Giardia and Pinworm Infections: What They Are, How They Spread, and How to Treat Them

Thousands of people in the U.S. get sick every year from parasites they never see. Two of the most common are giardia and pinworms. They don’t make headlines, but they cause real discomfort - diarrhea, itching, fatigue - and they spread easily. If you’ve had unexplained stomach issues or your child keeps scratching their bottom at night, it might not be a coincidence. These infections are treatable, but only if you know what you’re dealing with.

What Is Giardia, and How Do You Get It?

Giardia is a microscopic parasite called Giardia lamblia. It lives in the small intestine and causes giardiasis, one of the most common causes of traveler’s diarrhea and waterborne illness in developed countries. You don’t need to travel overseas to catch it. In the U.S., it’s often linked to contaminated water - think hiking streams, poorly maintained wells, or even tap water in older systems.

The parasite survives as a cyst, a tough shell that can live for months in cold water. All it takes is swallowing 10 to 25 of these cysts to get infected. That could happen from drinking untreated water, eating food washed in contaminated water, or touching a surface - like a bathroom doorknob or a child’s toy - and then putting your hands in your mouth.

Symptoms usually show up 1 to 14 days after exposure, with an average of 7 days. People often mistake giardiasis for a stomach bug. You get watery, greasy diarrhea that can last weeks. Bloating, gas, nausea, and fatigue are common. Some lose weight because their body can’t absorb nutrients properly. In rare cases, the infection becomes chronic, lasting months with recurring symptoms.

What Are Pinworms, and Why Do They Cause So Much Itching?

Pinworms, or Enterobius vermicularis, are tiny white worms about the length of a staple. They live in the colon and rectum. Unlike giardia, pinworms don’t cause diarrhea. Their main trick? Nighttime itching.

At night, the female pinworm crawls out of the anus and lays hundreds of eggs on the skin around the anal area. That’s when the itching starts - intense and worse at night. Kids often wake up scratching, and parents notice them rubbing their bottom or even pulling at their pajamas. The eggs can survive on bedding, clothing, toys, or even in dust for up to three weeks.

Transmission is simple: you touch something with eggs on it - a bedsheet, a pencil, a toilet seat - and then touch your mouth. Eggs can even become airborne when shaking out sheets or changing diapers. That’s why pinworms spread so fast in households, daycare centers, and schools. One person infected means the whole family is at risk. Studies show 75% of household members test positive when one person has it.

How Are These Infections Diagnosed?

Diagnosing giardia isn’t as simple as looking at a stool sample under a microscope. Traditional methods miss up to 30% of cases. The CDC recommends stool antigen tests, which detect parasite proteins. These are 95% accurate and give results in hours. If your doctor suspects giardia, they’ll order this test - not just a basic stool check.

For pinworms, the gold standard is the “tape test.” You press a piece of clear tape against the skin around the anus first thing in the morning, before bathing or using the toilet. The tape catches eggs. You take it to the doctor, who looks at it under a microscope. One test catches about half the cases. Three tests over three days catch 90%. That’s why doctors ask you to repeat it.

Many people go months undiagnosed because symptoms are mild or mistaken for other issues. If you’ve had persistent itching or diarrhea that won’t go away, ask for the right test. Don’t assume it’s just a virus.

How Are Giardia and Pinworms Treated?

Good news: both are treatable with simple, inexpensive drugs.

For giardia, the most common treatment is metronidazole. You take 250 mg three times a day for 5 to 7 days. Another option is tinidazole - just one 2-gram dose. Or nitazoxanide, taken twice daily for three days. Cure rates are 80% to 95% if you finish the full course. But side effects happen. About 78% of people on metronidazole report a strong metallic taste. Nausea is common. Some quit early because of it. Don’t. Stopping early means the infection comes back - and harder to treat.

For pinworms, the go-to drugs are mebendazole (100 mg single dose), albendazole (400 mg single dose), or pyrantel pamoate (11 mg per kg, max 1 gram). The CDC updated its guidelines in January 2024 to recommend a triple-dose regimen of albendazole for resistant cases - and it works 98% of the time.

But here’s the catch: treating the person isn’t enough. You have to treat the whole household. If you don’t, you’ll just get reinfected. That’s why doctors tell you to give the same dose to everyone living with you - even if they don’t have symptoms.

Preventing Reinfection Is Just as Important as Treatment

Treatment fails more often because of reinfection than because the drug doesn’t work. That’s why cleaning your environment is non-negotiable.

For pinworms: wash all bedding, towels, and clothing in hot water. Dry on high heat. Vacuum carpets and wipe down surfaces like doorknobs, toilet handles, and toys. Cut fingernails short. Don’t let anyone scratch and then touch their mouth. Shower in the morning to wash off any eggs laid overnight. Change underwear daily.

For giardia: avoid untreated water. If you’re hiking, boil water for at least one minute. Or use a filter with a pore size smaller than 1 micron. Bottled water is safe. Wash fruits and vegetables with clean water. Wash your hands after using the bathroom, changing diapers, or before eating. Handwashing with soap reduces transmission by 30% to 50%.

Children in daycare should stay home for at least two weeks after symptoms stop. Food handlers need the same rule. Giardia is highly contagious in group settings.

Who’s at Highest Risk?

Giardia hits kids, travelers, and people with weakened immune systems hardest. In developing countries, up to 30% of people have it. In the U.S., it’s most common in children under 5 and adults who camp or hike. People with HIV or other immune disorders can have giardia for months - not weeks.

Pinworms are the most common worm infection in the U.S. They’re mostly a childhood problem. About 40 to 80 million Americans have them at any time, mostly kids aged 5 to 10. But adults who care for kids - parents, teachers, daycare workers - are just as likely to get infected. Long-term care facilities also see outbreaks.

Climate change is making giardia worse. Warmer temperatures and more flooding increase water contamination. Experts predict giardia will spread into new areas in Europe and North America by 2040.

What Happens If You Don’t Treat It?

Pinworms rarely cause serious harm, but chronic itching can lead to skin infections from scratching. Sleep loss and stress are real problems for families.

Giardia is more dangerous long-term. If left untreated, it can cause malnutrition, weight loss, and lactose intolerance that lasts for months - even after the parasite is gone. In kids, it can slow growth and development. In adults, it can lead to chronic fatigue and digestive problems that mimic IBS.

Some people clear it on their own, but it’s risky to wait. The longer it lasts, the harder it is to treat. And you’re still spreading it to others.

What About Natural Remedies or Herbal Treatments?

You’ll find claims online about garlic, wormwood, or black walnut killing parasites. There’s no solid proof these work against giardia or pinworms in humans. Some herbal products might even be unsafe, especially for kids or pregnant women.

Prescription drugs like metronidazole and mebendazole have been tested in thousands of people. They’re safe, effective, and cheap. Don’t risk your health with unproven remedies. Talk to your doctor - not Google.

What’s New in 2026?

Research is moving fast. In 2023, a vaccine candidate called GID1 showed 70% success in early trials for giardia. It’s still years from being available, but it’s a first. For pinworms, the CDC’s 2024 update on triple-dose albendazole has already changed practice in many clinics.

Global water filters are improving. In Bangladesh, simple point-of-use filters cut giardia cases by 42%. That’s a win for communities without clean water.

But the biggest change? Awareness. More doctors now know to test for giardia with antigen tests, not just microscopy. More parents know to do the tape test for pinworms. That’s saving time, money, and sleep.