Giardia lamblia is a single-celled parasite that infects the small intestine and causes diarrheal illness. It’s one of the most common waterborne parasites in the world, responsible for an estimated 1.1 million illnesses per year in the United States alone. The infection it causes, called giardiasis, typically brings on watery diarrhea, cramping, and bloating that can last for weeks if untreated.
What Giardia Lamblia Looks Like
Giardia lamblia (also called Giardia intestinalis or Giardia duodenalis) is a microscopic organism, not a bacterium or virus. It’s a flagellated parasite, meaning it propels itself using whip-like tails. The active feeding form, called a trophozoite, is pear-shaped and tiny: roughly 12 to 15 micrometers long and 5 to 9 micrometers wide, far too small to see without a microscope.
What makes Giardia distinctive is a suction-cup-like disk on its underside. This concave disk covers the entire bottom surface of the parasite and lets it latch firmly onto the lining of your small intestine. It also has four pairs of flagella extending from different regions of its body, giving it the ability to swim through intestinal fluid. Under a microscope, its two large nuclei give it an appearance sometimes compared to a face with two eyes.
How It Lives and Spreads
Giardia exists in two forms. The trophozoite is the active form that feeds and reproduces inside your gut. The cyst is a dormant, armored form that the parasite builds around itself as it travels toward your colon. Cysts are what leave your body in stool, and they’re remarkably tough. They can survive for several months in cold water, waiting to be ingested by the next host.
When you swallow cysts through contaminated water, food, or contact with contaminated surfaces, they reach your small intestine and crack open. Each cyst releases two trophozoites, which immediately begin multiplying by splitting in half. They either float freely in the upper small intestine or attach to the intestinal wall with their suction disk. As some trophozoites drift toward the colon, they re-encyst, forming new cysts that pass out in stool and continue the cycle.
Giardia spreads easily. Swallowing just a few cysts can be enough to start an infection. The most common routes include drinking untreated water from lakes, rivers, or springs; close contact with an infected person (particularly in childcare settings); contaminated food; and contact with infected animals or their feces. It can also spread through sexual contact involving fecal exposure.
Who Gets Infected Most Often
In 2022, nearly 14,000 cases were reported to the CDC, with an overall incidence of about 5.2 cases per 100,000 people. The Northeast had the highest regional rate at 6.8 per 100,000, while Alaska had the highest state-level rate at 13.3 per 100,000. Men accounted for about 61.5% of reported cases. The age groups with the highest infection rates were children under 5 and adults in their early 30s.
People at elevated risk include those who work in or attend childcare centers, backpackers and campers who drink untreated surface water, international travelers visiting areas with limited water sanitation, and anyone in close contact with an infected person. Pet owners should also know that dogs and other animals can carry Giardia, though not all animal strains infect humans.
Symptoms and Timeline
Symptoms typically appear one to three weeks after swallowing the parasite. The hallmark signs are loose, watery stools that sometimes look greasy or soft, stomach cramps and bloating, excessive gas, and nausea. The stools often have a particularly foul smell. Unlike infections that cause bloody diarrhea, giardiasis usually does not involve blood or mucus in the stool.
Not everyone who swallows Giardia gets sick. Some people carry the parasite without symptoms but can still pass cysts to others. When symptoms do develop, they can last two to six weeks without treatment. In some cases, the infection becomes chronic, leading to ongoing digestive problems, weight loss, and difficulty absorbing nutrients. Temporary lactose intolerance is a well-known aftereffect, sometimes persisting for weeks or months after the parasite itself is gone.
How It’s Diagnosed
Diagnosis requires a stool sample, sometimes more than one. Labs examine the sample for Giardia cysts or trophozoites, but detection can be tricky. The parasite isn’t always present in every bowel movement, even during an active infection. Your provider may ask you to submit samples collected on different days to improve the chances of finding the organism. Labs can also test for Giardia-specific proteins in stool, which tends to be more reliable than looking for the parasite under a microscope.
Treatment
The standard treatment is an antiparasitic medication taken for five to seven days, which clears the infection in about 90% of cases. For infections acquired outside the United States, a single-dose antiparasitic option is widely available and commonly used. Most people start feeling better within a few days of beginning treatment, though digestive symptoms like bloating and loose stools can linger for a short time after the parasite is eliminated.
For people whose infection doesn’t respond to the first round, alternative medications or a longer course may be needed. Staying hydrated during the illness is important, since prolonged diarrhea can lead to significant fluid loss, especially in young children.
Preventing Giardia Infection
Because cysts are so resilient, standard levels of chlorine used in some water treatment systems don’t always kill them. If you’re relying on water from a lake, stream, spring, or shallow well, you have two reliable options: boil it at a full rolling boil for one minute, or filter it through a device with an absolute pore size of one micron or smaller. Filters that meet NSF/ANSI Standard 53 or 58 for cyst removal are also effective.
Beyond water, basic hygiene makes a big difference. Washing hands thoroughly after using the bathroom, changing diapers, and before preparing food reduces transmission in households and childcare settings. Swallowing water while swimming in lakes, rivers, or even pools is another common exposure route, since Giardia cysts can survive in recreational water despite chlorination. If you’re traveling in a region where tap water quality is uncertain, stick to bottled or boiled water and avoid ice made from tap water.