs hatch in the small intestine. Once swallowed, the eggs release tiny larvae.
3. The worms settle in the large intestine. The larvae move on and grow into adult worms in the colon, usually in its first section (the cecum).
4. The adults mature and mate. The CDC says the time from swallowing infective eggs to the adult females laying eggs is about one month.
5. Females lay eggs on the skin at night. While the person sleeps, egg-carrying females travel out through the anus and lay their eggs on the surrounding skin. This is what causes the itching that pinworm is known for.
6. The eggs become infective within hours. The CDC's public pinworm page says the eggs can infect another person two to three hours after they are laid; its parasite biology reference gives four to six hours under ideal conditions. Either way, they are ready to start a new cycle the same night or morning.

The CDC notes that humans are considered the only host for this pinworm. The cycle does not pass through soil, water or animals, which is why it spreads so easily between people who live together.

How long do pinworms live?

Adult pinworms live for about two months, according to the CDC. During that time the females keep laying eggs on the skin around the anus at night.

The adults are small. The CDC describes them as small, thin and white, about the size of a staple on average. Its parasite biology reference gives the adult female's length as about 8 to 13 millimeters and the male's as about 2 to 5 millimeters. If you have seen something in the stool or on the skin and are unsure what it was, our guide to white worms in stool explains what the common ones look like.

An infection can last much longer than one worm's life span. That is because new eggs keep being swallowed, so new worms keep replacing the old ones.

How long do pinworm eggs live?

Pinworm eggs do not need long to become infective, and they can last for a while once they leave the skin. The CDC says that if they are not washed or cleaned away, pinworm eggs can survive two to three weeks on objects such as bedding, clothes, towels and other items. Our guide to pinworm treatment for adults covers the household hygiene steps the CDC recommends for that reason.

People often ask whether pinworm eggs hatch outside the body. In the usual cycle, they hatch after being swallowed, in the small intestine. The CDC does describe one exception, called retroinfection: newly hatched larvae on the skin around the anus may move back into the rectum. The CDC says this may happen, but how often it happens is unknown.

Why the cycle keeps repeating

The life cycle explains why pinworm is common and why repeat infections are common too:

• Self-reinfection. Scratching at night puts eggs on the fingers and under the fingernails. If those eggs reach the mouth, the same person swallows them and a new cycle starts.
• Household spread. Eggs on pajamas, sheets, towels and toilet seats can reach other people in the home. The CDC says pinworm infections often occur in more than one person in a household. See how pinworms spread from person to person.
• Childcare and group settings. The CDC lists school-aged and preschool children, people in institutional settings, and the household members and caretakers of infected people as the groups most often infected.
• Eggs outlast medicine. The CDC explains that pinworm medicines kill the worms but cannot kill the eggs. Eggs that are already swallowed, or still on the skin or in the home, can grow into new worms later. Your clinician will explain how they plan to account for that.

If an infection is left alone, the cycle can keep going. Our page on what happens if pinworms go untreated covers that in more detail.

What the life cycle means for testing

Because the eggs are laid on the skin rather than released into the stool, stool tests are not the usual way to find pinworm. The CDC says healthcare providers generally do not use stool samples to diagnose it, because these samples usually don't contain enough worms or eggs to easily detect, and there is no blood test for pinworm.

Instead, the CDC describes three ways a provider can confirm it, with your help:

• Looking for worms around the anus two to three hours after the person falls asleep
• A pinworm tape test, pressing clear tape on the skin first thing in the morning to collect eggs
• Checking samples from under the fingernails

The tape test is timed for the morning because the eggs are laid overnight, and washing or using the toilet can remove them.

What the life cycle means for care

The CDC lists both over-the-counter and prescription medicines for pinworm, and advises treating everyone in the household and the caregivers of the person with pinworm at the same time. It also stresses handwashing with soap and warm water, especially after using the toilet and before handling food, as the best way to prevent infection and spread.

Itching around the anus has other causes as well. If you are not sure it is pinworm, our guide to itchy anus at night compares the common causes. See a clinician in person for severe belly pain, fever, blood in the stool, or symptoms in a child under 2 or during pregnancy.

If you and a provider decide a prescription is appropriate

Get It Prescribed connects adults with an independent licensed provider online. The service is available in all 50 states and the District of Columbia, and some states require a live phone or video visit. The cost is a one-time $50 provider consultation fee, plus $3.50 per capsule for mebendazole if a prescription is issued, with the quantity decided by the provider. No prescription is guaranteed, and the provider may recommend an in-person visit or testing instead. We don't publish doses or schedules on this site. See how it works for each step.

The mebendazole dispensed through Get It Prescribed is a 222 mg capsule prepared by a licensed compounding pharmacy. It is a compounded preparation. It is not FDA-approved, it has not been reviewed by the FDA for safety, effectiveness or quality, and it is not a generic of or substitute for the brand-name mebendazole chewable tablet.

This article is general education, not medical advice. It does not replace an evaluation by a licensed clinician who knows your history.

Related questions

How long do pinworms live in the body?

The CDC says an adult pinworm lives for about two months. An infection can last longer than that, because eggs that are swallowed again grow into new worms that replace the old ones.

How long after swallowing pinworm eggs do symptoms start?

The itching comes from female worms laying eggs on the skin around the anus. The CDC says it takes about one month from swallowing the eggs to the adult females laying eggs, so itching may not start right away. Many people never have symptoms at all.

How long do pinworm eggs live outside the body?

The CDC says pinworm eggs can survive two to three weeks on objects such as bedding, clothes and towels if they are not washed or cleaned away. They can infect someone within a few hours of being laid.

Can pinworm eggs hatch outside the body?

In the usual life cycle, the eggs hatch only after they are swallowed, in the small intestine. The CDC notes that newly hatched larvae on the skin around the anus may sometimes move back into the rectum, but how often that happens is unknown.

Can you get pinworms from pets?

No. The CDC considers humans the only host for the human pinworm, and its prevention guidance says you can't get pinworm from your pets. Pinworm spreads from person to person.

Sources

• CDC: DPDx — Enterobiasis — pinworm biology and life cycle: hatching, where adults live, one month to egg-laying, two-month adult life span, adult size, retroinfection and host.
• CDC: About Pinworm Infection — how pinworm spreads, when eggs become infective, how long eggs survive on objects, and how it is diagnosed and treated.
• CDC: Clinical Overview of Pinworm Infection — why stool and blood tests are not used, the tape test, and that medicines kill worms but not eggs.
• CDC: Preventing Pinworm Infection — hygiene steps, repeat infections, and that people can't catch pinworm from pets.
• MedlinePlus: Pinworms — consumer overview of pinworm infection.
• MedlinePlus: Pinworms (Medical Encyclopedia) — how pinworm spreads, symptoms and testing.
• DailyMed: mebendazole chewable tablet prescribing information — labeled uses, including pinworm.
• FDA: Compounding and the FDA — Questions and Answers — what compounded preparations are and how they are regulated.