A backpacker filters water from a clear mountain stream, hikes another six days, returns home healthy, and develops foul-smelling diarrhea two weeks later. By week three the diarrhea is sulfurous, the cramps wake them at night, they've lost six pounds, and the lab finally orders a stool antigen test. It comes back positive for Giardia. The trip was the source, even though they used a filter. The filter was the wrong type.
CDC estimates 1.2 million US Giardia infections per year, making it the most common intestinal parasite in North America. About 15,000 cases are formally reported, an enormous underestimate because mild cases never seek care and most clinicians do not test routinely. Outbreaks have been linked to drinking water systems, swimming pools, daycare centers, food handlers, and contaminated produce. Field-purification methods are covered in the emergency water purification guide, and outbreak patterns appear in waterborne disease outbreaks. The parent pillar is the infection prevention guide.
Key Takeaways
- Giardia duodenalis (also called G. lamblia or G. intestinalis) is a flagellated protozoan that infects the small intestine.
- Transmission is fecal-oral via contaminated water, food, surfaces, or direct person-to-person contact.
- Cysts are highly resistant to chlorine at standard pool concentrations; filtration or boiling reliably removes them.
- Symptoms include foul-smelling diarrhea, gas, abdominal cramps, weight loss, and fatigue, lasting 1 to 6 weeks untreated.
- Diagnosis is stool antigen testing or PCR; microscopy is less sensitive.
- Metronidazole, tinidazole, and nitazoxanide are first-line treatments; resistance is uncommon.
What is giardia?
Giardia duodenalis is a single-celled protozoan that lives in the small intestine, where it attaches to the epithelium with a ventral sucking disc. The trophozoite form (motile, flagellated) does the damage; the cyst form (oval, environmentally hardy) does the transmission.
Cysts shed in feces survive in cold water for weeks to months, in soil for months, and resist standard chlorination at swimming pool concentrations. As few as 10 cysts can establish infection in a human host. The combination of low infectious dose, environmental persistence, and chlorine resistance is what makes giardia the dominant waterborne pathogen in North American outbreaks.
The genus was first described by Antonie van Leeuwenhoek in 1681 from his own diarrhea, possibly the first protozoan ever observed under a microscope. Modern molecular work shows that G. duodenalis includes several assemblages (A through H), of which A and B infect humans. Other assemblages are largely host-restricted to dogs, cats, rodents, and livestock.
How does giardia spread?
Fecal-oral transmission has several routes:
| Route | Examples |
|---|---|
| Contaminated water (drinking) | Backcountry streams, lakes, wells with surface contamination, inadequately treated municipal water |
| Contaminated water (recreational) | Swimming pools (cyst chlorine resistance), water playgrounds, lakes, hot tubs |
| Person-to-person | Daycare centers, household contacts of infected children, men who have sex with men |
| Foodborne | Produce washed in contaminated water, food handlers with poor hand hygiene |
| Animal-to-person | Less common than once thought; most human strains are human-adapted |
Backcountry water has historically been blamed for the bulk of US cases, hence "beaver fever," but case-control studies suggest person-to-person, daycare, and recreational water account for more cases nationally than wilderness hiking. The risk is still real for backcountry trips, particularly from streams downstream of livestock pastures or heavily-used campsites.
What are the symptoms?
Symptoms begin 1 to 3 weeks after ingestion, often catching patients by surprise because they've forgotten the relevant exposure.
- Diarrhea (most patients): typically frequent loose stools, foul or sulfurous odor described as distinctive, occasional greasy or floating stools indicating fat malabsorption
- Gas and bloating: often severe, with abdominal cramping
- Abdominal pain: crampy, often relieved by defecation
- Weight loss: 5 to 10 pounds over a few weeks is common in symptomatic cases
- Fatigue: sometimes profound; can persist after parasite clearance
- Nausea and anorexia: common; vomiting is less common
- Low-grade fever: in a minority of cases
Roughly 25 to 50 percent of infected adults are asymptomatic and clear infection without diagnosis. Children, particularly under 5, more often have prolonged symptoms and growth faltering in heavily-exposed populations.
Untreated giardiasis lasts 1 to 6 weeks in most cases. A subset develops chronic symptoms: malabsorption, lactose intolerance, weight loss, and persistent diarrhea lasting months. Post-infectious irritable bowel syndrome and chronic fatigue are described after acute giardiasis and can persist for a year or more after parasite clearance.
How is giardia diagnosed?
Diagnostic options:
- Stool antigen test (immunoassay): ELISA or rapid lateral flow; sensitivity 85 to 95 percent on a single stool sample; widely available; the first-line test for most labs
- Stool PCR multiplex panels: highly sensitive and specific; also detects other diarrheal pathogens simultaneously; increasingly the standard at large hospitals
- Stool microscopy (ova and parasite exam): sensitivity only 60 to 80 percent on a single sample; three sequential samples improve yield; requires experienced microscopist
- Endoscopic biopsy: rarely needed; reserved for chronic symptoms with negative stool testing
The PCR vs antigen vs serology post covers the principles behind these assays. For most patients with compatible symptoms and possible exposure, stool antigen or PCR is the right first step.
How is giardia treated?
Several effective drugs exist. Choice depends on age, pregnancy status, and tolerance.
| Drug | Adult dose | Notes |
|---|---|---|
| Tinidazole | 2 g single dose | High cure rate; preferred for adults |
| Metronidazole | 500 mg three times daily for 5 to 7 days | Older but effective; metallic taste, alcohol interaction |
| Nitazoxanide | 500 mg twice daily for 3 days | Pediatric-friendly liquid; less GI side effect than metronidazole |
| Paromomycin | 25 to 35 mg/kg/day for 5 to 10 days | Pregnancy-safe; less effective than other agents |
| Albendazole | 400 mg daily for 5 days | Effective; useful when co-infection with other parasites suspected |
Cure rates run 85 to 95 percent with first-line therapy. Treatment failures are usually addressed by switching to a different drug class. Resistance is uncommon but documented; refractory cases may benefit from combination therapy.
Post-infectious lactose intolerance is common for weeks after treatment. Patients should expect gradual return of normal digestion, not instant resolution.
How do you prevent giardia from drinking water?
Water treatment matters more than the source. Filter pore size, chemical methods, and boiling each have their place.
- Boiling: roll for at least 1 minute (3 minutes at altitudes over 6,500 feet). Reliable for giardia, viruses, and bacteria.
- Filtration: absolute pore size 1 micron or smaller; many hiking filters now use 0.2 micron hollow-fiber technology that removes giardia, cryptosporidium, and bacteria. Squeeze filters and pump filters of this rating are effective.
- Chemical treatment: chlorine dioxide tablets (Aquamira, Katadyn Micropur) require 15 to 30 minutes contact time but kill giardia and cryptosporidium. Iodine tablets kill giardia but not reliably cryptosporidium; not recommended as sole method in some regions.
- UV light pens: SteriPEN-type UV devices kill giardia in clear water within 60 to 90 seconds. Less effective in cloudy water; require batteries.
Filters labeled "removes bacteria only" or with 5-micron rating do not reliably remove giardia. Older drugstore "purifying" tablets are unreliable; current EPA-registered products work when used correctly.
The emergency water purification guide covers field methods in detail.
How do you prevent person-to-person spread?
Most US giardia cases now come from person-to-person and recreational water sources rather than wilderness drinking water.
- Strict hand hygiene before food preparation and after toileting or diaper changes; the hand hygiene technique guide covers proper technique
- Daycare centers should exclude symptomatic children until diarrhea resolves and have rigorous diaper-changing protocols
- Public pools should follow CDC's Model Aquatic Health Code, including hyperchlorination after suspected fecal incidents
- Symptomatic patients should not prepare food for others or swim recreationally until symptoms resolve
- Households with one infected member often have asymptomatic carriage in others; testing and treatment of household contacts is sometimes warranted
FAQ
Can you get giardia from chlorinated swimming pools?
Yes. Giardia cysts survive standard pool chlorine concentrations long enough to infect other swimmers, especially in pools with high bather loads, inadequate filtration, or recent fecal incidents. Cryptosporidium is even more resistant. Recreational water outbreaks have been linked to splash pads, water playgrounds, and hotel pools. Avoid swallowing pool water.
Does giardia go away on its own?
In healthy adults, most acute giardiasis resolves within 1 to 6 weeks without treatment. The reasons to treat are to shorten symptoms, prevent transmission to household contacts, prevent post-infectious complications like irritable bowel syndrome, and avoid prolonged malabsorption. Treatment is fast, cheap, and well-tolerated, so empiric treatment of symptomatic confirmed cases is standard.
Can your dog give you giardia?
Possibly, though most human giardiasis is human-adapted strain assemblage A or B, while most canine giardiasis is assemblage C or D. A small proportion of cross-species transmission does occur. Households with an infected dog and an infected human are common, but the source is often shared environmental contamination rather than direct dog-to-human transfer. The pets and zoonotic disease post covers this in more detail.
Why does giardia cause such a distinctive smell?
Giardia disrupts fat absorption in the small intestine, producing steatorrhea (fatty stool). The undigested fats undergo bacterial fermentation in the colon, releasing volatile sulfur compounds that produce the characteristic foul or sulfurous odor patients and clinicians remember. Few other diarrheal pathogens reliably produce this smell.
Is giardia common in tap water?
US municipal water systems treat for giardia through filtration and disinfection. Modern surface water treatment plants are designed to achieve high log reduction of giardia, and outbreaks linked to municipal water are now rare in the US and Canada. Private wells, particularly shallow wells near septic systems or livestock, carry higher risk and are not routinely tested for giardia. See waterborne disease outbreaks for the broader picture.