Parasitic Infections: Giardia, Pinworms, and How to Treat Them Effectively

When you think of infections, you probably imagine colds, flu, or maybe even COVID-19. But millions of people - including kids in your child’s classroom and travelers back from abroad - are dealing with something far less talked about: parasitic infections. Two of the most common ones? Giardia and pinworms. They’re not glamorous, they don’t make headlines, but they cause real suffering: endless diarrhea, sleepless nights from itching, and families stuck in a cycle of reinfection.

What Exactly Is Giardia?

Giardia is a tiny, pear-shaped parasite that lives in your small intestine. It’s not a worm. It’s a protozoan - a single-celled organism that moves with whip-like tails called flagella. You can’t see it with the naked eye, but it’s powerful enough to wreck your digestion. It survives in water, soil, and on surfaces as a hard-shelled cyst, waiting to be swallowed.

How do you catch it? Usually through water. Hiking in the woods? Drinking from a stream? That’s a classic risk. But it’s not just the outdoors. Outbreaks happen in daycares, swimming pools, and even municipal water systems. The CDC says Giardia is the most common parasitic cause of diarrhea in the U.S. More than 1.2 million cases happen every year. And here’s the kicker: it’s often mistaken for a stomach bug. People think they had bad sushi. Turns out, they had a cyst in their water bottle.

Symptoms show up 1 to 14 days after swallowing the parasite - usually around day 7. You get watery, greasy diarrhea that smells awful. Bloating, gas, nausea, and fatigue follow. Some people lose weight because their gut can’t absorb nutrients anymore. In chronic cases, symptoms drag on for months. Kids are especially vulnerable. Adults might brush it off as “just a tummy ache.” But if diarrhea lasts longer than a week, especially after travel or camping, Giardia should be on the list.

What About Pinworms?

Pinworms are the opposite of Giardia. They’re visible. Tiny, white, thread-like worms about the length of a staple. They live in the colon and rectum. The female worm crawls out of the anus at night - yes, at night - to lay hundreds of eggs on the skin around the butt. That’s when the real problem starts: itching. Not just a little itch. A burning, unbearable itch that keeps kids (and sometimes adults) awake for hours.

Pinworms are the most common worm infection in the U.S. An estimated 40 to 80 million people have them at any given time. Most are children under 10. Why? Kids touch everything. They play on the floor, share toys, suck their thumbs. The eggs get on their fingers, then in their mouths. Eggs can live for up to three weeks on bedding, clothes, or even in the air. One infected child can spread it to half the household.

Unlike Giardia, pinworms rarely cause serious illness. No diarrhea. No fever. Just the itch. But that itch is enough to drive families crazy. And because it’s so common, many parents think it’s normal. It’s not. It’s a sign something’s wrong - and it’s treatable.

How Are These Infections Diagnosed?

Giardia is tricky. A regular stool sample under a microscope only catches it about 70% of the time. That’s why doctors now use a stool antigen test - a quick lab test that looks for Giardia proteins. It’s 95% accurate. If you’ve had diarrhea for more than a week and your doctor doesn’t test for Giardia, ask for it. Don’t wait.

Pinworms? There’s a simple trick called 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 eggs stick to the tape. You take it to the doctor, who looks under a microscope. One test misses half the cases. Three tests in a row catch 90% of them. If your child is scratching at night, do the tape test. It takes 30 seconds and costs nothing.

How Are They Treated?

Good news: both infections respond well to medication - if you take it right.

For Giardia, the go-to drug is metronidazole. You take 250 mg three times a day for 5 to 7 days. Side effects? A strong metallic taste (78% of people report it), nausea, and sometimes dizziness. Tinidazole is an alternative - one big pill instead of a week of doses. Nitazoxanide works too, and it’s approved for kids as young as 1. Cure rates are 80% to 95%. But here’s the catch: if you don’t finish the course, or if you get reinfected from contaminated water or a family member, it comes back.

Pinworms are even simpler. One pill: mebendazole (100 mg), albendazole (400 mg), or pyrantel pamoate. That’s it. But you need to take it again in two weeks. Why? Because the first pill kills the worms - but not the eggs. The eggs hatch, become adults, and the cycle starts again. That’s why the CDC now recommends a second dose, and in resistant cases, a third dose of albendazole.

And here’s what most people miss: you can’t just treat the person. You have to treat everyone in the house. If one person has pinworms, 75% of others in the same home are infected - even if they show no symptoms. Same with Giardia. If one kid gets it in daycare, the whole class needs to be watched.

How to Stop It From Coming Back

Medication alone won’t fix this. You need to break the cycle.

For Giardia:

• Boil water for at least 1 minute if you’re unsure of the source. Or use a filter with pores smaller than 1 micron.
• Wash hands with soap and water after using the bathroom, before eating, and after changing diapers.
• Avoid swallowing water in pools, lakes, or hot tubs.
• If you work with food or care for kids, stay home for two weeks after symptoms stop. You can still shed cysts.

For Pinworms:

• Wash all bedding, pajamas, and towels in hot water on the same day you take the first dose.
• Shower in the morning - not the night before - to wash off eggs laid overnight.
• Keep fingernails short. No thumb-sucking. No scratching.
• Wipe down doorknobs, toys, and countertops with a damp cloth daily for a week after treatment.
• Don’t shake out bedding or clothes - that sends eggs flying into the air.

One family on Reddit shared how they spent $300 on a professional steam cleaner after their daughter kept getting pinworms. They cleaned every mattress, couch, and stuffed animal. It worked. No recurrence in 18 months.

Who’s at Risk?

Giardia doesn’t care about wealth. It thrives where sanitation is weak - but it’s not just a problem in developing countries. Backpackers, hikers, campers, and even people drinking from public fountains are at risk. Immigrants, refugees, and travelers from high-risk areas bring it into the U.S. regularly. People with weakened immune systems - like those with HIV - get longer, more severe infections.

Pinworms? They’re the great equalizer. Rich or poor, urban or rural, if there are kids, there’s pinworms. Daycare centers, schools, and households with multiple children are hotspots. Adults who care for kids are just as likely to catch them.

What’s New in 2025?

Science is catching up. In January 2024, the CDC updated its pinworm guidelines to recommend a triple-dose treatment for stubborn cases - and it’s working. A recent trial showed 98% success with three doses of albendazole.

For Giardia, researchers are testing a vaccine called GID1. Early results show 70% of people developed protective antibodies. That’s promising - but it’s still years away.

Climate change is making things worse. Warmer temperatures and heavier rains mean more runoff into water sources. Experts predict Giardia will spread into new areas of the U.S. by 2040. What was once a summer camp problem might become a year-round issue.

And drug resistance? It’s real. In Southeast Asia, 15% of Giardia cases no longer respond to metronidazole. In North America, it’s still around 5%. But that number is climbing. That’s why it’s critical to take the full course - never skip doses.

What Happens If You Ignore It?

Pinworms? Usually, nothing serious. Just sleepless nights and frustration. But in rare cases, girls can get vaginal irritation if worms migrate.

Giardia? That’s different. Long-term, untreated Giardia can lead to malnutrition, lactose intolerance that lasts for years, and even damage to the gut lining. Some people never fully recover their digestion. It’s not fatal - but it can ruin your quality of life.

And here’s the hidden cost: missed school, missed work, doctor visits, and the emotional toll on families. One parent on MedHelp wrote: “I thought my son had a virus. Six months later, we found out it was Giardia. He lost 15 pounds. We didn’t know what we were dealing with.”

What Should You Do Next?

If you or your child has persistent diarrhea, especially after travel or swimming, ask your doctor for a Giardia stool test. Don’t wait. If there’s nighttime itching around the anus, do the tape test. It’s free and fast.

Treatment works - but only if you do it right. Take all the pills. Treat everyone in the house. Wash everything. Don’t assume it’s gone after one dose. And don’t be embarrassed. These infections are common, treatable, and nothing to be ashamed of.

Parasites don’t care about your income, your education, or your hygiene habits. They care about one thing: an opportunity to survive. Your job? Close the door before they walk in.