Giardia is the most commonly identified intestinal parasite in the United States and the classic cause of waterborne illness from untreated surface water — the streams, lakes, and shallow wells behind the folk name “beaver fever.” Where Cryptosporidium is the parasite that defeats a properly running municipal plant, and Cyclospora is the one that rides fresh produce, Giardia is the parasite of the water you didn’t treat: the mountain stream on a backpacking trip, the untested private well near a septic system or pasture, the daycare where hand-washing lapses.
The good news embedded in that framing is that Giardia is more manageable than its cousin Cryptosporidium. It is larger, it is easier to filter, and — unlike Crypto — it can be inactivated by chlorine given enough contact time. That combination is exactly why the first federal surface-water treatment rule was written around Giardia, and why properly treated municipal water reliably keeps it out. The risk is concentrated where that treatment is absent.
What Is Giardia?
Giardia duodenalis (also called Giardia lamblia or Giardia intestinalis) is a flagellated protozoan parasite that colonizes the small intestine and causes giardiasis. It exists in two forms. The trophozoite is the active, feeding, teardrop-shaped organism that attaches to the intestinal wall and multiplies. The cyst is the dormant, armored survival form — roughly 8 to 12 micrometers across, larger than a Cryptosporidium oocyst — that is shed in feces and can survive for weeks to months in cold water. A person swallows cysts; the cysts open in the gut and release trophozoites; the trophozoites multiply and eventually form new cysts that are shed to continue the cycle.
Two features make Giardia an effective waterborne pathogen. The cyst is immediately infectious when shed — no environmental maturation is required, unlike Cyclospora — and the infectious dose is very low, on the order of ten or fewer cysts. An infected human or animal can shed hundreds of millions to billions of cysts per day, so even a small amount of fecal contamination in a water source can carry an infective load. Because Giardia infects a wide range of mammals — beavers, muskrats, dogs, cattle, deer — a pristine-looking wilderness stream downstream of animal activity is a genuine risk, which is the origin of the “beaver fever” nickname.
How Giardia Gets Into Water
Giardia transmission is fecal-oral, and the cysts reach water through human and animal sewage:
- Untreated surface water is the signature route. Backcountry streams and lakes downstream of wildlife, grazing livestock, or upstream campers routinely carry Giardia cysts. Drinking untreated wilderness water is the textbook exposure.
- Private wells that are shallow, poorly sealed, or located near septic systems, barnyards, or pastures can draw in cyst-laden surface infiltration, especially after heavy rain. Private wells are not covered by federal treatment requirements, which puts the burden on the owner.
- Recreational water — lakes, rivers, and inadequately chlorinated pools and splash pads — spreads Giardia when swimmers swallow contaminated water.
- Person-to-person and daycare spread is significant. Because the cyst is immediately infectious, giardiasis passes directly between people through the fecal-oral route, and outbreaks in childcare settings and households are common. It is also a frequent cause of traveler’s diarrhea.
In a conventional municipal system, Giardia is controlled at two barriers: physical filtration removes the relatively large cysts efficiently, and disinfection finishes any that pass. Unlike Cryptosporidium, Giardia is susceptible to chlorine at achievable contact times, so a plant that meets its treatment targets reliably delivers Giardia-free water. The failures that cause municipal Giardia outbreaks are treatment lapses — inadequate filtration or insufficient disinfectant contact — and they are far less common than the untreated-source exposures that account for most US cases.
Health Effects
Symptoms
Giardiasis symptoms usually begin 1 to 3 weeks after infection — a longer incubation than most enteric illnesses — and this delay often disconnects the illness from the water exposure in a patient’s memory. The hallmark presentation is distinctive:
- Diarrhea that is greasy, foul-smelling, and tends to float (steatorrhea, from fat malabsorption) — a feature that helps distinguish giardiasis from watery viral diarrhea
- Prominent gas, bloating, and abdominal cramping
- Nausea and loss of appetite
- Weight loss and fatigue over time
Many infected people have no symptoms at all yet still shed cysts, which sustains transmission. Among those who do get sick, the illness can be prolonged: untreated giardiasis often lasts 2 to 6 weeks, and a subset of patients develop chronic symptoms with intermittent loose stools that persist for months. A well-recognized aftereffect is temporary lactose intolerance that can outlast the infection by weeks, along with, in some cases, a post-infectious irritable bowel pattern. In children, repeated or chronic giardiasis can impair nutrient absorption enough to affect growth.
Treatment
Giardiasis is treated with prescription antiparasitic drugs — most commonly metronidazole (Flagyl), tinidazole, or nitazoxanide. Treatment is effective, though relapses and reinfection occur, particularly where the source exposure continues. Because symptoms overlap with many gastrointestinal conditions and the incubation is long, diagnosis relies on a stool test for cysts or antigen; anyone with the greasy-diarrhea-plus-bloating pattern after drinking untreated water or traveling should raise Giardia specifically with their clinician.
Higher-Risk Groups
Dehydration is the main acute danger, so infants, young children, and older adults warrant closer attention. Immunocompromised individuals and people with certain immunoglobulin deficiencies may experience more severe or persistent disease. Pregnant people face the usual caution around drug selection and dehydration. For most healthy adults, giardiasis is miserable but self-limiting and fully treatable.
Giardia and Chlorine: The Key Difference From Cryptosporidium
The most consequential fact about Giardia for water treatment is that, unlike Cryptosporidium, it is vulnerable to chlorine. Giardia cysts are more resistant than bacteria and viruses, so they require a higher disinfectant dose and longer contact time (a higher “CT” value) than a routine bacterial kill — but that dose is achievable within a normally operated treatment plant. Cryptosporidium, by contrast, shrugs off practical chlorine doses entirely.
This single difference explains the regulatory history. When the EPA wrote the Surface Water Treatment Rule (SWTR) in 1989, it set the benchmark around Giardia: every filtered surface-water system must achieve at least 3-log (99.9%) removal or inactivation of Giardia lamblia cysts through the combined effect of filtration and disinfection. Giardia was the right yardstick precisely because the filtration-plus-chlorine barrier could reliably control it. It was only after the 1993 Milwaukee Cryptosporidium disaster — where that same barrier failed against a chlorine-immune parasite — that the EPA layered on the Crypto-specific rules (IESWTR, LT2ESWTR) described in our Cryptosporidium profile. In short: the drinking-water rulebook was built on Giardia first, then hardened for Crypto.
How WaterVerge Tracks Giardia
Like the other protozoa, Giardia is regulated through a Treatment Technique rather than a Maximum Contaminant Level — utilities are required to achieve removal and inactivation credit, not to test finished water for cysts. So Giardia does not appear as a direct reading on WaterVerge city pages. We track the signals that reflect whether that treatment barrier is intact:
- Surface Water Treatment Rule compliance — turbidity performance and treatment-technique violations that indicate filtration or disinfection problems relevant to Giardia removal.
- Coliform bacteria and E. coli detections and Total Coliform Rule violations, which flag possible fecal intrusion into a system.
A utility with a clean surface-water treatment and coliform record is one that is reliably controlling Giardia. The higher-risk situations are the ones outside that framework entirely: private wells and untreated recreational or backcountry sources, where no compliance data exists because no utility is responsible.
How to Remove Giardia
Municipal Scale
Conventional treatment — coagulation, flocculation, sedimentation, and filtration — removes the relatively large Giardia cysts efficiently, and chlorine disinfection at adequate contact time inactivates the remainder. Properly operated plants meet the SWTR’s 3-log Giardia requirement as a matter of routine. UV disinfection and ozone provide additional inactivation credit.
Point-of-Use and Field Treatment
For private wells, backcountry water, and boil-water advisories, several barriers are effective. Because Giardia cysts are larger than Cryptosporidium oocysts, they are the easier of the two to filter — any device rated to stop Crypto stops Giardia:
| Method | Effective? | Certification to look for | Notes |
|---|---|---|---|
| Boiling (1 minute) | Yes — complete | N/A | Definitive; 3 minutes above 6,500 ft |
| Reverse osmosis | Yes | NSF/ANSI 58 | Membrane pores far below cyst size |
| 1-micron absolute filter | Yes | NSF/ANSI 53 (cyst reduction) | “Absolute,” not “nominal,” pore size |
| Backcountry filter (0.2–1 µm) | Yes | NSF/ANSI 53; look for “cyst” claim | Standard for wilderness use |
| UV (point-of-use / SteriPEN) | Yes | NSF/ANSI 55 Class A | Requires clear, pre-filtered water |
| Chlorine tablets | Partial / slow | — | Works but needs long contact; cold water slows it |
| Iodine tablets | Partial / slow | — | Similar limits to chlorine; not for long-term use |
| Standard carbon pitcher | No | — | Removes taste, not cysts |
Boiling is the most reliable field measure — one minute at a rolling boil destroys cysts. For a permanent tap barrier, a certified reverse osmosis system or a filter carrying NSF/ANSI 53 cyst reduction with a 1-micron absolute pore size physically excludes Giardia. Backpackers should choose a filter with an explicit cyst or 0.2–1 micron rating; unlike for Cryptosporidium, halogen tablets do eventually work against Giardia, but they act slowly and are hampered by cold water, so filtration or boiling is the more dependable field choice.
If you are on a private well, testing is the foundation of any decision — a well near a septic system, pasture, or subject to surface infiltration should be tested, and repeat coliform positives are a warning sign for the kind of contamination that also carries Giardia. Our well water testing guide covers what to test for and how often, and the how to test your tap water guide compares lab and home options.
Frequently Asked Questions
Does boiling kill Giardia?
Yes. Bringing water to a rolling boil for one minute (three minutes above 6,500 feet) reliably destroys Giardia cysts. Boiling is the most dependable way to make backcountry or advisory water safe from this parasite.
Does chlorine kill Giardia?
Yes, but more slowly than it kills bacteria. Giardia cysts need a higher chlorine dose and longer contact time, which a municipal plant achieves routinely. Chlorine tablets for field use do work against Giardia, but they act slowly — especially in cold water — so filtering or boiling is more reliable when you need water quickly.
How long does giardiasis last?
Symptoms usually start 1 to 3 weeks after infection and, untreated, often last 2 to 6 weeks. Some people develop chronic symptoms lasting months, and temporary lactose intolerance can linger after the infection clears. Prescription antiparasitic treatment shortens the course.
Can you get Giardia from tap water?
From properly treated municipal tap water, it is very unlikely — the parasite is efficiently filtered and inactivated by standard treatment, and the EPA’s Surface Water Treatment Rule is built around removing it. The real risk is untreated water: wilderness streams, shallow or poorly protected private wells, and recreational water that people accidentally swallow.
Is Giardia the same as Cryptosporidium?
No, though both are chlorine-tolerant intestinal parasites removed by fine filtration. Giardia cysts are larger, easier to filter, and — unlike Cryptosporidium — can be inactivated by chlorine at achievable contact times. That is why municipal treatment controls Giardia so reliably. Our waterborne parasites guide compares Giardia, Cryptosporidium, and Cyclospora side by side.
Check Your City
Giardia is controlled by the treatment barriers your utility is required to maintain, so the meaningful question for tap water is whether those barriers are intact — and that is measurable.
Search your city on WaterVerge to review your utility’s surface-water treatment compliance and coliform history. If you draw from a private well or spend time drinking from backcountry sources, the risk calculus is different: test your well, carry a cyst-rated filter or the means to boil, and treat every untreated source as a potential Giardia exposure regardless of how clean it looks.