Giardia: the water-borne parasite behind the weeks of diarrhea and bloating
Last updated September 3, 2026.
Giardia (giardiasis) is a tiny parasite caught from contaminated water (the streams, the lakes, the pools, the travel destinations): causing the prolonged diarrhea, the dramatic bloating and gas, the sulfur-tasting burps, and the cramps. Unlike the usual stomach bugs, it drags on for weeks without treatment (the fatigue and the weight loss accumulate), it is diagnosed by the stool test, and it clears with a short antibiotic course.
What does it feel like?
The pattern, arriving one to two weeks after the exposure: the watery or greasy, foul-smelling diarrhea (often floating and hard to flush: the fat-malabsorption sign), the dramatic bloating and gas, the rotten-egg (sulfur) burps, the cramping, and the nausea, without much vomiting or fever (which distinguishes it from the bacterial bugs). Instead of clearing in days, it grinds on: weeks of the loose stools, the exhaustion, and the weight quietly falling. The hiker's diarrhea and the traveller's lingering bug are its famous forms.
How is it caught?
The parasite's cysts travel in water: the mountain streams (beautiful, and contaminated by the animals upstream: the hiker's classic), the lakes and the rivers, the inadequately-treated pools and the hot tubs, the travel destinations' tap water and ice, the daycare centers (the children pass it hand-to-mouth), and the person-to-person spread in households. The cysts survive the ordinary chlorine doses and the cold: the streams that look pristine are the classic trap, and the filter-or-boil rule is the hiker's answer.
How is it treated?
- The stool test: the sample confirming it (the antigen test now, genuinely sensitive): worth doing for any diarrhea lingering past a week or two, especially post-travel or post-stream.
- The antibiotic course: the metronidazole or tinidazole (often a single dose or a few days): genuinely curative.
- The household hygiene: the handwashing (soap, not gel), the towels separate, the swimming paused until the course finishes: it spreads.
- The lactose aftermath: the gut's brush border is stripped: the temporary dairy intolerance for the weeks after is common and passes.
- The retreatment question: the persistence or relapse happens (the course repeated or the drug switched).
When is it urgent?
Giardia is clinic medicine; the dehydration signs are the urgent tier (the very dry mouth, the no-urine day, the dizziness standing: in the children and the elderly especially), and the blood in the stool or the high fever suggests a different bug entirely (the bacterial colitis: different door). The weight falling fast or the symptoms past two weeks warrant the test promptly. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
The stream looked clean. How could it make me ill?
The oldest trap in outdoor medicine: the giardia cysts come from the feces of the animals and the people upstream (the beavers are the famous carriers: the old beaver-fever name), they are microscopic (the clearest, coldest, fastest mountain stream carries them invisibly), and they survive the cold and the ordinary chlorine doses that kill the bacteria. The pristine look is precisely irrelevant (the contamination is upstream and microscopic), which is why the hiking rule is absolute: filter, boil, or treat all surface water (the filters rated for cysts, the rolling boil for a minute, the purification tablets), however clean it looks. The backpackers' diarrhea of the guidebooks is this parasite, and your camping trip's stream was the ordinary way to meet it.
Why has this lasted three weeks when stomach bugs clear in days?
Because giardia is a parasite, not a virus or a quick bacterium: it sets up residence on the small intestine's wall (coating the brush border that absorbs the food), it does not burn out the way the stomach-flu viruses do, and without treatment it grinds on for weeks to months (the loose stools, the bloating, the gas, and the slow weight loss from the malabsorption), which is why the duration itself is the diagnostic clue (your three weeks is the classic asking-for-the-test point: the ordinary bugs are gone by day five). The eggy burps are its signature (the sulfur gases of the parasite's metabolism). The antibiotic course is what ends it: this one does not resolve by waiting, and the test-plus-treatment is quick and genuinely curative.
What is the test like?
The stool sample, and the modern test is good: you collect a small sample in the provided pot (at home, into the container, genuinely undramatic), and the lab runs the antigen test (detecting the parasite's proteins: sensitive from a single sample, unlike the old microscopy that needed three samples over days to catch the cysts shedding intermittently). The result returns in days, and the treatment follows immediately on the positive (and given your classic picture, the doctor may treat on the story alone while the test processes: the empiric treatment is reasonable for the textbook presentation). The test's other job: the negative result redirects the hunt (the other prolonged-diarrhea causes), so it earns its place either way.
Why have I lost weight, and will it come back?
The malabsorption mechanism: the parasite coats the small intestine's absorptive surface (the brush border where the fats and the nutrients cross), so for the weeks of the infection the food passes through partially undigested (the greasy, floating, foul stools are the unabsorbed fat), and the weight drifts down as the calories leak. It reverses completely: once the antibiotic clears the parasite, the gut lining regrows its brush border over the weeks (the weight returns with the appetite), with the one common temporary wrinkle: the dairy intolerance for a few weeks after (the lactase enzyme lives on the stripped border and regrows last: the milk and the ice cream may bloat you temporarily: it passes). Four pounds at three weeks is the expected scale of this and recovers.
Can I pass it to my family?
Yes, genuinely, and the hygiene window matters: giardia spreads fecal-oral (the cysts from the hands to the mouths: the households and the daycare centers are its highways), so until the course is finished: the handwashing with soap (the alcohol gels do not kill the cysts: soap and water, genuinely), the towels and the bedding kept separate, the toilet surfaces cleaned, the food-preparing ideally handed to someone else, and the swimming pools avoided (the cysts survive the chlorine: the pool rule is two weeks after the symptoms stop). The young children and the babies are the genuine caution (the diaper-changing handwashing is the critical moment). Once the course is done and the stools have normalized, the contagious window closes.
Do I need to tell anyone, and will it come back?
The notification question: in the UK, giardiasis is a reportable infection (the lab notifies the public-health team automatically: nothing for you to do, and the occasional follow-up call is routine), and the food-handlers and the healthcare workers get the return-to-work guidance. The relapse question: the single course cures the great majority, but the relapse happens (the course repeated or the drug switched: genuinely manageable), and the symptoms returning after a clear week or two earn the re-test rather than the shrug. The future prevention is the one durable lesson: the filter-or-boil rule for every stream, the bottled-or-boiled water in the travel destinations, and the pool hygiene: the camping trips continue, the stream water does not.