Giardiasis is an intestinal infection caused by a microscopic parasite called Giardia. It’s one of the most common waterborne illnesses worldwide, causing over 1.1 million infections per year in the United States alone. The infection targets the small intestine, leading to watery diarrhea, cramping, and bloating that can last anywhere from a couple of weeks to several months.
How Giardia Infects the Body
Giardia exists in two forms during its life cycle. The first is the cyst, a tough, shell-like stage that survives outside the body in water, soil, and on surfaces. Cysts thrive in cool, moist environments and have a metabolic rate only 10% to 20% of the active parasite, which lets them persist in the environment for extended periods. This durability is a big part of why giardiasis spreads so effectively.
When you swallow even a few cysts, they travel to your small intestine and transform into trophozoites, the active form of the parasite. Trophozoites use a suction-cup-like disk on their underside to latch onto the intestinal wall, where they feed and multiply. As they move further down the intestine, they re-form into cysts that pass out in stool, ready to infect the next person. This is why giardiasis cycles so efficiently through communities, daycare centers, and contaminated water sources.
How It Spreads
Giardia spreads through the fecal-oral route, meaning the cysts shed in an infected person’s (or animal’s) stool end up being swallowed by someone else. Swallowing just a few cysts is enough to cause infection. The most common ways this happens include:
- Contaminated water: Drinking from lakes, streams, or pools that contain Giardia, or drinking untreated tap water in areas with poor sanitation
- Person-to-person contact: Especially in childcare settings where diaper changing creates opportunities for spread
- Contaminated food: Produce washed with contaminated water or handled by someone carrying the parasite
- Contact with infected animals: Dogs, cats, cattle, and wildlife can all carry Giardia
- Contaminated surfaces: Bathroom fixtures, toys, and other objects can harbor cysts
Giardiasis affects roughly 2% to 5% of the population in developed countries and 20% to 30% in developing nations, where access to clean water and sanitation is limited. In the U.S., about 13,800 cases were reported to the CDC in 2022, though the true number of infections is far higher since many people never get tested.
Symptoms and Timeline
After swallowing the parasite, symptoms typically appear within 1 to 3 weeks, with an average incubation period of about 7 days. The hallmark symptoms are watery, foul-smelling diarrhea, stomach cramps, bloating, gas, nausea, and fatigue. Some people also experience weight loss and a general feeling of being unwell. The diarrhea in giardiasis tends to be greasy or soft rather than bloody, which helps distinguish it from some other intestinal infections.
Acute giardiasis usually lasts 1 to 3 weeks, though symptoms can persist for 2 to 6 weeks in some cases. A significant number of people carry the parasite without ever developing symptoms at all. For others, the infection becomes chronic, with symptoms that come and go over weeks or months. Chronic giardiasis can lead to poor nutrient absorption and ongoing fatigue, which is particularly concerning for young children, who may experience slowed growth and development if the infection isn’t treated.
How Giardiasis Is Diagnosed
If your doctor suspects giardiasis, they’ll ask you to provide one or more stool samples. Because the parasite isn’t always shed consistently, you may need to collect samples on several different days to improve the chances of detection. The lab checks your stool for Giardia cysts or proteins produced by the parasite. A single negative test doesn’t necessarily rule out infection, which is why repeat testing is sometimes needed.
Treatment
Giardiasis is treated with a short course of antiparasitic medication. The most commonly used options include a single-dose treatment with cure rates ranging from 80% to 100%, or a multi-day course taken two to three times daily for five to seven days. The multi-day option is effective but tends to cause more stomach-related side effects like nausea and a metallic taste. Most people start feeling better within a few days of beginning treatment.
Even after the parasite is cleared, some people notice lingering digestive symptoms. Temporary difficulty digesting dairy products is common because the infection can irritate the lining of the small intestine where lactose is normally broken down. This usually resolves on its own over a few weeks, but avoiding dairy during recovery can help with comfort.
Preventing Giardia Infection
Water safety is the single most important factor in preventing giardiasis. If you’re hiking, camping, or traveling somewhere with uncertain water quality, don’t drink untreated water from lakes, rivers, or streams, no matter how clean they look. Boiling water for at least one minute kills Giardia cysts reliably. If boiling isn’t practical, a portable water filter with an absolute pore size of 1 micron or smaller will physically remove the cysts. Look for filters labeled NSF 53 or NSF 58 for added assurance. Reverse osmosis systems also work.
Beyond water treatment, thorough handwashing with soap and water makes a significant difference, particularly after using the bathroom, changing diapers, and before preparing food. If you swim in pools, lakes, or water parks, try not to swallow the water. Giardia is resistant to standard chlorine levels used in many pools, so chlorination alone isn’t a guarantee. When traveling internationally, stick to bottled or treated water and avoid ice, raw salads, and peeled fruit washed in local water.