Key takeaways
- Giardia is a microscopic waterborne parasite that causes the gut infection giardiasis.
- It spreads through untreated water and fecal contamination (poor hygiene).
- Symptoms (diarrhea, gas, bloating, cramps, nausea, fatigue) appear ~1–3 weeks after exposure and can linger.
- Prevent it by treating all backcountry water and practicing good hygiene; treat it with prescription medication.
Named after the parasite Giardia lamblia.
This is general educational information, not medical advice. See a healthcare provider for diagnosis and treatment of suspected giardiasis.
What giardia is
Giardia (Giardia lamblia) is a microscopic parasite found in untreated backcountry water — lakes, streams, rivers contaminated by the feces of infected humans or animals — that causes the gut infection giardiasis. It’s one of the most common waterborne causes of illness for hikers who drink untreated water.
Symptoms
Giardiasis typically causes diarrhea (often foul-smelling), gas, bloating, cramps, nausea, greasy stools, and fatigue. A notable feature is the incubation period — symptoms usually appear about one to three weeks after ingestion, so people often don’t connect the illness to a past trip’s water. Symptoms can last weeks and sometimes recur; some people have mild or no symptoms while still being contagious.
Ten days after a backpacking trip where they drank straight from a ‘clean-looking’ stream, a hiker develops persistent diarrhea, bloating, and bad gas. A doctor diagnoses giardiasis and prescribes medication — and the hiker resolves to always treat their water from then on.
Prevention and treatment
Prevent it with water treatment and hygiene: treat all backcountry drinking water (filtering, boiling, UV, or chemicals all remove or kill the protozoan), and wash hands or sanitize after toileting and before food, since it also spreads person-to-person. Bury waste well away from water (use a proper cat hole) to protect sources. Treatment is prescription anti-parasitic medication — see a doctor for persistent GI symptoms after a trip. It’s the main reason hikers rely on water purification via a squeeze filter or chemical treatment.
The bottom line
Giardia is a microscopic waterborne parasite that causes giardiasis — diarrhea, gas, bloating, cramps, and fatigue that appear ~1–3 weeks after drinking contaminated untreated water and can linger for weeks. A leading reason hikers treat their water, it's prevented by purifying all drinking water (filters, boiling, UV, or chemicals all work) and good hygiene, and treated with prescription medication from a doctor.
Frequently asked questions
What is giardia?
Giardia is a microscopic parasite (Giardia lamblia/intestinalis) that infects the intestines and causes an illness called giardiasis. It's found in untreated water sources in the backcountry — lakes, streams, rivers — that have been contaminated by the feces of infected humans or animals, and is one of the most common waterborne causes of illness for hikers who drink untreated water.
What are the symptoms of giardiasis?
Giardiasis typically causes gastrointestinal symptoms: diarrhea (often foul-smelling), excessive gas, bloating, stomach cramps, nausea, greasy stools, and fatigue, sometimes with weight loss. A notable feature is the incubation period — symptoms usually appear about one to three weeks after you ingest the parasite, so people often don't connect the illness to a water source from a past trip. Symptoms can last for a couple of weeks or longer and sometimes recur, and some infected people have mild or no symptoms while still being contagious.
How do you prevent and treat giardia?
Prevention is about water and hygiene: treat all backcountry drinking water (by filtering, boiling, UV, or chemical treatment — filters and most methods remove or kill Giardia, which is a protozoan), and practice good hygiene, especially washing hands or using sanitizer after toileting and before handling food, since Giardia also spreads person-to-person via the fecal-oral route. Proper backcountry sanitation (like burying waste well away from water) protects water sources. Treatment for giardiasis is prescription anti-parasitic medication from a doctor, so see a healthcare provider if you develop persistent GI symptoms after backcountry travel.
Sources
- Giardia & giardiasis — Centers for Disease Control and Prevention
- Backcountry water safety — The Mountaineers