Almost everyone that drinks backcountry water knows there are microscopic organisms in untreated water that can make people ill. Unfortunately, there is lack of agreement about how common the problem is, or what people should do about it. It is not unusual to see articles in outdoor magazines that combine correct with incorrect information about how to deal with backcountry drinking water. The problem here is: outdoor magazine editors don’t have to have science degrees, and the writers submitting articles are generally only partly informed about microorganisms in water and their safe removal.

The information contained in this report has been compiled mainly from web-pages available on-line from the U.S. Center For Disease Control (CDC) and the U.S. Food & Drug Administration (FDA). Other sources are available on the internet and credited at the end of the report. I have done my best to be un-biased, relative to information I have read, or know from practical use (I also have a Forestry degree from the U. of Idaho with a minor in Biology). My inspiration has been the best ever report on “What’s in the Water?” by Mark Jenkins in the December 1996 issue of Backpacker. That article hit a journalistic high for articles in outdoor magazines on problems with outdoor H2O. In the last 13 years things have changed and a new report on “What’s in the Water” is needed. This is my much-researched attempt to amend the Mark Jenkins report.

There are three general classifications of waterborne disease-causing microorganisms. From largest to smallest, they are: Protozoa, Bacteria, and Virus. Very few of these microorganisms naturally occur in outdoor water—–they instead are primarily introduced into H2O from animal and human feces.

Protozoa are by far the most common disease-causing microorganism in back-country water. Protozoa are also the largest and easiest microorganism to remove with microfiltration, but the hardest to kill with chemicals.

Articles on microorganisms in backcountry water usually warn about two protozoa: Girardia and Cryptosporidium. Both these protozoa are common worldwide and in North America—–a 1992 study found that 97% of U.S. rivers and lakes contain one or both of them. Besides being carried in the intestines of many mammal species, a significant percent of humans also carry these protozoa. Testing of human feces found in archaeological sites has revealed both Giardia and Cryptosporidium were carried by Native Americans over 2,000 ago. Giardia was first identified under the microscope in 1681 (from human feces).

The prevalence of Giardia in human stool specimens submitted for examination ranges from 2% to 5% in industrialized countries, and from 20% to 30% in developing countries. From 1979 through 1988, an estimated 4,600 hospitalizations per year in the United States resulted from severe Giardiasis (the disease caused by Giardia) and its complications. The CDC reports during 1991-2000 Giardia was identified as a causal agent of 16.2% of reported drinking-water associated outbreaks of gastroenteritis of known or suspected infectious etiology.

Cryptosporidium may be just as common as Giardia. U.S. FDA-Bad Bug Book Direct Human Surveys: indicate a prevalence of Cryptosporidium in about 2% of the population in North America. In 1991 two studies found Cryptosporidium present in between 65% to 87% of U.S. surface-water samples.

One problem in identifying protozoan infections is they typically have a long incubation period, before symptoms are noticeable—–5 to 28 days after you drink untreated water——you finally notice you are ill.

Although both Giardia and Cryptosporidium are relatively common, symptoms can be quite severe—— as per the following table.¹

Table 2. Symptoms of 205 patients with confirmed cases of cryptosporidiosis during the Milwaukee outbreak


Symptom Percent(%)


Watery diarrhea 93%

Abdominal Cramps 84%

Weight Loss (Median 10 lbs) 75%

Fever—- Median 100.9 F., range: 98.7-104.9 F. 57%

Vomiting 48%

Although Giardia symptoms are similar to Cryptosporidium symptoms: generally there is not a fever. Some Giardia victims (less than 4%) remain symptomatic more than 2 weeks. In them, it becomes a chronic intestinal condition that can last for years and be life-threatening, due to severe weight loss. Giardia does generally respond to treatment with several different drugs.

Waterborne protozoa can be removed by microfiltration with any Katadyn or MSR microfilter.

However, I must note that recent testing at University of Arizona led the researchers to issue the following statement about using iodine tablets to treat water.9

“These data strongly suggest that iodine disinfection is not effective in inactivating Cryptosporidium oocysts in water. Because this organism is common in all surface waters, it is recommended that another method of treatment be used before ingestion.”

Chlorine Dioxide at a concentration of 4 PPM (parts per million) has been demonstrated to kill Cryptosporidium, but it can take up to 4 hours in cold dirty water. Katadyn Micropur is the best known brand of Chlorine Dioxide Tablets.

Until 2005, Cryptosporidium sufferers had to let the disease run it’s course—–which can be several weeks of suffering. The drug Alinia, which contains the active ingredient, nitazoxanide, is now prescribed for both Cryptosporidium and Giardia infections. Alinia effects a cure in about a week in a high percentage of those infected.

Likely, many serious outdoor people contracted Giardia or Cryptosporidum during a long-ago adventure—or in the most common place that people are infected: their daycare center, during infancy. They may now have resistance to those protozoa from earlier infections, or carry one or both of these protozoa in their intestines without showing serious symptoms (estimates are up to 15% of infected people may be asymptomatic carriers).

If, in the distant past, you have consumed untreated backcountry water, or if you have a whitewater boating background, or if you are someone who claims: “there ain’t no bugs in that there water”, it’s probable you are —-“one of them”—- an asymptomatic protozoa carrier.

The protozoa that causes Ameobic Dysentery (Entamoeba histololytica) is uncommon in North America, and very unlikely to affect outdoor people here. However in South America, Africa, and S.E. Asia: it causes more deaths than any other parasite, except those that cause Malaria. One study indicated that up to 12% of the world’s population may be infected by E. histololytica with 10% of those having clinical symptoms.² The most dramatic incident in the USA was the Chicago World’s Fair outbreak in 1933 caused by contaminated drinking-water. There were 1,000 cases (with 58 deaths). There have been scattered cases of Ameobic Dysentery reported in urban areas in the U.S. since 1988.

Other waterborne protozoa that infect humans are: Cyclospora cayetanensis, Isospora belli, and Microsporidia (five genera of this huge group have been implicated in human disease). These other protozoa are not often tested for or identified, but they can cause severe diarrhea and other more profound symptoms. Cyclospora and Microsporidia are common worldwide and are carried by many animals species, while Isospora belli is believed to only infect humans and is found in tropical areas worldwide.


Waterborne bacteria that infect humans: are the same that we are warned about in meat that has not been properly processed and cooked. The method of transmission is the same—-animal or human feces contamination.

People drinking water downstream from where livestock, or in some cases where wild animals, have been defecating: are also at risk of catching these bacteria. Generally only a small number of bacteria need to be ingested to cause illness. Some outdoors-people may also have a natural resistance to a particular strain of water-carried bacteria, while others may carry these bacteria in their intestines and infect others without showing symptoms (asymptomatic carriers).

All these bacteria can cause severe diarrhea, and fever. Other more severe symptoms such as nausea, vomiting, and abdominal pain are not unusual. Without antibiotic treatment some victims may suffer sufficient fluid loss from diarrhea to cause death.

Campylobacter jejuni: appears to be the most common bacteria in backcountry water. During a study in the early 1980’s: Campylobacter jejuni was isolated from 25% of the people with diarrheal disease acquired in the area of Grand Teton National Park Wyoming. Campylobacter were found in a stream at 7,700 ft. (2,350 m.) in Grand Teton National Park.

The authors of the study believed the water sample and the number of people infected from drinking surface water in the mountains “suggests that a C. jejuni serotype—-can survive in alpine water. In vitro studies have demonstrated that Campylobacter remains viable for months in surface water kept at 4° C. (39° F.).”³ Campylobacter is commonly carried by poultry, horses, and cows; it is found in most migratory waterfowl, some rodents, and may be present in deer, elk, and bears.

Campylobacter is the most common bacterial causes of diarrheal illness in the United States. Campylobacter infections are estimated to affect over 1 million persons every year, or 0.5% of the general population. Most people who become ill with Campylobacter: get diarrhea, cramping, abdominal pain, and fever within 2 to 5 days after exposure to the organism. The diarrhea may be bloody and can be accompanied by nausea and vomiting. The illness typically lasts 1 week, but there can be serious long-term problems. Some persons who are infected with Campylobacter are asymptomatic.

Shigella: which causes bacillary dysentery, is also thought to be transmitted from wild animal feces in some places. Every year, about 18,000 cases of Shigellosis are reported in the United States. Because many milder cases are not diagnosed or reported, the actual number of infections may be twenty times greater. Children, especially toddlers aged 2 to 4, are the most likely to be infected.

E. Coli Currently, there are four recognized classes of enterovirulent E. coli (collectively referred to as the EEC group) that cause gastroenteritis in humans. The most frequent clinical syndrome of infection includes watery diarrhea, abdominal cramps, low-grade fever, nausea and malaise. E. coli is a normal inhabitant of the intestines of all animals, including humans. A minority of E. coli strains are capable of causing human illness.

Salmonella are a large group of similar species of bacteria and are common worldwide in water that is contaminated by livestock or human feces. Although the CDC states there are over 40,000 infections reported yearly in the U.S.—-most of these are from food, rather than contaminated water.

Salmonella typhi. Travelers to Asia, Africa, and South America can fear Typhoid Fever (which now has antibiotic resistant strains) from untreated water. Typhoid fever is a life-threatening diarrheal illness caused by the bacterium Salmonella typhi. In the United States about 400 cases occur each year, and 75% of these are acquired while traveling internationally. Typhoid fever is still common in the developing world, where it affects about 21.5 million persons each year. Immunization shots or boosters are strongly recommended for international travelers.

Cholera: Cholera is an acute, diarrheal illness caused by infection of the intestine with the bacterium Vibrio cholerae. The infection is often mild or without symptoms, but sometimes it can be severe. Approximately one in 20 infected persons has severe disease characterized by profuse watery diarrhea, vomiting, and leg cramps. In these persons, rapid loss of body fluids leads to dehydration and shock. Without treatment, death can occur within hours. It is very uncommon in North America and Europe, but continues to kill many people in S. America, Africa and Asia.

Other Bacteria: Plesiomonas shigelloides has been identified in outbreaks of diarrhea and fever in the U.S from contaminated water. It is relatively rare. Yersinia enterocolitica has caused waterborne disease outbreaks in Alaska & Japan. It is a common bacteria and has caused various out- breaks in food and dairy products. It produces symptoms as gastroenteritis with diarrhea and/or vomiting; however, fever and abdominal pain are the hallmark symptoms. Both these bacteria have been isolated from such animals as pigs, birds, beavers, cats, and dogs.


Waterborne viruses, that infect humans, have only one source——human feces from infected carriers. These viruses include: rotavirus, caliciviruses-which now include the norovirus group with Norwalk virus, adenovirus, astrovirus, Hepatitis A & E, and enteroviruses (which include those that cause Polio and Viral-Meningitis). More than half of the infections with these viruses only cause mild or asymptomatic illness—but they can cause a wide range of serious and even life-threatening illnesses—-especially in children.

The two most common water-borne viruses in North America—-rotovirus and noroviruses are responsible for numerous outbreaks in restaurants each year of what is commonly called “food-poisoning”. Outbreaks of both have occurred from well or river water contaminated with raw sewage. The Center for Disease Control (CDC) has noted outbreaks of norovirus in the Grand Canyon of the Colorado River. The Grand Canyon outbreaks have been traced to a sewage treatment facility at the base of Glen Canyon Dam—–just above the Grand Canyon.

Norovirus and the calcivirus group: Symptoms of norovirus are usually over within 48 hours, and include headache, fever, vomiting and diarrhea. Noroviruses are relatively resistant to environmental challenge from cold or heat. Moreover, noroviruses can survive in up to 10 ppm chlorine: well in excess of levels routinely present in public water systems (they are however destroyed by Chlorine Dioxide at 1ppm in 30 minutes.)

Rotavirus: The incubation period for rotavirus disease is approximately 2 days. The disease is characterized by vomiting and watery diarrhea for 3 – 8 days, and fever and abdominal pain occur frequently. Rotavirus is the most common cause of severe diarrhea among children, resulting in the hospitalization of approximately 55,000 children each year in the United States and the death of over 600,000 children annually worldwide.

Hepatitis A: Although hepatitis A, does cause occasional outbreaks in food in the U.S., it has not been recognized as a culprit in backcountry water disease here. However in South America, Africa, and Asia; it and other viruses can be life-threatening in contaminated water. Hepatitis A affects liver function and be a very serious disease, or a long-lasting low-grade infection. Immunization shots are strongly recommended for international travelers.

Hepatitis E has not been a problem in the U.S. water supply. It is estimated to cause 2% mortality in the general population and up to 30% mortality in pregnant women. Major waterborne epidemics have occurred in India (1955 and 1975-1976), USSR (1955-1956), Nepal (1973), Burma (1976-1977), Algeria (1980-1981), Ivory Coast (1983-1984), in refugee camps in Eastern Sudan and Somalia (1985-6), and in Borneo (1987). The first outbreaks reported in the American continents occurred in Mexico in late 1986.

Enteroviruses: The enterovirus family contains at least 62 different viruses that infect humans. The most dangerous cause Polio (now eradicated in the Americas); while others cause aseptic (viral) meningitis-symptoms include fever, severe headache, stiff neck, and nausea and vomiting. Most of the enteroviruses cause cold-like or flu-like symptoms. The CDC states: “Enteroviruses may be found in water sources such as private wells. Wells become contaminated when feces from infected humans enter the water through various means, including sewage.”

Enteric adenovirus, astroviruses: These commonly infect young children with viral gastroenteritis. They may be transmitted by infected food or water. Viral gastroenteritis is characterized by nausea, vomiting, diarrhea, malaise, abdominal pain, headache, and fever. The infectious dose is not known but is presumed to be low. Symptoms last 2 to 9 days.

Enteric adenovirus is the most common virus in this group and causes 5-20% of the gastroenteritis in young children. Adenoviruses most commonly cause respiratory illness; however, depending on the infecting serotype, they may also cause various other illnesses, such as gastroenteritis, conjunctivitis, cystitis, and rash illness. A recent study notes that adenovirus is not destroyed by UV light disinfection.

Other Things to worry about in water!

Cyanobacteria, also known as blue-green algae, can produce toxins that may taint drinking water. Humans who drink water that contains high concentrations of cyanobacteria or cyanobacterial toxins may experience gastroenteritis, allergic responses, or liver damage, or in the long run —cancer.

The CDC is worried enough about cyanobacteria to have a web-page for it. There have been numerous problems with cyanobacteria poisoning worldwide, generally with livestock or dogs.

Cyanobacteria blooms generally occur late summer or early fall in warm stagnant or slow-moving waters that are rich in nutrients such as fertilizers or septic system overflows. The blooms can look like foam, scum, or mats; and can be blue, bright green, brown, or red. Some blooms may not affect the water’s appearance. The water may smell bad.

Given a choice—-water suspected of having an algae bloom should be avoided. If you must drink it: first try to obtain your water where the algae are not as thick. Second: microfilter to remove cyanobacteria, and clean your filter frequently to remove toxins from the filter. Thirdly, and most important, run your microfiltered water through activated charcoal, (part of most, but not all microfilter systems) which will remove most of the toxins. Please note: that activated charcoal in micro-filters may only be effective for approximately 6 months from first use.

Liver Flukes: Faciola hepatica-common name – sheep liver fluke. These are a parasitic flatworm (trematode). They commonly infect livestock in North America, Europe, and South America and can infect humans as well: if eaten in vegetation growing in fresh water, like water cress, eaten in leafy vegetables irrigated with infected water, or (more rarely) ingested in untreated drinking water. In some rural areas of Peru up to 50% of the population is infected. Similar species occur worldwide and are estimated to infect between 40 million and 100 million people. Many who are infected are asymptomatic, but others can have abdominal pain, headache, and a host of other symptoms.

Mineral contamination: We all know this type of water—–there’s a small sign with a skull & crossbones and bleached skeletons near the pool. Water like this does exist in the U.S.—- but is not common. The dangers to worry about here are: selenium and other dissolved metals that cause long-term problems, high concentrations of dissolved carbonates that can cause intestinal upset, and salts.

Clues to mineral contamination may be some or all of the following: a lack of vegetation around the pool, residues of salts or carbonates on the banks or in the pool, nearby evidence of mining, strong mineral odor, un-natural colors like bright blues or greens, metallic, or salty taste. Hot spring water should be avoided, since it frequently contains high concentrations of dissolved minerals. No standard portable treatment methods make these liquids safe for long-term use. If you have a choice—move on to better water.

Chemical Contamination: The E.P.A. currently lists 50 different organic chemical contaminants of water that they monitor and regulate. These are a potential problem downstream from agriculture or industrial areas. Activated charcoal will remove many organic chemicals from water, but does not remove most metals, salts, or carbonates: this includes another 16 inorganic chemicals that the EPA lists as potential dangers.

THE NAY-SAYERS: There is an increasing amount of misinformation from sources that tell you that most backcountry water is not going to cause illness. An article, “Giardia Lamblia and Giardiasis by Robert Rockwell, makes an eloquent case for Giardia not being common in the Sierra Nevada range. The report is widely available on the internet. His primarily point is: Giardia and its cysts are killed by water temperatures below 5 degrees C. (41 degrees F.) for a span of 3 months or more——which means all Giardia is killed out of Sierra Nevada mountains every winter.

Although Mr. Rockwell’s article dismisses Giardia as a problem in the Sierras—-he makes frequent references to other waterborne diseases as a possible problem there and elsewhere. He also discusses Giardia as a problem in other places, including high mountain environments. Unfortunately, many people have heard or seen small parts of Mr. Rockwell’s article and then erroneously reached the conclusion that microorganisms in back-country water are not a problem——anywhere.

Some people may have an immunity to some waterborne pathogens, due to resistance from a previous infection, or because they may already be an asymptomatic carrier of a protozoa, bacteria, or virus. This can be a temporary immunity, and also doesn’t mean they are immune to all other microorganisms. Unfortunately, because these people can drink some backcountry water without becoming ill: they either assume there are not infectious microorganisms in the water, or they believe whatever unsafe method of treatment they employ removes infectious microorganisms.

Every year, while doing employee clinics for Katadyn in retail stores in the western U.S., I hear from various people who have been infected with disease causing organisms from backcountry water sources. Most of these people have contracted Giardia or Cryptosporidium, but I have talked with several who caught Campylobacter. I see these people as a very small percentage of those that have been infected, since for the most part only those males who become extremely ill go to a doctor and have the disease diagnosed. Many males, “tough out” their illness (it appears women are brighter, and more readily seek medical care).

Water Treatment Methods:

What do I recommend? My employers are very tolerant of my wife and me making long trips into the outdoors “to test equipment”. I enjoy car-camping, backpacking and multi-day whitewater rafting trips. I always assume all backcountry water is contaminated.

When I go with a group, I offer up my expertise (and free filters) for back-country water treatment. Highpoints for me include an 18 day private Grand Canyon raft trip, where I perversely pumped most of my group’s water out of the very dirty Colorado River “just to see if I can trash my microfilters” and a 16 day trek in the Everest area of Nepal, where I convinced the owners of the trekking company to let me handle water purification. (All clients kept healthy digestive systems: for the first time ever).

For North America, I recommend a Katadyn or MSR microfilter that suits your technical inclination and the size of your group. I really believe pumping water with one of these products is faster and easier than other methods. A good microfilter is a very liberating piece of outdoor equipment——you don’t have to carry much water in most situations——or limit the length of your trips.

For areas where there is concern about waterborne viruses: I recommend first microfiltration, then treatment for 15 minutes with chlorine dioxide from Katadyn Micropur tablets, or the MSR Miox.

Treating backcountry water is a lot like wearing a seatbelt—–it gets to be a habit —–and you never know when that habit is going to save you from some real grief.

Copyright Ray Brooks 2010

References/The Sources

My thanks to: the very valuable information on waterborne diseases on the web sites of the Center for Disease Control (CDC) and the Food & Drug Administration (FDA), especially their “Bad Bug Book” site. I also sampled information on chemicals in water from the Environmental Protective Agency (E.P.A.) web site.

Other specific references by number:

1. MacKenzie WR, Hoxie NJ, Proctor ME, Gradus MS, Blair KA, Peterson DE, et al. A massive outbreak in Milwaukee of Cryptosporidium infection transmitted through the public water supply. N Engl J Med 1994;331:161-7.

2. Waterborne Protozoan Pathogens, Marshall, Naumovitz, Ortega, & Sterling, U. of Arizona. Clinical Microbiology Reviews Jan 1997.

3. Waterborne Transmission of Campylobacter enteritis, Taylor, Brown, & McDermott, Microbial Ecology, (1982) 8:347-354

4. Small rodents & other mammals associated with mountain meadows as reservoirs of Giardia spp. and Campylobacter spp. Pacha, Clark, Williams, Carter, etc, Applied and Environmental Microbiology v.53 (7); (July 1987)

5. Inactivation of Enteric Adenovirus and Feline Calicivirus by Chlorine Dioxide: Thurston-Enriquex, Haas, Jacangelo, Gerba. Applied and Environmental. Microbiology. 2005 June; v.71 (6) 3100-3105.

6. Inactivation of Feline Calcivirus and Adenovirius Type 40 by UV Radiation, Thurston-Enriquex, Haas, Jacangelo, Riley, and Gerba. Applied and Environmental. Microbiology, 2003 Jan, v.69 p. 577-582

7. Cyanobacterial Toxins: Removal during Drinking Water Treatment and Human Risk Assessment. Hitzfild, Hoger, Dietrich; Environmental Health Perspectives, Vol 108, Supplement 1, March 2000.

8. Fascioliasis in Relatives of Patients with Fasciola hepatica Infection in Peru: Marcos, Maco, Terashima, Samalvides, Espinoza, Gotuzzo; Review Int. Med. Trop. S. Paulo 47(4) 219-222, July-August 2005.

9. From: Wilderness and Environmental Medicine: Vol. 8, No. 2, pp. 96-100.

Efficacy of iodine water purification tablets against Cryptosporidium oocysts and Giardia cysts