Giardia lamblia
Giardia·Affects skin,
A waterborne parasite that causes the persistent diarrheal illness known as giardiasis or 'beaver fever.' The most commonly diagnosed intestinal parasite in the United States.
Giardia
EPA limit
99.9% removal required (Treatment Technique)
Most at risk
Infants, children, elderly, immunocompromised, well-water households, hikers
Found in
Well water
AquaOx filters
YesViqua UV sterilizer (downstream pairing)
What it does to you
Health effects
Giardiasis symptoms appear one to three weeks after exposure and include explosive diarrhea, greasy floating stools, abdominal cramps, bloating, excessive gas, fatigue, and weight loss. The illness typically lasts two to six weeks but can recur or become chronic, especially in children and the immunocompromised.
Long-term complications include lactose intolerance that can persist for months after the infection clears, malabsorption of fats and fat-soluble vitamins, and failure-to-thrive syndromes in young children. Reactive arthritis and chronic fatigue have been documented in a subset of patients.
Giardia is treatable with prescription antiparasitics such as metronidazole or tinidazole, but reinfection from contaminated water is common and some strains have developed drug resistance. Asymptomatic carriers can shed cysts for months without knowing they are infected.
Where it comes from
How it gets into your water
Giardia exists in water as cysts, a dormant form roughly 8 to 12 microns across with a protective wall that lets the parasite survive in cold water for months. Cysts are shed in the feces of infected humans, beavers, muskrats, dogs, cattle, and wildlife, hence the trail-hiker nickname "beaver fever."
Contamination enters drinking water through agricultural runoff, sewage discharge, septic failures near wells, and surface water intakes downstream of wildlife habitat. Spring snowmelt and heavy rain events flush accumulated cysts into streams, lakes, and shallow groundwater all at once.
Giardia cysts are more chlorine-susceptible than Cryptosporidium oocysts but still resistant enough that conventional municipal disinfection cannot rely on chlorine alone. The EPA Surface Water Treatment Rule requires 99.9 percent (3-log) removal or inactivation through filtration and disinfection combined.
Wildlife contamination
Beavers, muskrats, and deer shed cysts into surface water
Septic system leakage
Human sewage from failed septic near wellheads
Surface water intakes
Lakes and rivers serving municipal utilities
Untreated camping water
Streams and springs are a primary infection route for hikers
Find out for yourself
Is it in your water?
Municipal utilities are required to meet the 99.9 percent Giardia removal/inactivation standard under the EPA Surface Water Treatment Rule. Boil-water advisories are issued whenever filtration performance drops below the required log-removal threshold or whenever direct detection is reported.
Private well owners get no such warning. CDC data consistently shows that surface-influenced wells (shallow wells, dug wells, springs) in rural and agricultural areas carry the highest Giardia burden. Annual testing for total coliforms is the recommended proxy, since the same pathways that bring coliforms in usually bring cysts as well.
If household members have prolonged GI symptoms with greasy stools and significant weight loss, ask your doctor to order a stool Giardia antigen test specifically. Standard ova-and-parasite panels miss it about half the time because cyst shedding is intermittent.
The fix
How AquaOx filters it
AquaOx filters Giardia in two complementary ways. The whole-house system's 1-micron absolute sediment pre-filter physically captures Giardia cysts, which measure 8 to 12 microns and cannot pass through an absolute 1-micron barrier. NSF/ANSI Standard 53 certifies 1-micron absolute filters for cyst filtration. The carbon and KDF stages downstream do not inactivate parasites chemically.
For a true microbial barrier, especially on well water, AquaOx pairs the whole-house filtration with a Viqua UV sterilizer downstream. UV at 254 nm damages cyst DNA and prevents replication even if a cyst slips through. Giardia is more UV-susceptible than Crypto, so a properly sized residential UV chamber inactivates it reliably.
The filtration-plus-UV combination is the standard AquaOx configuration for well-water customers. Filtration without UV leaves a backup gap; UV without pre-filtration loses dose to turbidity shielding. Together they form the redundant barrier that municipal plants use at industrial scale.