Short definition
Giardia is a single-celled parasite (protozoan) that causes severe diarrhea and is transmitted via the fecal-oral route from wild and domestic animal hosts. It’s common in backcountry surface water and shallow rural wells. Boiling, NSF/ANSI 53 absolute 1-micron filtration, and UV all reliably handle it.
What it is
Giardia (Giardia intestinalis, formerly G. lamblia) is a protozoan parasite that infects mammals including humans, beavers, dogs, cats, livestock, and wild rodents. The transmissible form — the cyst — is 8 to 12 microns long and survives in cold surface water for weeks to months.
Infection causes a flu-like illness with severe diarrhea, bloating, fatigue, and weight loss, typically appearing 1 to 2 weeks after exposure. Untreated giardiasis can run 2 to 6 weeks; definitive treatment is a prescription antiparasitic (typically metronidazole or tinidazole). It’s nicknamed “beaver fever” because beavers are a recognized reservoir host in North American watersheds.
Unlike cryptosporidium, giardia is more chlorine-susceptible — utility-scale disinfection (CT around 50 mg·min/L of free chlorine at 1 ppm) gets 99.9% inactivation. UV at about 5 mJ/cm² also gets 3-log inactivation. Boiling for at least 1 minute kills it reliably (3 minutes above 6,500 feet elevation). Filtration to NSF/ANSI 53 absolute 1-micron blocks the cyst.
Why it matters to a homeowner
For most Washington homeowners on Group A municipal supplies, giardia is a backcountry concern, not a home-tap concern. Public utilities running standard treatment (filtration + chlorination, or UV + chlorination for unfiltered systems) reliably control giardia. Outbreaks from WA utilities are rare.
For private-well users — particularly anyone on a shallow well, a well influenced by surface water, or a well in a livestock-grazing area — giardia is a real risk. Standard well-water tests for coliform and nitrates do not detect giardia. A clean coliform test does not rule it out. The protective answer for surface-influenced supplies is point-of-entry UV plus sub-micron filtration, not just routine bacterial testing.
For backcountry water (Cascades, Olympics, Methow, San Juans), assume giardia is present in any surface source. The Pacific Northwest watershed is a documented giardia reservoir.
When you’ll encounter this term
- Rural shallow well user notices a long-running unexplained GI illness — giardiasis on the doctor’s differential
- Cabin opening in spring after long absence — flush, then test or boil first-use water if the well is surface-influenced
- Daycare or school in a rural Group B water system — small systems sometimes have giardia events
- Backpacking in WA — every Cascades or Olympics surface source is treated as giardia-positive
Mitigation hardware
| Method | Cost | Notes |
|---|---|---|
| Boiling | Free | ≥1 minute at sea level, 3 minutes above 6,500 ft |
| Backcountry filter (Sawyer, Katadyn, MSR) | $25–$120 | 0.1–1 micron absolute pore size |
| Chemical treatment (chlorine dioxide, iodine tablets) | $5–$15 per pack | Kill rate drops in cold water; extend hold time |
| Whole-house point-of-entry filtration (NSF 53) | $300–$1,200 hardware | For surface-influenced wells |
| Whole-house UV at point-of-entry | $400–$1,200 hardware + install | Annual lamp replacement essential |
Common failure modes (in homeowner / backcountry context)
- Untreated rural surface water consumed. Typical hiker or camper exposure; symptoms 1–2 weeks later.
- Cracked private well casing letting surface water in. Giardia, cryptosporidium, and coliform all become risks together.
- Ineffective filter (>1 micron pore size or no cyst rating). Doesn’t reliably remove giardia.
- Iodine drops on cold water. Kill rate drops with temperature; extend the hold time per package directions.
- Conflating coliform-clean with safe. A clean coliform test does not rule out giardia.
Common variants and disambiguation
- Giardia vs. cryptosporidium. Both protozoa, both filter-removable at NSF 53 absolute 1 micron. Giardia is more chlorine-susceptible than crypto. Same homeowner mitigation strategy works for both — and any filter rated for crypto is also rated for giardia.
- Giardia vs. coliform. Different organism class; coliform tests do not detect giardia.
- Giardia in tap water vs. recreational water. Public utilities generally control giardia well; outbreaks are rare from utilities and more common from rural or recreational sources.
Washington note
WA Group A surface-water utilities comply with EPA’s Surface Water Treatment Rule (SWTR), which requires 3-log (99.9%) Giardia inactivation at the point of distribution. Standard treatment trains used by Seattle, Tacoma, Bellingham, Everett, and other utilities meet this requirement. Outbreaks from Group A systems are uncommon.
WA Department of Health treats giardia as a routine public-health concern but does not require giardia-specific testing on private wells. The default homeowner posture for surface-influenced private wells is preventive: install whole-house UV + sub-micron filter rather than waiting for a sick-family event.
For backcountry use, every Cascades and Olympics water source is treated as giardia + crypto positive. Use a filter with 0.1–1 micron absolute rating, chemical treatment with adequate contact time for the water temperature, or boil.