What You Need to Know About Giardia and Pinworms
Most people think parasites are something that only happens in faraway countries or to people who don’t wash their hands. But here in the U.S., giardia and pinworms are more common than you think. One in ten kids in daycare gets pinworms. Giardia causes more diarrhea than any other parasite in the country. And both can stick around for weeks-or even months-if you don’t treat them right.
Giardia is a tiny, swimming germ that lives in water. Pinworms are small white worms that crawl out of your butt at night to lay eggs. They’re not scary, but they’re annoying. And they don’t go away on their own.
How Giardia Spreads and What It Does
Giardia doesn’t come from dirty hands alone. It comes from water. A single drop of contaminated water-maybe from a stream, a backyard well, or even a poorly filtered tap-can make you sick. You don’t need to swallow a lot. Just 10 to 25 cysts, the hard-shelled form of the parasite, is enough.
Once inside, the cysts hatch into trophozoites. These little things latch onto the lining of your small intestine and mess with your digestion. They flatten your gut’s microvilli-the tiny finger-like projections that absorb nutrients. That’s why people with giardia lose weight, feel tired, and get bloated. Diarrhea isn’t just loose stools-it’s watery, foul-smelling, and lasts for days.
Symptoms show up 1 to 14 days after swallowing the cysts, usually around day 7. Some people feel fine. Others can’t leave the bathroom. In kids, it can stunt growth. In older adults or people with weak immune systems, it can turn chronic. And yes, you can spread it even if you don’t feel sick.
Pinworms: The Itch That Won’t Quit
Pinworms are different. They don’t live in your water. They live in your house. The female worm is about the length of a staple. At night, she crawls out of your rectum, lays thousands of eggs around your anus, and dies. That’s when the itching starts-sharp, burning, and worst when you’re trying to sleep.
That’s also when the eggs get everywhere. Kids scratch. Their fingers get coated. Then they touch doorknobs, toys, blankets, or food. Eggs can live on surfaces for up to three weeks. They can even float in the air. You don’t need to touch feces. Just breathing in dust that carries eggs can infect you.
It’s why pinworms spread like wildfire in households with kids. One child gets it. Then the whole family does. Parents think it’s just a rash or allergies. By the time they realize it’s pinworms, the eggs are already on the sheets, the couch, the toothbrushes.
How Doctors Diagnose These Infections
For giardia, a stool test used to be the go-to. But it misses half the cases. Now, doctors use a stool antigen test-it looks for giardia proteins. It’s 95% accurate. If you’ve had diarrhea for more than a week, especially after camping, traveling, or drinking from a stream, ask for this test.
For pinworms, it’s the scotch tape test. First thing in the morning, before you shower or use the bathroom, press a piece of clear tape against the skin around the anus. Peel it off and stick it to a slide. A lab looks for eggs under a microscope. One test catches about half the cases. Three tests in a row catch 90%. No blood work. No imaging. Just tape and time.
Don’t wait for symptoms to get worse. If your child wakes up scratching their bottom, or you’ve had unexplained diarrhea after drinking tap water, get tested. Early diagnosis stops the spread.
How to Treat Giardia
There are three main drugs used for giardia: metronidazole, tinidazole, and nitazoxanide.
- Metronidazole (250 mg three times a day for 5-7 days) is the most common. But it has side effects: a metallic taste in your mouth (78% of people), nausea (65%), and dizziness. Don’t drink alcohol while taking it-it can make you violently ill.
- Tinidazole is a single 2-gram dose. Same effectiveness, fewer days. Fewer side effects. But it’s more expensive.
- Nitazoxanide (500 mg twice a day for 3 days) works for kids as young as 1. It’s gentler on the stomach. Good if you can’t tolerate metronidazole.
Cure rates are 80-95% with the right drug. But if you don’t finish the course-or if you get reinfected-symptoms come back. And some strains are starting to resist metronidazole. In Southeast Asia, 15% of cases don’t respond. In the U.S., it’s still under 5%.
After treatment, stay home from work or school for two weeks. Even if you feel fine, you can still shed cysts. Kids shouldn’t go to daycare until they’ve been clean for 14 days. And don’t share towels or utensils.
How to Treat Pinworms
Pinworms are easier to kill. But harder to eliminate because the eggs stick around.
- Mebendazole (100 mg, one pill, repeat in two weeks)
- Albendazole (400 mg, one pill, repeat in two weeks)
- Pyrantel pamoate (11 mg per kg, one dose, repeat in two weeks)
It’s not enough to treat just the person who’s itchy. Everyone in the house gets treated at the same time. Even if they feel fine. The CDC says 75% of household members are infected when one person shows symptoms.
And here’s the part most people skip: cleaning. Wash all bedding, pajamas, and towels in hot water. Dry on high heat. Vacuum carpets, mattresses, and upholstered furniture. Wipe down doorknobs, light switches, and toys with disinfectant. Eggs survive on surfaces for weeks. If you don’t clean, you’re just re-infecting yourself.
In January 2024, the CDC updated guidelines to recommend a triple dose of albendazole (400 mg on days 1, 8, and 15) for stubborn cases. That works 98% of the time.
How to Prevent These Infections
Handwashing with soap and water reduces transmission by 30-50%. That’s not a suggestion-it’s science. Wash after using the bathroom, before eating, and after changing diapers. Don’t just rinse. Scrub for 20 seconds. Sing the birthday song twice.
For giardia, don’t drink untreated water. Even in the U.S., mountain streams and backyard wells can be contaminated. Boil water for one minute. Or use a filter with a pore size smaller than 1 micron. Regular coffee filters? Useless. Cheap camping filters? Often useless. Look for ones labeled “NSF 53” or “cyst removal.”
For pinworms, keep fingernails short. Don’t let kids suck their thumbs. Change underwear daily. Shower in the morning to wash away any eggs laid overnight. Avoid shaking out bedding or clothes-egg dust can become airborne.
Water treatment plants in the U.S. have cut giardia cases by more than half since the 1990s. But climate change is making things worse. More floods mean more contaminated runoff. The Merck Manual predicts giardia will spread to 20-30% more areas in temperate zones by 2040.
What Happens If You Don’t Treat Them
Giardia can turn into a long-term problem. Malabsorption. Weight loss. Vitamin deficiencies. In kids, it can delay growth. In people with HIV or other immune issues, it can last for months. Some people develop long-lasting gut sensitivity-even after the parasite is gone.
Pinworms won’t kill you. But the itching can wreck sleep. Kids become irritable, distracted, or fall behind in school. The constant scratching can break the skin and cause bacterial infections. And the shame? It’s real. Parents feel guilty. Kids feel dirty. They shouldn’t.
These aren’t “bad hygiene” infections. They’re common, easily spread, and treatable. But they need to be treated correctly-and completely.
What’s Next? Vaccines and New Treatments
There’s no vaccine yet for giardia or pinworms. But researchers are trying. A giardia vaccine called GID1 showed 70% immune response in early trials in 2023. That’s promising. If it works, it could change how we protect kids in daycare and travelers in developing countries.
Drug resistance is a growing worry. In parts of Asia, metronidazole is failing more often. Scientists are testing new drugs and combinations. For now, sticking to the CDC guidelines and cleaning your environment is the best defense.
Can you get giardia from a swimming pool?
Yes, if the pool isn’t properly chlorinated. Giardia cysts survive chlorine longer than most bacteria. A single infected person can contaminate a pool. That’s why public pools require swimmers to shower before entering and ban kids with diarrhea.
Do pinworms lay eggs inside the body?
No. Female pinworms crawl out of the anus at night to lay eggs on the skin around it. The eggs are not laid inside the intestines. That’s why the scotch tape test works-it catches eggs on the skin, not in stool.
Can pets give you giardia or pinworms?
Dogs and cats can get their own versions of giardia, but the strains that infect them rarely infect humans. Pinworms are human-only parasites. You can’t get them from pets. So don’t blame your dog.
Is it safe to treat pinworms while pregnant?
Mebendazole and albendazole are not recommended during pregnancy unless the infection is severe. Pyrantel pamoate is considered safer in the second and third trimesters. Always talk to your doctor. But don’t wait-untreated pinworms can cause sleep loss and stress, which also affect pregnancy.
Why do symptoms come back after treatment?
Reinfection is the most common reason. If you don’t treat everyone in the house or clean contaminated surfaces, you’re just getting the same eggs back. Sometimes, the first treatment didn’t kill all the worms or cysts. A second dose, given two weeks later, fixes most of these cases.
