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Revision as of 08:57, 18 October 2008 editBob K31416 (talk | contribs)Extended confirmed users, Pending changes reviewers22,010 edits →Epidemiology: deleted Crouse1993 article; see Talk page - Appalachian trail studies← Previous edit Revision as of 08:57, 18 October 2008 edit undoBob K31416 (talk | contribs)Extended confirmed users, Pending changes reviewers22,010 edits rewrote 1st sentenceNext edit →
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⚫ '''Wilderness-acquired diarrhea''' (WAD) is ] acquired in the ] and is sometimes called '''wilderness diarrhea''' (WD), or '''backcountry diarrhea''', and is a much-discussed hazard among backpackers, hikers, campers and other outdoor recreationalists who visit ] areas in temperate climates of the developed world.<ref name="Zell1992">{{cite journal

⚫ ] acquired in the ], sometimes called '''wilderness diarrhea''' (WD), '''wilderness-acquired diarrhea''' (WAD) or '''backcountry diarrhea''', and is a much-discussed hazard among backpackers, hikers, campers and other outdoor recreationalists who visit ] areas in temperate climates of the developed world.<ref name="Zell1992">{{cite journal
|author=Zell SC |author=Zell SC
|title=Epidemiology of Wilderness-acquired Diarrhea: Implications for Prevention and Treatment |title=Epidemiology of Wilderness-acquired Diarrhea: Implications for Prevention and Treatment
Line 121: Line 120:
}}</ref> and the study methodology. }}</ref> and the study methodology.


Surveys among hikers who completed the ] reported more than half experienced at least one episode of diarrhea, usually of two days or less in duration.<ref name="Boulware2003">{{cite journal |author=Boulware DR, Forgey WW, Martin WJ 2nd |title=Medical Risks of Wilderness Hiking |journal=Am J Med |volume=114 |issue=4 |pages=288–93 |year=2003 |pmid=12681456 |doi= }}</ref><ref name="Boulware2004">{{cite journal |author=Boulware DR |title=Influence of Hygiene on Gastrointestinal Illness Among Wilderness Backpackers |journal=J Travel Med |volume=11 |issue=1 |pages=27–33 |year=2004 |pmid=14769284 |doi= }}</ref> Treating water and routinely washing hands with soap and water after defecation reduced the incidence. Surveys among hikers who completed the ] reported more than half experienced at least one episode of diarrhea, usually of two days or less in duration.<ref name="Boulware2003">{{cite journal |author=Boulware DR, Forgey WW, Martin WJ 2nd |title=Medical Risks of Wilderness Hiking |journal=Am J Med |volume=114 |issue=4 |pages=288–93 |year=2003 |pmid=12681456 |doi= }}</ref><ref name="Boulware2004">{{cite journal |author=Boulware DR |title=Influence of Hygiene on Gastrointestinal Illness Among Wilderness Backpackers |journal=J Travel Med |volume=11 |issue=1 |pages=27–33 |year=2004 |pmid=14769284 |doi= }}</ref><ref name="Crouse1993">{{cite journal
|author=Crouse BJ, Josephs D
|title=Health care needs of Appalachian trail hikers
|journal=The Journal of family practice
|volume=36
|issue=5
|pages=521–5
|year=1993
|month=May
|pmid=8482936
|doi=
|url=http://findarticles.com/p/articles/mi_m0689/is_n5_v36/ai_13928819/pg_3?tag=artBody;col1
}}</ref> Treating water and routinely washing hands with soap and water after defecation reduced the incidence.


Backpackers may be carriers of Giardia without being symptomatic.<ref name="Zell1993">{{cite journal |author=Zell SC, Sorenson SK |title=Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe |journal=Journal of Wilderness Medicine |year=1993 |volume=4 |issue=2 |pages=147–54 |url=http://www.wemjournal.org/pdfserv/i0953-9859-004-02-0147.pdf |format=PDF}}</ref> 16.7% of subjects in this study developed mild gastrointestinal illness other than giardiasis.<ref name="Zell1993" /> Backpackers may be carriers of Giardia without being symptomatic.<ref name="Zell1993">{{cite journal |author=Zell SC, Sorenson SK |title=Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe |journal=Journal of Wilderness Medicine |year=1993 |volume=4 |issue=2 |pages=147–54 |url=http://www.wemjournal.org/pdfserv/i0953-9859-004-02-0147.pdf |format=PDF}}</ref> 16.7% of subjects in this study developed mild gastrointestinal illness other than giardiasis.<ref name="Zell1993" />

Revision as of 08:57, 18 October 2008

Wilderness-acquired diarrhea (WAD) is diarrhea acquired in the wilderness and is sometimes called wilderness diarrhea (WD), or backcountry diarrhea, and is a much-discussed hazard among backpackers, hikers, campers and other outdoor recreationalists who visit wilderness areas in temperate climates of the developed world. Risk factors include drinking untreated surface water and failure by the individual and his or her companions to maintain personal hygiene practices and clean cookware. Most cases are self-limited and the cause is most often never known. Some medical and public health researchers believe that the risks of WAD have been over-stated and are poorly understood by the public.

Names and definitions

Diarrhea acquired in the wilderness is typcally a form of infectious diarrhea, itself classified as a type of secretory diarrhea. These are all considered forms of gastroenteritis. The term may be applied in various remote areas of non-tropical developed countries (U.S., Canada, western Europe, etc.), but is less applicable in developing countries, and in the tropics, because of the different pathogens that are most likely to cause infection.

Causes

Infectious diarrhea acquired in the wilderness is caused by various bacteria, viruses, parasites and protozoa. The most commonly reported are the protozoa Giardia and Cryptosporidium. Other infectious agents may play a larger role than generally believed and include Campylobacter, hepatitis A virus, hepatitis E virus, enterotoxogenic E. coli, e. coli 0157:H7, Shigella, and various viruses. More rarely, Yersinia enterocolitica, Aeromonas hydrophila, and Cyanobacterium may also cause disease. Vectors for all of these causes are limited to fecal-oral transmission, and contaminated water and food. The major factor governing pathogen content of surface water is human and animal activity in the watershed..

Giardia lamblia does not tolerate freezing but can remain viable for nearly three months in river water when the temperature is 10°C and about one month at 15–20°C in lake water. Cryptosporidium may survive in cold waters (4°C) for up to 18 months, and can even withstand freezing, although its viability is thereby greatly reduced. Many other varieties of diarrhea-causing organisms, including Shigella and Salmonella typhi, and hepatitis A virus, can survive freezing for weeks to months. Virologists believe all surface water in the United States and Canada has the potential to contain human viruses, which cause a wide range of illnesses including diarrhea, polio and meningitis.

Symptoms

The average incubation periods for giardiasis and cryptosporidiosis are each 7 days. Certain other bacterial and viral agents have shorter incubation periods, although hepatitis may take weeks to manifest itself. The onset usually occurs within the first week of return from the field, but may also occur at any time while hiking.

Most cases begin abruptly and usually result in increased frequency, volume, and weight of stool. Typically, a hiker experiences at least four to five loose or watery bowel movements each day. Other commonly associated symptoms are nausea, vomiting, abdominal cramping, bloating, low fever, urgency, and malaise, and usually the appetite is affected. The condition is much more serious if there is blood or mucus in stools, abdominal pain, or high fever. Dehydration is a possibility. Life-threatening illness resulting from WAD is extremely rare.

Diagnosis

It may be difficult to associate a particular case of diarrhea with a recent wilderness trip of a few days because incubation of the disease may outlast the trip. Studies of trips that are much longer than the average incubation period, e.g. a week for Cryptosporidium and Giardia, are less susceptible to these errors since there is enough time for the diarrhea to occur during the trip. Other bacterial and viral agents have shorter incubation periods, although hepatitis may require weeks.

A suspected case of wilderness diarrhea may be assessed within the general context of intestinal complaints. During any given four-week period, as many as 7.2% of Americans may experience some form of infectious or non-infectious diarrhea. There are an estimated 99 million annual cases of intestinal infectious disease in the United States, most commonly from viruses, followed by bacteria and parasites, including Giardia and Cryptosporidium. Giardia alone may infect up to 10% of Americans at any one time. However, because most carriers are asymptomatic, there are only an estimated 2 million U.S. cases of symptomatic giardiasis annually spread mostly by fecal-oral or food-borne transmission.

Treatment

WAD is typically self-limited, generally resolving without specific treatment. Oral rehydration therapy with rehydration salts is often beneficial to replace lost fluids and electrolytes. Clear, disinfected water or other liquids are routinely recommended.

Hikers who develop three or more loose stools in a 24-hour period — especially if associated with nausea, vomiting, abdominal cramps, fever, or blood in stools — should be treated by a doctor and may benefit from antibiotics, usually given for 3–5 days. Alternately, a single dose azithromycin or levofloxacin may be prescribed. If diarrhea persists despite therapy, travelers should be evaluated and treated for possible parasitic infection.

There is no effective antibiotic against Cryptosporidium, which can be quite dangerous to patients with compromised immune systems.

Prevention

Since wilderness-acquired diarrhea can be caused by fecal-oral transmission, contaminated water, and contaminated food, prevention methods should address these causes. The risk of fecal-oral transmission of pathogens can be reduced by good hygiene, including washing hands eating utensils with warm soapy water on a timely basis. Additionally a three-bowl system can be used for washing eating utensils. The risk from contaminated water can be addressed by various water treatment methods.

Treating water

Water can be treated in the wilderness through filtering, chemical disinfectants, a portable ultraviolet light device, pasteurizing or boiling. Factors in choice may include the number of people involved, space and weight considerations, the quality of available water, personal taste and preferences, and fuel availability.

In a study of long-distance backpacking, it was found that water filters were used more consistently than chemical disinfectants. Inconsistent use of iodine or chlorine may be due to disagreeable taste, extended treatment time or treatment complexity due to water temperature and turbidity.

Because methods based on halogens, such as iodine and chlorine, do not kill Cryptosporidium, and because filtration misses some viruses, the best protection may require a two-step process of either filtration or coagulation-flocculation, followed by halogenation. Boiling is effective in all situations.

Iodine resins, if combined with microfiltration to remove resistant cysts, are also a viable single-step process, but may not be effective under all conditions. New one-step techniques using chlorine dioxide, ozone, and UV radiation may prove effective, but still require validation.

Ultraviolet (UV) light for water disinfection is well established and widely used for large applications, like municipal water systems. A small portable UV device, called a Steri-pen, is now available for hikers. According to the manufacturer, it meets standards set forth in the U.S. EPA Guide Standard and Protocol for Testing Microbiological Water Purifiers.

The backcountry medical text Wilderness Medicine, describes water disinfection and personal hygiene as one key to preventing wilderness diarrhea. Drinking untreated backcountry water is "like Russian roulette," according to its editor, Paul S. Auerbach, in an interview with REI.com. Auerbach added that while research has shown Giardia in wilderness water sources is "probably not as prevalent as once believed...it's still present". Howard Backer, author of Wilderness Medicine's guidelines on water disinfection, told the same publication that if a wilderness visitor is confident of untainted alpine water, "you're probably safe to drink it untreated," but that in areas with significant human or animal activity, the risk of infection makes precautionary disinfection a wise precaution..

Two standard textbooks on backcountry medical issues promote water disinfection as a key means of preventing infection. Various commercial water treatment devices are described in detail, and most infectious diarrhea acquired in the U.S. wilderness is considered to be from water-borne pathogens, and effective prevention is therefore thought to require treatment of drinking water as well as proper sanitiation.

One researcher, in discussing evidence for Giardia contamination of backountry water sources, suggested that if Giardia lamblia is an uncommon isolate and wilderness-acquired diarrhea is infrequent, then it may not be cost effective to recommend filtering for recreationalists involved in short-duration trips. In terms of preventing giardiasis, handwashing is more important than treating water.

Drinking untreated water is something like Russian roulette. Giardia in wilderness water sources may probably not be as prevalent as once believed but it's still present. If a wilderness visitor is confident of untainted alpine water, it's probably safe to drink it untreated, but in areas with significant human or animal activity, treatment is critical in prevention.

Contamination of U.S. backcountry water sources is widespread and disinfection is necessary but exaggerated concern has been raised about the issue. An example is an alleged case where Government agencies have filtered hundreds of gallons of water from wilderness streams, found one or two organisms (far less than enough to be infective), and erected garish signs proclaiming the water hazardous.

Personal hygiene

The text Medicine for Mountaineering and Other Wilderness Activities , also advises that disinfection and hygiene are necessary to avoid risk, although elsewhere, the same work notes that "exaggerated concern" has been raised about the issue.

Other commentators place relatively greater emphasis on preventative hygiene. Thomas R. Welch in a 2004 editorial in the journal Wilderness Medicine expressed the view that "stopping hand-to-mounth spread is the key to preventing gastrointestinal infection" and that routine universal treatment of water should be de-emphasized. Careful attention to personal hygiene can help prevent the spread of infection.

Other measures

Also, one study suggests that on very long trips, taking multivitamins may reduce the incidence of WAD.

Epidemiology

The risk of acquiring infectious diarrhea in the wilderness arises from inadvertent ingestion of pathogens. Various studies have sought to estimate diarrhea rates among wilderness travelers and results have ranged widely. A number of these have either focused on only one pathogen, Giardia lamblia, or provided scant data.

The rate of diarrhea varies dramatically between studies and may depend on the time of year, the location of the study, the length of time the hikers were in the wilderness, and the study methodology.

Surveys among hikers who completed the Appalachian Trail reported more than half experienced at least one episode of diarrhea, usually of two days or less in duration. Treating water and routinely washing hands with soap and water after defecation reduced the incidence.

Backpackers may be carriers of Giardia without being symptomatic. 16.7% of subjects in this study developed mild gastrointestinal illness other than giardiasis.

In 1992 a review of four separate field studies, concluded that the general rate of infectious diarrhea acquired in the wilderness "is probably less than 10%, with scant data placing it in the 3% to 5% range." Fecal-oral transmission may be the most common vector for backcountry Giardia infection. There are differing opinions regarding the importance of routine disinfection of water during relatively brief backcountry visits.

Among National Outdoor Leadership Schools (NOLS) participants in various wilderness settings (primarily in North America, but also in South America, Asia, and Australia), gastrointestinal illnesses occurred at rates of 0.44 per 1000 days in 1984-1989 and and 0.20 in 1999-2002. The rates in 2002-2005 of 0.26 rates per 1000 days comprised 23% of all illness reported in the programs. NOLS emphasized strict hand-washing techniques, water disinfection and washing of common cooking utensils.

A study at Grand Teton National Park, found 69% of diarrhea affected visitors had no identifiable cause, with 23% having Campylobacter and 8% Giardia; Campylobacter enteritis occurred most frequently in young adults who had hiked in wilderness areas and drunk untreated surface water in the week prior.

Backcountry water quality surveys

Infection by fecal coliform bacteria, which indicate fecal pollution, represent a much higher risk than giardiasis. Risks are highest in surface water near trails used by pack animals and cattle pastures.

Most surveys of backcountry water in the United States have found very low or no Giardia cysts. However, very few studies have addressed the issue of transient contamination. For example, the likely model for the risk of Giardia from wilderness water is pulse contamination, that is, a brief period of high cyst concentration from fecal contamination.

Other studies

In 2000 a North America literature review found a high incidence of confirmed giardiasis among outdoor recreationalists, but little association between drinking backcountry water and acquiring giardiasis.

In a 1991 giardiasis survey of 48 state health departments in the United States, 34,348 cases were reported. Of these, 19 were attributed to contaminated drinking water and 2 were reported among campers and backpackers.

See also

References

  • Backer, Howard D. (2007). "Chapter 61: Field Water Disinfection". In Auerbach, Paul S. ed. (ed.). Wilderness Medicine (5 ed.). Philadelphia, PA: Mosby Elsevier. pp. 1368–417. ISBN 9780323032285. {{cite book}}: |editor= has generic name (help); Cite has empty unknown parameter: |coauthors= (help)

Footnotes

  1. ^ Zell SC (1992). "Epidemiology of Wilderness-acquired Diarrhea: Implications for Prevention and Treatment" (PDF). J Wilderness Med. 3 (3): 241–9.
  2. ^ Hargreaves JS (2006). "Laboratory evaluation of the 3-bowl system used for washing-up eating utensils in the field". Wilderness Environ Med. 17 (2): 94–102. PMID 16805145. Diarrhea is a common illness of wilderness travelers, occurring in about one third of expedition participants and participants on wilderness recreation courses. The incidence of diarrhea may be as high as 74% on adventure trips. …Wilderness diarrhea is not caused solely by waterborne pathogens, … poor hygiene, with fecal-oral transmission, is also a contributing factor
  3. ^ Rockwell, Robert L. (2003). "Giardia Lamblia and Giardiasis With Particular Attention to the Sierra Nevada". Peak Climbing Section, Loma Prieta Chapter, Sierra Club. Archived from the original (PDF) on 2008-10-16.
  4. ^ Backer, Howard (1992). "Wilderness acquired diarrhea (editorial)" (PDF). Journal of Wilderness Medicine. 3: 237–240. Cite error: The named reference "Backer editorial 1992" was defined multiple times with different content (see the help page).
  5. (Backer 2007, p. 1371)
  6. (Backer 2007, p. 1369)
  7. (Backer 2007, p. 1374)
  8. Prepared by Federal-Provincial-Territorial Committee on Drinking Water of the Federal-Provincial-Territorial Committee on Health and the Environment (2004) (2004), "Protozoa: Giardia and Cryptosporidium" (PDF), Guidelines for Canadian Drinking Water Quality: Supporting Documentation, Health Canada, retrieved 2008-08-07 {{citation}}: Cite has empty unknown parameters: |coeditors= and |coauthors= (help)CS1 maint: numeric names: authors list (link)
  9. Dickens DL, DuPont HL, Johnson PC (1985). "Survival of bacterial enteropathogens in the ice of popular drinks". JAMA. 253 (21): 3141–3. PMID 3889393. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  10. Backer H (2000). "In search of the perfect water treatment method" (PDF). Wilderness Environ Med. 11 (1): 1–4. PMID 10731899.
  11. Gerba C, Rose J (1990). "Viruses in Source and Drinking Water". In McFeters, Gordon A. ed. (ed.). Drinking water microbiology: progress and recent developments. Berlin: Springer-Verlag. pp. 380–99. ISBN 0-387-97162-9. {{cite book}}: |editor= has generic name (help)
  12. White, George W. (1992). The handbook of chlorination and alternative disinfectants (3rd edition ed.). New York: Van Nostrand Reinhold. ISBN 0-442-00693-4. {{cite book}}: |edition= has extra text (help)
  13. ^ CDC Division of Parasitic Diseases (2004). "CDC Fact sheet: Giardiasis". Centers for Disease Control. Retrieved 2008-10-13.
  14. ^ National Center for Zoonotic, Vector-Borne, and Enteric Diseases (2008-04-16). ""Crypto" - Cryptosporiodosis". Centers for Disease Control. Retrieved 2008-10-13.{{cite web}}: CS1 maint: multiple names: authors list (link)
  15. ^ Boulware DR, Forgey WW, Martin WJ 2nd (2003). "Medical Risks of Wilderness Hiking". Am J Med. 114 (4): 288–93. PMID 12681456.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  16. ^ Boulware DR (2004). "Influence of Hygiene on Gastrointestinal Illness Among Wilderness Backpackers". J Travel Med. 11 (1): 27–33. PMID 14769284.
  17. Scallan, E. J. (>=2002). "Prevalence of Diarrhea in the Community in Australia, Canada, Ireland and the United States" (PDF). CDC. Retrieved 2008-10-15. {{cite web}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  18. Garthright WE, Archer DL, Kvenberg JE (1988). "Estimates of incidence and costs of intestinal infectious diseases in the United States". Public Health Rep. 103 (2): 107–15. PMC 1477958. PMID 3128825.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  19. Brody, Jane E. (2008-10-15). "HEALTH: Diagnostics; Test Unmasks a Parasitic Disease". New York Times. New York Times Company. Retrieved 2008-10-15. {{cite news}}: Cite has empty unknown parameter: |coauthors= (help)
  20. Hlavsa, Michele C. (2005-01-28). "Giardiasis Surveillance --- United States, 1998--2002". Morbidity and Mortality Weekly Report. Centers for Disease Control and Prevention. Retrieved 2008-10-15. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  21. Sanders JW, Frenck RW, Putnam SD; et al. (2007). "Azithromycin and loperamide are comparable to levofloxacin and loperamide for the treatment of traveler's diarrhea in United States military personnel in Turkey". Clin. Infect. Dis. 45 (3): 294–301. PMID 18688944. {{cite journal}}: Explicit use of et al. in: |author= (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  22. ^ (Backer 2007, p. 1368-417)
  23. Johnson, Mark (2003). The Ultimate Desert Handbook : A Manual for Desert Hikers, Campers and Travelers. International Marine/Ragged Mountain Press. p. 46. ISBN 0-07-139303-X.
  24. Backer H (2002). "Water disinfection for international and wilderness travelers". Clin. Infect. Dis. 34 (3): 355–64. PMID 11774083. {{cite journal}}: Unknown parameter |month= ignored (help)
  25. (Backer 2007, p. 1411)
  26. "Steripen - Proven Technology". Hydro-Photon, Inc. 2008. Retrieved 2008-10-14.
  27. "Steripen - Microbiological Testing". Hydro-Photon, Inc. 2008. Retrieved 2008-10-14.
  28. "Expert Advice: Water: What Are the Risks? from REI".
  29. ^ Welch TP (2000). "Risk of giardiasis from consumption of wilderness water in North America: a systematic review of epidemiologic data". International journal of infectious diseases. 4 (2): 100–3. PMID 10737847.
  30. Wood, T. D. (2008). "Water: What Are the Risks?". REI.com. Retrieved 2008-08-07.
  31. ^ Seattle, Washington: The Mountaineers Books, 5th edition, 2001
  32. Welch TR (2004). "Evidence-based medicine in the wilderness: the safety of backcountry water (editorial)" (PDF). Wilderness & environmental medicine. 15 (4): 235–7. PMID 15636372.
  33. Gardner TB, Hill DR (2002). "Illness and injury among long-distance hikers on the Long Trail, Vermont" (PDF). Wilderness & environmental medicine. 13 (2): 131–4. PMID 12092966.
  34. Crouse BJ, Josephs D (1993). "Health care needs of Appalachian trail hikers". The Journal of family practice. 36 (5): 521–5. PMID 8482936. {{cite journal}}: Unknown parameter |month= ignored (help)
  35. ^ Zell SC, Sorenson SK (1993). "Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe" (PDF). Journal of Wilderness Medicine. 4 (2): 147–54.
  36. McIntosh, Scott E. (2007). "Medical incidents and evacuations on wilderness expeditions" (PDF). Wilderness and Environmental Medicine. 18 (4): 298–304. PMID 18076301. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  37. Taylor, D. N. (1983). "Campylobacter enteritis from untreated water in the Rocky Mountains". Ann Intern Med. 99 (1): 38–40. PMID 6859722. Retrieved 2008-10-16. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  38. Derlet, Robert W. (>=2003). "Sierra Nevada Water: Is it safe to drink? - Analysis of Yosemite National Park Wilderness water for Coliform and Pathologic Bacteria". SierraNevadaWild.gov. Sierra Wilderness Education Project. Retrieved 2008-10-15. {{cite web}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  39. Derlet RW (2008). "Backpacking in Yosemite and Kings Canyon National Parks and neighboring wilderness areas: how safe is the water to drink?". Journal of travel medicine. 15 (4): 209–15. doi:10.1111/j.1708-8305.2008.00201.x. PMID 18666919. {{cite journal}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysummary= ignored (help)
  40. Derlet, Robert W. (April 2004). "High Sierra Water: What is in the H20?". Yosemite Association.
  41. (Backer 2007, p. 1372)
  42. Welch TR, Welch TP (1995). "Giardiasis as a threat to backpackers in the United States: a survey of state health departments". Wilderness Environ Med. 6 (2): 162–6. PMID 11995903. {{cite journal}}: Unknown parameter |month= ignored (help)
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