Giardiasis is a gastrointestinal illness caused by the microscopic parasite Giardia lamblia. This parasite is a common concern for campers, hikers, and travelers who rely on natural water sources, and for those in areas with inadequate water treatment. Giardiasis is one of the most frequently diagnosed intestinal parasitic infections in the United States. Many people wonder if simple methods like freezing can eliminate this threat. This article addresses that belief and outlines proven methods for ensuring water safety.
Understanding Giardia Cysts
Giardia exists in two life stages: the active trophozoite form and the dormant cyst form. The cyst is the infectious stage, excreted in feces, and responsible for transmission, often through contaminated water. Cysts measure about 8 to 19 micrometers in diameter, making them removable by specialized filtration.
The cyst stage is hardy, surviving for extended periods outside a host, sometimes for several months in cool, moist environments. This resistance is due to a thick, protective outer wall and metabolic dormancy, which protects the parasite from environmental stressors and some chemical disinfectants.
Freezing and Giardia Viability
Freezing is considered an unreliable method for killing Giardia cysts in water. While extreme cold can inactivate the parasite, standard household freezer temperatures or natural environmental freezing are often insufficient for guaranteed destruction. Studies show that cysts demonstrate cold tolerance and can withstand a single freeze-thaw cycle.
Viable cysts can survive in water for two to three months at temperatures below 50°F (10°C), with optimal survival around 41°F (5°C). To ensure complete inactivation, the temperature must be significantly low and maintained for an extended period. Freezing often only suspends metabolic activity, meaning that once the ice thaws, the cysts can become active and infectious again. Relying on freezing alone for water purification is a risk.
Proven Water Disinfection Techniques
Proven disinfection methods must be used to reliably remove or inactivate Giardia cysts.
Boiling
Boiling is the most dependable method for deactivating Giardia and other waterborne pathogens. Bringing water to a full, rolling boil for at least one minute is sufficient to kill the cysts at sea level. At high altitudes above 6,500 feet, the boiling time should be extended to three minutes.
Filtration
Filtration physically removes the parasite from the water. To reliably remove Giardia cysts, a filter must have an absolute pore size of 1 micron or smaller. Filters certified for “cyst reduction” under standards like NSF/ANSI 53 meet this requirement. Since cysts measure 8 to 19 micrometers, a 1-micron filter physically blocks them.
Chemical Treatment
Chemical treatment using halogens like chlorine or iodine can inactivate Giardia cysts, but requires careful attention to dosage and contact time. Cysts are more resistant to standard chlorination than bacteria or viruses, necessitating higher concentrations and longer contact periods. Water temperature is a factor, as disinfection is less effective in cold water (e.g., 37°F or 3°C). Cloudy or turbid water also reduces the chemical’s effectiveness. For instance, at 41°F (5°C), a chlorine concentration of 4 milligrams per liter may require a 60-minute contact time for inactivation.
Preventing Giardiasis Infection
Preventing Giardiasis involves treating water and adopting hygiene practices to avoid the fecal-oral route of transmission. The infectious dose is very low, as few as 10 cysts can cause illness. The most effective behavioral measure is practicing thorough hand hygiene with soap and running water.
Handwashing is important after using the toilet, changing diapers, and before preparing or eating food. Individuals should avoid drinking untreated water from natural sources like streams, lakes, or springs when hiking or camping. Safe food preparation, including avoiding cross-contamination and washing raw produce with safe water, also reduces infection risk. Avoiding swallowing water during recreational activities, such as swimming in contaminated pools or lakes, is another preventive step.