Key facts
- What it is: a protozoan parasite (Giardia duodenalis) of the small intestine.
- Tell-tale sign: greasy, foul-smelling, floating diarrhea.
- Waterborne: cysts tolerate routine chlorine; classic in hikers/well water.
- Lingers: strongest post-infectious IBS link of the common parasites.
Giardia is one of the most common intestinal parasites — nicknamed “beaver fever” for its link to contaminated wilderness water. Its hardy cysts tolerate the chlorine levels in routine drinking water, so it spreads through lakes, streams, wells and municipal water, and also person-to-person (childcare and households), through food, and from animals.
Symptoms
The classic picture is greasy, foul-smelling, floating diarrhea with bloating, gas, abdominal cramps, nausea, fatigue and unintended weight loss, typically starting one to three weeks after exposure. Fever is usually absent, and many infections cause no symptoms at all. Acute illness often lasts two to six weeks, and a minority become chronically infected with malabsorption.
Why stool tests miss Giardia
Giardia is shed intermittently, so a single stool sample is often negative even when infection is present — the CDC recommends collecting several samples on different days. Stool antigen tests, direct fluorescent antibody (DFA) microscopy, and PCR panels outperform a single routine O&P. The GI-MAP detects Giardia by qPCR, and more on the wider picture is in our parasite testing guide.
Treatment
Effective prescription options include metronidazole, tinidazole, nitazoxanide and secnidazole; paromomycin is generally preferred in pregnancy. Asymptomatic carriers often don’t need treatment unless there’s a transmission risk, and some symptomatic cases resolve on their own over weeks.
Why symptoms can linger — the part most pages skip
Giardia has the best-documented link to post-infectious IBS of the common parasites. A meaningful share of people report lingering gut symptoms and fatigue for months after the parasite clears, and Giardia commonly triggers a temporary secondary lactose intolerance that can persist for weeks. This is where a measure-adjust-remeasure approach helps — see rebuilding your gut after an infection.
Still not right after Giardia? The GI-MAP™ helps assess the microbiome and gut markers during recovery.
Order Your GI-MAP™ Test → How it worksFrequently asked questions
How do you get Giardia?
By swallowing cysts, most often from contaminated water (cysts tolerate routine chlorine), but also person-to-person, from food, or animals.
What are the symptoms?
Greasy, foul-smelling, floating diarrhea with bloating, gas, cramps, fatigue and weight loss, starting 1–3 weeks after exposure. Many cases are asymptomatic.
Why do stool tests miss Giardia?
It's shed intermittently, so single samples can be negative — collect several on different days; antigen, DFA and PCR are more reliable.
Why do symptoms linger?
Giardia has the strongest post-infectious IBS link of the common parasites and can cause temporary lactose intolerance.
Sources & further reading
- CDC — Giardia testing & diagnosis.
- Mayo Clinic — Giardia infection: symptoms & causes.
- CDC Emerging Infectious Diseases — post-infectious syndromes after Giardia infection.
- Diagnostic Solutions Laboratory — GI-MAP test overview & methodology.