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The need to treat backcountry water is often hotly debated. However, there is a clear scientific consensus that there is significant risk in drinking untreated backcountry water, no matter how safe it may appear.

Below are some of the most important papers on the topic of backcountry pathogens, risk assessment and water treatment, with selected quotes from those papers. Some of those citations and quotes are followed by my comments or clarifications where I thought they were needed.

I invite your comments and suggestions for further papers to be cited.

Adam, E. A., et al. “Giardiasis outbreaks in the United States, 1971–2011.” Epidemiology & Infection 144.13 (2016): 2790-2801.
For waterborne outbreaks associated with water other than drinking water or recreational water, the most commonly implicated water type was a river/stream… [My comment: 98% of individual cases were waterborne, and and only 2.1% were foodborne or person-to-person. According to the CDC less than 1% of reported giardiasis cases are associated with outbreaks.]

Barbour, Alan G., Craig R. Nichols, and Taira Fukushima. “An outbreak of giardiasis in a group of campers.” The American journal of tropical medicine and hygiene 25.3 (1976): 384-389.
Twenty-five other campers had stools examined before and after a subsequent hiking trip in another area of Utah; none had Giardia cysts before, but 6 (24%) had them after return. Questionnaires returned by 133 of the campers showed that 5% had an illness compatible with giardiasis within 2 weeks after their trip. These surveys show that campers exposed to mountain stream water are at risk of acquiring giardiasis.

Backer, Howard, and Robert W. Derlet, MDVincent R. Hill, PhD . “Wilderness Medical Society Clinical Practice Guidelines for Water Disinfection for Wilderness, International Travel, and Austere Situations” Wilderness Medical Society Clinical Practice Guidelines| Volume 30, ISSUE 4, SUPPLEMENT , S100-S120, December 01, 2019 Public health agencies regularly report outbreaks of disease associated with surface water from backcountry and parks… [My comment: Includes a good discussion of water treatment methods.]

Beer, Karlyn D., et al. “Outbreaks Associated With Environmental and Undetermined Water Exposures-United States, 2011-2012.” MMWR. Morbidity and mortality weekly report 64.31 (2015): 849-851.

State Disease Year Cases Water Type Exposure Setting
Colorado Giardiasis 2011 2 River/Stream Camp/Cabin
Idaho Giardiasis 2012 4 River/Stream National forest
Utah Giardiasis 2011 20 River/Stream Public Outdoor Area

Outbreaks associated with untreated water sources highlight the importance of properly treating water for drinking in backcountry settings. [My comment: according to the CDC less than 1% of reported giardiasis cases are associated with outbreaks.]

Benedict, K. M., et al. “Case–case analyses of cryptosporidiosis and giardiasis using routine national surveillance data in the United States–2005–2015.” Epidemiology & Infection 147 (2019). The highest odds ratios in this analyses for reported exposures associated with giardiasis were for untreated water, both drinking (aOR 4.07, 95% CI 3.56–4.65) and recreational (aOR 4.08, 95% CI 3.69–4.51) exposures. [My note: this new study shows again that giardiasis is primarily a waterborne disease.]

Boulware, David R., William W. Forgey, and William J. Martin. “Medical risks of wilderness hiking.” The American journal of medicine 114.4 (2003): 288-293. [a prospective surveillance study of 334 persons who hiked the Appalachian Trail]
The risk of diarrhea was greater among those who frequently drank untreated water from streams or ponds (odds ratio [OR] = 7.7; 95% confidence interval [CI]: 2.7 to 23; P <0.0001), whereas practicing “good hygiene” (defined as routine cleaning of cooking utensils and cleaning hands after bowel movements) was associated with a decreased risk (OR = 0.46; 95% CI: 0.22 to 0.97; P =0.04). Conclusion: Diarrhea is the most common illness limiting long-distance hikers. Hikers should purify water routinely, avoiding using untreated surface water. The risk of gastrointestinal illness can also be reduced by maintaining personal hygiene practices and cleaning cookware. Molecular biologist Dr. Christie Wilcox interpreted the study this way “Those that practiced good hand-washing were about 50% less likely to have gotten sick, while those that drank untreated water were almost 8 times as likely to have had to pack it in because of diarrhea.”

Boulware, David R. “Influence of hygiene on gastrointestinal illness among wilderness backpackers.” Journal of travel medicine 11.1 (2004): 27-33.
The most significant factor in preventing gastrointestinal illness was the regularity of water disinfection…CONCLUSIONS: Lack of hygiene, specifically handwashing and cleaning of cookware, should be recognized as a significant contributor to wilderness gastrointestinal illness. Hikers should routinely disinfect water and avoid untreated surface water.

BRYAN, JOHN A., et al. “An outbreak of hepatitis-A associated with recreational lake water.” American Journal of Epidemiology 99.2 (1974): 145-154. Raw lake water, which was grossly contaminated and inadvertently consumed by many campers, was associated with a statistically significant risk of hepatitis-A.

Carter, A. M., et al. “Seasonal occurrence of Campylobacter spp. in surface waters and their correlation with standard indicator bacteria.” Applied and Environmental Microbiology 53.3 (1987): 523-526.
Campylobacter spp. were isolated from a variety of aquatic habitats in central Washington. The wide distribution of the bacteria in this region suggests that no water should be considered free of the organism, including pristine mountain streams.

Centers for Disease Control and Prevention “Drinking Water Treatment Methods for Backcountry and Travel Use”
[My note: Simple, easy to read chart comparing commonly used water treatment methods including boiling, filtering, chlorine dioxide, iodine and chlorine. (Chlorine dioxide is much different than chlorine.)]

Chatterjee, Aparajita et al. “Ethanol and Isopropanol in Concentrations Present in Hand Sanitizers Sharply Reduce Excystation of Giardia and Entamoeba and Eliminate Oral Infectivity of Giardia Cysts in Gerbils.” Antimicrobial Agents and Chemotherapy 59.11 (2015): 6749–6754. PMC. Web. 13 Mar. 2018 [My note: this doesn’t apply specifically to the backcountry, but although there is no evidence that hand hygiene is MORE important than water treatment in the backcountry, there is no doubt it plays a significant role, especially in larger groups.]

Chute, Christopher G., Robert P. Smith, and John A. Baron. “Risk factors for endemic giardiasis.American journal of public health 77.5 (1987): 585-587.
Households with shallow well or surface water sources had an odds ratio (OR) for giardiasis of 2.1 (95% confidence interval (95%CI) 1.3-3.2) compared with households with drilled well or municipal water supply…Previously reported risks such as travel out of country (OR 3.2, 95%CI 1.5-7.2) and camping (OR 1.7, 95%CI 0.9-3.2) were also observed.

Clow, David W., et al. “Effects of stock use and backpackers on water quality in wilderness in Sequoia and Kings Canyon National Parks, USA.” Environmental management 52.6 (2013): 1400-1414.
The intensive-monitoring data indicated that nutrient and E. coli concentrations normally were low, except during storms, when notable increases in concentrations of E. coli, nutrients, dissolved organic carbon, and turbidity occurred. In summary, results from this study indicate that water quality in SEKI wilderness generally is good, except during storms; and visitor use appears to have a small, but statistically significant influence on stream water quality.

Craun, Gunther F., and Rebecca L. Calderon. “Observational epidemiologic studies of endemic waterborne risks: cohort, case-control, time-series, and ecologic studies.” Journal of water and health 4.Suppl 2 (2006): 101-19.
In several studies, the consumption of untreated water from a lake, river, or spring and rainwater was significantly associated with cryptosporidiosis.

Craun, Gunther F., et al. “Causes of outbreaks associated with drinking water in the United States from 1971 to 2006.Clinical Microbiology Reviews 23.3 (2010): 507-528.
The outbreaks [in untreated water purposefully drunk] occurred primarily among hikers, campers, and other persons who drank untreated water from streams and lakes (n = 17, 36.2%).

Craun, Gunther F. “Waterborne outbreaks of giardiasis.” Giardia and Giardiasis. Springer US, 1984. 243-261.
A number of single cases of [waterborne] giardiasis occur in backpackers where a common source cannot be investigated and when investigated these outbreaks are often reported in an anecdotal manner…There was a cluster of 10 cases where hikers had consumed water from a stream later found to be contaminated with Giardia cysts. Fourteen cases had a common exposure to a work camp where all had consumed water from a stream contaminated with Giardia cysts…Twenty clusters of 43 cases were composed of loggers, game workers, campers, etc., who drank untreated water while on an outing as a
group or during their work.
[My note: This paper refutes Giardiasis as a threat to backpackers in the United States: a survey of state health departments.]

Crouse, Byron J., and D. Josephs. “Health care needs of Appalachian trail hikers.” The Journal of family practice 36.5 (1993): 521-525.
Methods to purify water need to be used regularly, and adequate preparation of food needs to be a priority.

Derlet, Robert W., et al. “Risk factors for coliform bacteria in backcountry lakes and streams in the Sierra Nevada mountains: a 5-year study.” Wilderness & environmental medicine 19.2 (2008): 82-90.
Surface water from watersheds below cattle areas and those used by pack animals is at high risk for containing coliform organisms. Water from Wild, Day Hike, or Backpack sites poses far less risk for contamination by coliforms.

Derlet, Robert W., James R. Carlson, and Mikla N. Noponen. “Coliform and pathologic bacteria in Sierra Nevada national forest wilderness area lakes and streams.” Wilderness & environmental medicine 15.4 (2004): 245-249.
Coliform bacteria were detected in 14 of 31 sites (45%).

Dennis, David T., et al. “Endemic giardiasis in New Hampshire: a case-control study of environmental risks.” Journal of Infectious Diseases 167.6 (1993): 1391-1395.
Giardiasis is the most frequently reported diarrheal disease in northern New England. A case control study of endemic giardiasis and environmental risk factors among residents of New Hampshire involved 273 cases from the state’s 1984 disease registry and 375 controls. Giardiasis was associated with … a recent history of drinking untreated surface water (OR = 3.4; CI, 2.1-5.5), a history of swimming in a lake or pond (OR = 4.6; CI, 2.4-86.0) or swimming in any natural body of fresh water (OR = 4.0; CI, 2.3-70.0)

Environmental Protection Agency “EPA Guidance Manual” CT (contact time) for inactivation of giardia and viruses by ozone, chlorine, chloramine, and chorine doxide (Aquamira).

Environmental Protection Agency “Chlorine Dioxide” …chlorine dioxide has been known as a powerful disinfectant of water

Environmental Protection Agency “Giardia: Drinking Water Health Advisory”
[giardia] Cysts have been found all months of the year in surface waters from the Arctic to the tropics in even the most pristine of surface waters… Hibler (1988) found Giardia cysts in 19% of springs…Hancock et al. (1997) collected 463 groundwater samples from 199 sites in 23 states in the United States; Giardia cysts were found in 14% of the springs…Roach et al. (1993) found Giardia cysts in 32% of samples collected from pristine streams in the Canadian North…Rose et al. (1991a) detected Giardia cysts in surface water sources…The geometric mean levels found in…pristine rivers were 0.35 cysts/100 L (maximum = 12/100 L)… The geometric mean was …0.5 cysts/100 L (maximum = 7/100 L) for pristine lakes…In Wisconsin, Archer et al. -5- (1995) found that all 18 streams sampled were positive for Giardia ; the highest level was 2,601 cysts/100 L from a stream characterized as pristine…In three pristine watersheds in Washington, Ongerth (1989) found assessed Giardia cysts in 43% of 222 samples during a nine- month period. Cyst levels ranged from 10 to 520 cysts/100 L [My note: This is a great paper for an overview of the topic. Use the “search” function to find your topic of interest.]

Ericsson, Charles D., Robert Steffen, and Howard Backer. “Water disinfection for international and wilderness travelers.” Clinical infectious diseases 34.3 (2002): 355-364.
Estimations of water safety cannot reliably be made on the basis of the look, smell, and taste of water. In fact, travelers have no reliable resources for evaluating the quality of local water systems. Even less information is available for determination of the quality of remote (i.e., wilderness) surface-water sources. [My note: great information on water treatment methods including boiling, chlorine dioxide, filtration, iodine and UV.]

Fashina, Lukman, Ingrid Luffman, T. Andrew Joyner, and Arpita Nandi. 2022. “Geospatial Assessment of Karst Spring Water Quality in Northeast Tennessee, USAGeosciences 12, no. 8: 303. https://doi.org/10.3390/geosciences12080303  While springs are often regarded as high-quality water sources, they are also highly susceptible to pathogenic bacteria and contamination…All 50 sampled springs in the study area contained fecal coliform and 90% contained E. coli.

Flack, J. Ernest, Allen J. Medine, and Katherine J. Hansen-Bristow. “Stream water quality in a mountain recreation area.” Mountain Research and Development (1988): 11-22.
Water purification prior to drinking appears essential.

Frost, Floyd. Giardiasis in Washington State. EPA-600/1 (USA) (1982).
…[in one study] a follow-up of human giardiasis cases identified through medical diagnostic laboratories, contacted a total of 865 Giardia infected Washington State residents… clusters of cases were linked to… backpacker groups or sites for drawing water on outings… [another] study was a case-control study to identify risk factors for giardiasis. Factors which appeared to place a person at increased risk included consumption of untreated water

Gallaher, Margaret M., et al. “Cryptosporidiosis and surface water” American journal of public health 79.1 (1989): 39-42.
There was a strong association between drinking surface water and illness: five of the 24 case-patients, but none of the 46 controls drank untreated surface water.

Gretsch, Stephanie. “Outbreak of Giardiasis Associated with Drinking Surface Water Along a Hiking Trail.” 2015 CSTE Annual Conference. Cste, 2015.
This was a waterborne outbreak of giardiasis associated with drinking surface water at multiple points along a hiking trail. Chlorine dioxide can be an effective pathogen reduction method; however, the product used by the campers is not certified for pathogen reduction, although it is frequently marketed as such. Consequently, while the campers followed the package instructions, the treatment appears to have been ineffective at inactivating Giardia cysts in the water. Future efforts to prevent illness from the consumption of backcountry water need to include creation of clear guidelines recreationalists can use to choose effective water treatment methods.

Harter, Lucy, Floyd Frost, and Walter Jakubowski. “Giardia prevalence among 1-to-3-year-old children in two Washington State counties.” American journal of public health 72.4 (1982): 386-388. Identified risk factors for Giardia infection include a history of drinking untreated surface water

Hopkins, Richard S., Russell Olmsted, and Gregory R. Istre. “Endemic Campylobacter jejuni infection in Colorado: identified risk factors.American Journal of Public Health 74.3 (1984): 249-250. Ill persons were more likely than controls to have drunk raw water (untreated water taken from a stream, river, or lake)

Jakubowski, Walter, and John C. Hoff, eds. Waterborne Transmission of Giardiasis: Proceedings of a Symposium September 18-20, 1978. Vol. 79. No. 1. US Environmental Protection Agency, Office of Research and Development, Environmental Research Center, 1979.
[My comment: This linked image, which appears in the paper, shows the results of the original Rendtorff “prisoner test,” of giardia cysts.]

Khan, I. D., V. Sashindran, and G. S. Sandhu. “Outbreak of wilderness/backcountry/travelers’ diarrhea at a Himalayan Base Camp at 4000 m/13,125 ft.” Int J Travel Med Glob Health 6.1 (2018): 25-29. Results: Sixty-two personnel presented with wilderness diarrhea of 5.58 days mean duration with onset after mean stay of 10.26 days, despite provisions for adequate camp hygiene and sanitation… Conclusion: Wilderness diarrhea can present in outbreak proportions from formerly safe water sources due to variable microbial contamination…

Kellogg, Dylan S., et al. “High fecal hand contamination among wilderness hikers.” American journal of infection control 40.9 (2012): 893-895. The prevalence of fecal hand contamination in entering and exiting hikers was similar (33% and 27%, respectively) [My notes: no surprise that Dr. Thomas Welch is behind this one and the way the results were spun. Note that hikers hands were CLEANER at the end of the trip than when they started. The largest study of non-hikers hands that I could find, Isolation of bacteria of faecal origin on commuter’s hands: a preliminary study, involving British commuters, found (28%) had fecal bacteria present, HIGHER than hikers leaving the field.]

Kivistö, R.; Kämäräinen, S.; Huitu, O.; Niemimaa, J.; Henttonen, H. Zoonotic Cryptosporidium spp. in Wild Rodents and Shrews. Microorganisms 2021, 9, 2242.  Cryptosporidium protozoan are present in mouse, vole and shrew populations around Finland and the highest zoonotic risk is associated with C. ditrichi in Apodemus flavicollis and C. andersoni in Craseomys rufocanus. [My comment: those who think that it’s safe to drink untreated water above an arbitrarily selected elevation might consider that mice have been found up to 22,000 feet.]

Krometis, Leigh-Anne, et al. “Water Scavenging from Roadside Springs in Appalachia.Journal of Contemporary Water Research & Education 166 (2019): 46-56. Water Quality at Springs: All samples except for one (99%) were positive for total coliform bacteria, sometimes at very high levels.

Laxer, M. A. “Potential exposure of Utah Army National Guard personnel to giardiasis during field training exercises: a preliminary survey.” Military medicine 150.1 (1985): 23.
A survey of 383 Utah National Guard members showed that 15% had symptoms suggestive of giardiasis and that the guardsmen were at risk of contracting giardiasis by drinking contaminated water during field exercises in the state (Laxter, 1985); 62% of the men who had symptoms drank untreated water from lakes, streams, and a cattle watering trough.

Leemon, Drew, and Tod Schimelpfenig. “Wilderness injury, illness, and evacuation: National Outdoor Leadership School’s incident profiles, 1999–2002.” Wilderness & environmental medicine 14.3 (2003): 174-182.
The rates of gastrointestinal and flu-like illness have decreased since the 1980S…We believe the incidence of these illnesses is influenced by hygiene, and the long-term decline in the rate reflects our concerted efforts to improve hygiene. These efforts include emphasizing hand washing and camp hygiene, educating students and instructors on food-borne illness, expecting all wilderness drinking water to be disinfected, and repeatedly restating our expectations in these areas throughout the course. [My note: few, if any outdoor organizations have more data and a better track record of good gastrointestinal health.]

Marder, Ellyn. “Risk Factors for Cryptosporidiosis and Giardiasis Infections in the United States: A Case-Case Comparison in 16 States.” (2012). Cryptosporidiosis infection was associated with livestock contact, day care association, drinking untreated water, having a private well as a water source, knowing another ill person, recreational water exposure and travel when compared to salmonellosis infection. Giardiasis infection was associated with day care association, drinking untreated water, knowing other ill persons, recreational water exposures, and travel when compared to salmonellosis infection.

McClung, R. Paul. “Waterborne Disease Outbreaks Associated With Environmental and Undetermined Exposures to Water—United States, 2013–2014.” MMWR. Morbidity and Mortality Weekly Report 66 (2017).

State Disease Year Cases Water Type Exposure Setting
Colorado Giardiasis 2014 9 River/Stream Park
Illinois Giardiasis 2013 69 River/Stream Park
Minnesota Giardiasis 2014 6 River/Stream Public Outdoor Area
New Mexico Giardiasis 2013 3 River/Stream Camp/Cabin
New York Giardiasis 2013 5 Spring Private, Rural
Utah Giardiasis 2014 4 River/Spring Backcountry

[My comment: according to the CDC less than 1% of reported giardiasis cases are associated with outbreaks. These are selected outbreaks. See the full chart at the above link.]

Waterborne disease outbreaks in the United States are associated with a wide variety of water exposures and are reported annually to CDC on a voluntary basis…Outbreaks [included] six outbreaks of giardiasis caused by ingestion of water from a river, stream, or spring. To prevent illnesses and outbreaks in backcountry settings, it is important to appropriately treat water obtained from the natural environment before consuming it.

Meyer, Derek, Amber Costantino, and Susanne Spano. “An Assessment of Diarrhea among Long-Distance Backpackers in the Sierra Nevada.” Wilderness & Environmental Medicine 27.3 (2016): 431-432.
No statistical significance was found between diarrhea incidence as related to hiker demographics or compliance with standard hygiene recommendations [water filtration, hand hygiene, dish washing]. Regular hand sanitizer use was significantly correlated with having more severe diarrhea, but had no effect on incidence…Diarrhea incidence is unaffected by backpacker demographics or hygiene compliance regularity on the JMT. [My comment: Clean water, hands and dishes made no difference? Hand sanitizer made people MORE sick? This surprising result might be explained by self-selecting and self-reporting.]

Minnesota Department of Health “Giardiasis, 2003”
One waterborne outbreak was documented among Minnesota residents who were exposed to contaminated drinking water while camping in a national park in another state

National Park Service “Giardia in Denali National Park : a preliminary study”
The author identified giardia cysts in both water and fecal samples from Denali National Park in 1987. Caribou and arctic ground squirrel have been shown to carry Giardia spp. While this study does not determine if the giardia found is human infective, circumstantial evidence in the form of confirmed cases of giardiasis associated with Park waters suggest that it is. The U.S. National Park Services warning to treat the water is appropriate.

Naumova, Elena N. “Mystery of seasonality: getting the rhythm of nature.Journal of public health policy 27.1 (2006): 2-12.
Although spread of pathogens via food is certainly possible, contaminated water is the dominant source of exposure for enteric infections caused by protozoa Cryptosporidium and Giardia. Depending on locality, outbreaks of cryptosporidiosis and giardiasis associated with drinking or recreational water frequently occur during warm or wet seasons

Ongerth, Jerry E., et al. “Back-country water treatment to prevent giardiasis.” American Journal of Public Health 79.12 (1989): 1633-1637.
This study was conducted to provide current information on the effectiveness of water treatment chemicals and filters for control of Giardia cysts in areas where treated water is not available. [My note: Chlorine Dioxide is probably the most effective chemical treatment for backpackers now.]

ONGERTH, JERRY E. Cryptosporidium and Giardia—Levels and Distribution in Surface Water throughout Europe

Ongerth JE, Plutzer J, Karanis P.  Cryptosporidium and Giardia—Levels and Distribution in Surface Water throughout Europe. In Multidisciplinary Digital Publishing Institute Proceedings 2018 (Vol. 2, No. 11, p. 690).  ...analysis clearly demonstrate the distribution of these waterborne pathogens across all parts of the continent. The presence of these organisms at readily detectable levels in surface water used for public water supply corresponds to epidemiological evidence describing the prevalence of cryptosporidiosis and of giardiasis in all countries of Europe

ONGERTH, JERRY E. “Giardia cyst concentrations in river water.” (1989).
Giardia cysts appear to be continuously present, though at low concentrations, even in relatively pristine rivers.

Ongerth, J. E., G. D. Hunter, and F. B. DeWalle. “Watershed use and Giardia cyst presence.” Water Research 29.5 (1995): 1295-1299.
Based on the water samples analyzed, a calculated median cyst concentration of 1 cyst per 20 l (0.05 cysts/l) can be expected in relatively pristine rivers.

Painter, Julia E., et al. “Giardiasis Surveillance—United States, 2011–2012.” Morbidity and mortality weekly report. Surveillance summaries (Washington, DC: 2002) 64 (2015): 15-25.
The summer peak coincides with increased outdoor activities (e.g., camping and swimming) that likely increase exposure to contaminated water…Untreated drinking water has been identified as a risk factor for sporadic giardiasis in the United States … Do not drink untreated water from lakes, rivers, springs, ponds, streams, or shallow wells.

Palmieri, James R., Chasi F. Skinner, and Shaadi F. Elswaifi. “Giardiasis Revisited: An Underappreciated Reemerging Zoonotic Disease.” The Journal of the American Osteopathic Association 112.10 (2012): 649-651.
certain groups within a population may be at greater risk for acquiring an infection. These groups include international travelers, back-packers, hikers, and campers who drink unfiltered or untreated water and people who drink from shallow wells

Petersen, Lyle R., Matthew L. Cartter, and James L. Hadler. “A food-borne outbreak of Giardia lamblia.The Journal of infectious diseases (1988): 846-848.
Common-source outbreaks of giardiasis are almost exclusively waterborne… [My comment: according to the CDC less than 1% of reported giardiasis cases are associated with outbreaks.]

Reed, Brian C., and Mark S. Rasnake. “An assessment of coliform bacteria in water sources near Appalachian Trail shelters within the Great Smoky Mountains National Park.” Wilderness & environmental medicine 27.1 (2016): 107-110.
This analysis of water samples along the Appalachian Trail emphasizes that the majority of water access points require treatment during the summer season. Coliform burden was not as high through the fall months. These data suggest one infectious disease risk for wilderness travelers.

Reses, H. E., et al. “Risk factors for sporadic Giardia infection in the USA: a case-control study in Colorado and Minnesota.” Epidemiology and infection (2018): 1.
Identified risk factors included… drinking water from a river, lake, stream, or spring (aOR = 6.5; 95% CI 2.0-20.6), swimming in a natural body of water (aOR = 3.3; 95% CI 1.5-7.0) [My comment: this study showed drinking untreated water was about 6.5 times more risky.}

Rockwell, Robert L. “Giardia lamblia and giardiasis with particular attention to the Sierra Nevada.” Sierra Nature Notes May 2.2 (2002).
One conclusion of this paper is that you can indeed contract giardiasis on visits to the Sierra Nevada, but it won’t be from the water. So drink freely and confidently…
[My comment: I got giardia in the Sierra after reading and believing this paper. The conclusions of this 2002 non peer-reviewed paper are refuted by the CDC, FDA, EPA, Mayo Clinic etc. I have written a rebuttal to Rockwell: Backcountry Giardia, Why Rockwell is Wrong ]

Rose, Joan B., Charles N. Haas, and Stig Regli. “Risk assessment and control of waterborne giardiasis.American journal of public health 81.6 (1991): 709-713.
In water samples originating from pristine watersheds (protected from all human activity) Giarda cyst levels averaged 0.6 to 5/100L. [My notes: This is the definitive study of the infectious dose for giardiasis. Mathematicians can click on the link to see the formulas for calculating annual and single event risk for the ingestion of giardia cysts. In layman’s terms, there is about a 2% risk of infection with a SINGLE cyst. It does NOT require the ingestion of 10 cysts to produce an infection as is often erroneously stated.]

Silsbee, David G., and Gary L. Larson. “Bacterial water quality: springs and streams in the Great Smoky Mountains National Park.” Environmental Management 6.4 (1982): 353-359.
Water samples from streams and springs in the Great Smoky Mountains National Park were analyzed… Levels of fecal coliform and total coliform in most water samples were unsuitable for drinking without treatment. As a result of these findings, park managers increased efforts to inform visitors of the need to treat drinking water and removed improvements at backcountry springs which tended to give the springs the image of safe, maintained water sources.

Suk, Thomas J., Stephen K. Sorenson, and Peter D. Dileanis. “The Relation between Human Presence and Occurrence of Glardia Cysts in Streams in the Sierra Nevada, California.Journal of Freshwater Ecology 4.1 (1987): 71-75.
A portable apparatus was used to filter large quantities of water from streams in California’s Sierra Nevada. Samples were processed and examined for Giardia spp. Cysts of Giardia were detected in 22 of 49 (44.9 percent) samples collected at sites downstream from areas of high recreational use and in 5 of 29 (17.2 percent) samples collected at sites downstream from areas of low recreational use

Swirski, A. L., et al. “A comparison of exposure to risk factors for giardiasis in non-travellers, domestic travellers and international travellers in a Canadian community, 2006–2012.” Epidemiology and Infection (2015): 1-20.
Travel-related cases (both international and domestic) were more likely to go camping or kayaking, and consume untreated water compared to endemic cases.

Swistock, Bryan, et al. “Issues Associated with the Use of Untreated Roadside Springs as a Source of Drinking Water.” Issues 156 (2015): 78-85.
A survey of over 1,000 Pennsylvania residents found that 30% have consumed water from a roadside spring and 12% consume water every year, mostly because they perceive the water as natural with a good taste. A synoptic survey of 37 springs in 2013-2014 found that more than 90% failed one or more health-based drinking water standards. A more intensive follow-up study in 2014-2015 on ten of the 37 roadside springs found that they consistently failed drinking water standards throughout the year, including some presence of both Giardia and Cryptosporidium cysts. [My notes: this testing included piped, developed springs that were “coming out of the ground.”]

TAYLOR, DAVID N., et al. “Campylobacter enteritis from untreated water in the Rocky Mountains.” Annals of internal medicine 99.1 (1983): 38-40.
These studies show that backcountry surface water can be an important source of C. jejuni and that infection with Campylobacter, as well as Giardia lamblia, should be considered as a cause of diarrhea in those who have recently returned from wilderness areas.

Viau, Emily J., et al. “Bacterial pathogens in Hawaiian coastal streams—associations with fecal indicators, land cover, and water quality.” water research 45.11 (2011): 3279-3290.
Twenty-two streams were sampled four times (in December and March, before sunrise and at high noon) to capture seasonal and time of day variation. Salmonella, Campylobacter, Staphylococcus aureus, Vibrio vulnificus, and V. parahaemolyticus were widespread —12 of 22 O’ahu streams had all five pathogens. All stream waters also had detectable concentrations of four fecal indicators…

Water Research Foundation “Waterborne Pathogens”
This website presents water-relevant information of important bacterial, protozoan, and viral waterborne pathogens. It is actually a detailed scientific review, which was put on the web for reasons of accessibility instead of being published in form of a book. [My notes: I highly recommend this site as a starting point for your research. Written by true experts and the information is well organized, contains many citations, and is easy to read.]

Welch, Timothy P. “Risk of giardiasis from consumption of wilderness water in North America: a systematic review of epidemiologic data.International journal of infectious diseases 4.2 (2000): 100-103. Published reports of confirmed giardiasis among outdoor recreationists clearly demonstrate a high incidence among this population. However, the evidence for an association between drinking backcountry water and acquiring giardiasis is minimal. [My comments: Dr. Welch arrived at this remarkable conclusion in 1999 by throwing out 95 of the 104 studies he considered for this paper, and in my opinion, by egregious cherry-picking and misrepresentation of the data in the remaining papers. As of 2015, the conclusions of this paper are refuted by the CDC, FDA, EPA, Mayo Clinic etc.]

Welch, Thomas R., and Timothy P. Welch. “Giardiasis as a threat to backpackers in the United States: a survey of state health departments.” Wilderness & environmental medicine 6.2 (1995): 162-166.
…[state health departments] do not, however, provide any evidence that wilderness water is an important cause of the disease in this country. [My comment: the conclusions of this 1995 paper are refuted as of 2015 by the CDC, FDA, EPA, Mayo Clinic etc. I have written a rebuttal entitled Backpacker Giardia: Debunking a Skeptical Paper Also, see Craun’s paper Waterborne outbreaks of giardiasis, above.]

Wolfe, Martin S. “Giardiasis.” Clinical microbiology reviews 5.1 (1992): 93-100.
In the United States, most infections are sporadic, especially in campers and hikers who drink untreated stream water [My note: Full paper available by clicking link.]

WRIGHT, RICHARD A., et al. “Giardiasis in Colorado: an epidemiologic study.” American journal of epidemiology 105.4 (1977): 330-336.
…an increased incidence of giardiasis [was shown for those who] camped out overnight (38% vs. 18%), and drank untreated mountain water (50% vs. 17%), p less than .001. Also identified was a correlation between the seasonal distribution of cases and degree of fecal contamination of mountain streams….drinking untreated mountain water is an important cause of endemic infection

Yoder, Jonathan, et al. “Surveillance for waterborne disease and outbreaks associated with drinking water and water not intended for drinking–United States, 2005-2006.” Morbidity and mortality weekly report. Surveillance summaries (Washington, DC: 2002) 57.9 (2008): 39-62.
Although the advice to universally filter and disinfect backcountry drinking water to prevent disease has been debated, the health consequences of ignoring that standard water treatment advice have been documented…

Zell, Steven C. “Epidemiology of wilderness-acquired diarrhea: implications for prevention and treatment.” Journal of Wilderness Medicine 3.3 (1992): 241-249.
Backcountry travelers should filter water to remove protozoan cysts. This should be followed by halogenation to disinfect against bacterial and viral pathogens. [My response: The second sentence is  dated. Chlorine dioxide  (Aquamira, Portable Aqua, Katadyn Micropur) is more effective.]

Zell, S. C., and S. K. Sorenson. “Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe.” Journal of Wilderness Medicine 4.2 (1993): 147-154.
The incidence of Giardia cyst acquisition in backcountry travelers was only 5.7% [My response: 5.7% on a SINGLE TRIP? Seems pretty high to me!]

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