12 May 2002
Giardia Lamblia and Giardiasis
With Particular Attention to the Sierra Nevada
By Robert L. Rockwell
Figure 1. Giardia lamblia trophozoites as they appear with the scanning electron microscope.*
Ask the average outdoors person about Giardia lamblia or giardiasis, and they have certainly heard
about it. Almost always, however, they are considerably misinformed about both the organism?s
prevalence in wilderness water, and the seriousness of the disease if contracted.
With the advent of the Internet, the amount of information one can easily find on the subject is
voluminous. Unfortunately, most of it is flawed in important aspects, being unsubstantiated, anecdotal,
or merely quoting other unsubstantiated and anecdotal articles. Official sources, such as many
informational publications put out by the US government, are not immune to this criticism.
This paper is the result of a critical distillation of relevant articles, retaining only those from
scholarly, peer-reviewed, or otherwise professional and trustworthy sources.
One conclusion of this paper is that you can indeed contract giardiasis on visits to the Sierra Nevada,
but it almost certainly won?t be from the water. So drink freely and confidently: Proper personal
hygiene is far more important in avoiding giardiasis than treating the water.
First, an excerpt written by a highly regarded wilderness physician:
?In recent years, frantic alarms about the perils of giardiasis have aroused exaggerated
concern about this infestation. Government agencies, particularly the United States Park
Service and the National Forest Service, 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.??1
* Original image by Arturo Gonzalez, CINVESTAV, Mexico. From http://www.biosci.ohiostate.
And another, by researchers who surveyed the health departments in all 50 states and scanned the
medical literature looking for evidence that giardiasis is a significant threat to outdoor folk:
?Neither health department surveillance nor the medical literature supports the widely
held perception that giardiasis is a significant risk to backpackers in the United States.
In some respects, this situation resembles (the threat to beachgoers of a) shark attack:
an extraordinarily rare event to which the public and press have seemingly devoted
I explored this subject in 19873 and again in 19964 with an update in 1997.5 The conclusions have
always been that the ?Giardia problem? in Sierra Nevada water is grossly exaggerated, and that the
cases of giardiasis subsequent to wilderness visits are wrongly blamed on the water. After incorporating
the recent information for this paper, those prior conclusions are not only still valid but also
Just who is this little guy, anyway? 6, 7, 8, 9, 10
Giardia lamblia, now also known as G. intestinalis or G. duodenalis, was first observed in 1681 by
Anton van Leeuwenhoek, inventor of the microscope. The parasite was named in 1915 for two
scientists who studied it: Prof. A. Giard in Paris and Dr. F. Lambl in Prague.
Giardia is a flagellated (having whip-like appendages for locomotion) protozoan that, in the
trophozoite (active) form, attaches itself with an adhesive disk to the lining of the upper intestinal tract
of the host animal. There, it feeds and reproduces. Trophozoites divide by binary fission about every
12 hours. Thus, a single parasite can theoretically result in more than a million in 10 days and a billion
in 15 days.
At some time in its active life, the trophozoite releases its hold on the bowel wall and floats in the
fecal stream. As it makes its journey, it transforms into an egg-like structure called a cyst, which is
eventually passed in the stool. Duration of cyst excretion, called shedding, may persist for months.
Once outside the body, the cysts can be ingested by another animal. Then, they ?hatch? into
trophozoites due to stomach acid action and digestive enzymes, and the cycle repeats.
The trophozoite is 9 - 15 µm long, 5 - 15 µm wide, and 2 - 4 µm thick. Unlike the cyst, it cannot live
for long outside a host. Cysts are 8 - 12 µm long by 6 - 9 µm in diameter; so a million could fit under a
fingernail. Cysts can survive for as long as 2 to 3 months in cold water,10, 11 but they cannot tolerate
drying12 or freezing.9, 12, 13, 14
A significant infestation can leave millions of trophozoites stuck tight to the intestinal lining. There,
they cripple the gut?s ability to secrete enzymes and absorb food, especially fats, thereby producing the
disease?s symptoms. The symptoms typically appear one to two weeks after ingestion, with an average
of nine days, but four weeks is not uncommon. Symptoms can vanish suddenly and then reappear.
They may hide for months. They may not appear at all.10, 15
There are three ways that giardiasis, the disease caused by ingesting Giardia cysts, can be
contracted: contaminated water, contaminated food, and direct fecal-oral. A person who has just come
down with the disease and who wishes to identify the source needs to reflect on not only the possibility
of each of these avenues, but in a suspect period ranging from typically one week to four weeks earlier.
The bad news: Giardia lamblia is almost everywhere 1, 6, 7, 8, 9, 10, 16, 17, 18, 19, 20, 21
Giardiasis has been most often associated with travel to such places as Latin America, Africa, Asia,
and the former Soviet Union. However, Giardia has always been present in wilderness streams, in the
water supplies of most cities around the world, and even in the municipal water of large US cities. In
fact, in the 1930s and 1940s, before regulated municipal water treatment plants, everyone was drinking
Giardia all the time.22
Currently, Giardia lamblia is the most commonly diagnosed intestinal parasite in North America.24
It is the most frequently identified cause of diarrheal outbreaks associated with drinking water in this
country. Fully 20 percent of the world?s population is infected, and up to 7 percent of Americans, most
without any symptoms at all.10, 23 The Centers for Disease Control and Prevention (CDC) estimates that
as many as 2,500,000 cases occur annually in the US or about one for every 100 persons.24
Infestation rates of 60 percent of the children in day care centers across the country have been noted.
Institutions for mentally retarded persons can have high rates. Other high-rate populations include
promiscuous male homosexuals, international travelers, and patients with cystic fibrosis. And family
members of these individuals.
In an incident in New Jersey a child had a ?fecal accident? in a 700,000-liter swimming pool, and
nine swimmers came down with the disease.25 How many Giardia cysts might have been involved?
The number of cysts shed in feces is highly variable but has been estimated as high as 900 million per
day for a human.
Municipal water utilities must use filters to remove the organism. San Francisco water, coming
primarily from the Hetch Hetchy watershed in Yosemite National Park, tests positive for Giardia about
23 percent of the time, although at very low levels: fewer than 0.12 cysts per liter?. This water is of
such high quality that the US Environmental Protection Agency and the California Department of
Health Services have granted Hetch Hetchy water a filtration exemption, meaning that filtration
treatment to ensure its safety from Giardia and other organisms is not required.26 The Los Angeles
Aqueduct, which transports water to that city from the eastern slopes of the Sierra Nevada, averages
0.03 cysts per liter before treatment.27
Drinking highly contaminated water is one way to get the disease. Less common in developed
countries is direct passage from stool to the hands of a food preparer and then to the food itself. When
16 people got sick from the salad at a Connecticut picnic, the CDC tracked the source to a woman who
had mixed the salad with her hands. She didn?t have giardiasis, but one of her children did?without
any symptoms.15 A similar situation occurred in New Jersey, with the salad preparer testing positive for
Giardia along with her child and pet rabbit.28
Contaminated food may be a less-common source for the general population, but for wilderness
visitors, it may be the most important one. Put another way: If the water is clean, food-borne and direct
fecal-oral routes are the only pathways.
On a recent climbing expedition to Tibet, members of our party came down again and again with
what was undoubtedly giardiasis. Our water came from glacial melt, but all our food in advanced base
camp and below was prepared by Sherpa cooks. Much of the food they prepared?potatoes, rice,
cauliflower, cabbage, onions?came from Nepal. We were continually assured that the cooks were
practicing good hygiene, yet we had major intestinal problems that prevented many of the participants
from getting high on the mountain.
The disease has been referred to as ?beaver fever? because of a presumed link to those waterdwelling
animals known to be carriers. However, it has been suggested that it is more likely that
humans have carried the parasite into the wilderness and that beavers may actually be the victims. In
particular, there is a growing amount of data showing that beavers living downstream from
campgrounds have a high Giardia infection rate compared with a near-zero rate for beavers living in
more remote areas.
In any case, beavers can and do contract giardiasis. Being water-dwellers, they are thus able to
contaminate water more directly than an animal that defecates on the ground.
? The referenced sources use a variety of units for portraying cyst concentration: cysts per 100 liters, per
100 gallons, etc. For uniformity, all have been converted to cysts per liter since that is the size of the
typical backpacker?s canteen.
Other animals that can harbor Giardia are bighorn sheep, cats, cattle, coyotes, deer, dogs, elk,
muskrats, pet rabbits, raccoons, and squirrels. But naturally occurring infections have not been found in
most wild animals including badgers, bears, bobcats, ferrets, lynxes, marmots, moose, porcupines,
rabbits, and skunks. In the past, horses and domestic sheep were thought to be Giardia-free, but more
recent studies have shown that they can sometimes be infected.12, 29 Indeed, in some cases the issue
seems to one of degree and not a black-or-white situation.
If ?It?s everywhere!? why is it not more of a problem?
The good news: Most of the time, the concentration of Giardia cysts is very low 1, 6, 8
Outside of places where fecal accidents occur, dirty diapers congregate, and cities where water
treatment plants break down or are ineffective, there is little room to worry. A few Giardia cysts now
and then will cause no harm, and in fact may be useful in developing an immunity as will be expanded
How many cysts does it take to get the disease? Theoretically only one, but volunteer studies have
shown that 10 or so are required to have a reasonable probability of contracting giardiasis: About onethird
of persons ingesting 10 ? 25 cysts get detectable cysts in their stools. 6, 7, 8, 10, 30, 31
But be careful with statistics: Animal droppings containing 100,000 Giardia cysts deposited at the
edge of a 10 million liter lake may be an average of only 0.01 per liter for the lake as a whole, but in the
immediate vicinity of the deposit, the concentration can be much higher.
A comforting observation is that significant cyst inactivation, as high as 99.9 percent, can occur as a
result of anaerobic digestion in sewage sludge.32 Of course, using a simple cat hole is not exactly a
good approximation to the sewage plant process, but this points out the wisdom of doing something
better than just leaving it on the ground or under a couple of rocks. On the other hand, cysts perish in a
day on dry surfaces,12 so smearing it over a rock slab makes some sense when burial is not feasible.
Since cysts that ?winter over? in the Sierra Nevada are either in liquid water for considerably more
than 2 to 3 months, or exposed to freezing temperatures, it would appear that few will survive the harsh
Sierra winters. So, except for pollution by winter mountaineers and non-hibernating animals, Giardia
contamination in the high country must begin essentially anew each spring.
More good news: If you have a Giardia infestation, you will likely have no symptoms 1, 6, 7, 17, 18, 33,
The symptoms of giardiasis vary widely. Characteristic symptoms, when they occur, are mild to
moderate abdominal discomfort, abdominal distention due to increased intestinal gas, sulfurous or
?rotten egg? burps, highly offensive flatulence, and mild to moderate diarrhea. Stools are soft (but not
liquid), bulky, and foul smelling. They have been described as greasy and frothy, and they float on the
surface of water. Nausea, weakness, and loss of appetite may occur. Studies have shown that giardiasis
can be suspected when the illness lasts seven or more days with at least two of the above symptoms.7
However, most infected individuals have no symptoms at all! In one incident1 studied by the CDC,
disruption in a major city?s water disinfection system allowed the entire population to consume water
heavily contaminated with Giardia. Yet only 11 percent of the exposed population developed
symptoms even though 46 percent had organisms in their stools. These figures suggest that (a) even
when ingesting large amounts of the parasite, the chance of contracting giardiasis is less than 1 in 2, and
(b) if you are one of the unlucky ones to contract it, the chance of having symptoms is less than 1 in 4.
But perhaps the most telling statistic is that drinking heavily contaminated water resulted in symptoms
of giardiasis in only 1 case in 9.
If you have symptoms it may not be giardiasis 1, 7, 15, 18, 35
Many people claim that they ?got it? on a particular trip into the wilderness. Yet upon questioning,
they usually report that the presence of Giardia was not confirmed in the laboratory. (Only 8 percent of
persons with a diarrheal illness in this country seek medical care.24) Depending on the situation, more
likely offenders are Campylobacter, Cryptosporidium, Salmonella, Shigella, Yersinia, Aeromonas,
Clostridium, and Escherichia coli, with the last being the most common cause of traveler?s diarrhea
worldwide. Food poisoning is also a possibility.
Cryptosporidiosis, in particular, is a growing problem in this country, and currently, there is no
effective treatment for it. An outbreak in Milwaukee in 1993 caused 403,000 people to become ill and
100 to die. A year later, 43 people in Las Vegas died from the same disease.36 The severity of
cryptosporidiosis depends on the condition of the host?s immune system. In immunologically normal
people, symptoms and duration are similar to those of giardiasis. But in persons whose immune systems
have been compromised (e.g., AIDS victims), symptoms can be profound: Frequent (6 to 25),
voluminous (1 to 25 liters) daily bowel movements, serious weight loss, and cyst shedding often persist
The diarrhea being blamed on Giardia from that climbing trip a week ago may instead be due to
some spoiled food eaten last night or Campylobacter in undercooked chicken four days ago. Or,
because the incubation period is usually from one to four weeks, even if it is giardiasis the uncertainty
range indicates that the culprits could have been ingested anytime during a full three weeks worth of
meals and beverages. People in high-risk groups for Giardia, such as family members of children in day
care centers or promiscuous male homosexuals, have even more possible sources to consider. To indict
a particular stream or lake under such circumstances, without being able to at least verify that cysts are
indeed there at all, is illogical at best.
The type of diarrhea can help in the diagnosis: If it is liquid and mixes readily with water rather than
floating on top and is not particularly foul smelling, the problem is likely something other than
giardiasis. Diarrhea which lasts less than a week, untreated, is probably not from giardiasis.
Almost always, giardiasis goes away without treatment 1, 6, 7, 15, 16, 17, 33, 37
If you are unlucky enough to get giardiasis with symptoms, the symptoms will probably go away in
a week or so without treatment. You may still be harboring the cysts, however, and can unknowingly
spread the disease. Thus, practicing commonly recommended wilderness sanitary habits?defecating
100 feet from water, burying or packing out feces and toilet paper, washing before handling food, etc.?
is an excellent idea.
The US Food and Drug Administration, observing that giardiasis is more prevalent in children than
adults, suggests that many individuals seem to have a lasting immunity after infection.38 Furthermore,
citizens of cities and countries where the parasite is numerous clearly seem to have few if any problems
with their own water, which also points to an acquired immunity. So there is a possible bright side to
contracting the disease.
Looking for cysts and trophozoites in stool specimens under the microscopic has been the traditional
method for diagnosing giardiasis, but it is notoriously unreliable. Now, however, an immunologic test
(enzyme-linked immunosorbent assay, or ELISA) for the detection of Giardia antigens in stool samples
is available. The antigens are present only if there is a Giardia infection. ELISA is a big improvement
over the microscopic search, with detection sensitivities of 90 percent or more.
Rare individuals not only do not spontaneously rid themselves of the organisms but instead develop
serious symptoms of malabsorption, weight loss, ulcer-like stomach pain, and other chronic
disturbances. Fortunately, this occurs in fewer than 1 percent of those with infestations. These unlucky
people need medical treatment.
Metronidazole (Flagyl) has been the standard medication, with about a 92 percent cure rate.
Recommended by the CDC, it is not approved by the FDA for giardiasis because it can have some
serious side effects and is potentially carcinogenic. Quinacrine (Atabrine) and furazolidone (Furoxone)
are also prescribed. Tinidazole (Tinebah) is highly effective in single doses and is widely used
throughout the world, but it is not available in the US; it can be purchased over-the-counter in many
developing countries.7, 24
Giardiasis has been called a disease of ?somes.? Some people do not contract it even from heavily
contaminated sources. Some infestations vanish with no treatment at all. Some people become asymptomatic
carriers. Some evidence suggests that some people acquire a natural immunity to some strains.
And some strains seem more virulent than others.15
The problem may not be whether you are infected with the parasite but how harmoniously you both
can live together. And how to get rid of the parasite when the harmony does not exist or is lost.6
So, what about the Sierra Nevada? 1, 6, 7, 8, 16, 18
In 1984, the US Geological Survey in cooperation with the California Department of Public Health
examined water at 69 Sierra Nevada stream sites that were selected in consultation with Park Service
and National Forest managers.39, 40 Forty-two of the stream sites were considered ?high-use? (high
probability of human fecal contamination), and 27 were ?low-use.? Cysts were found at only 18 (43
percent) of the high-use sites and at 5 (19 percent) of the low-use sites. The highest concentration of
Giardia cysts was 0.108 per liter of water in Susie Lake, south of Lake Tahoe. The next highest was
0.037 per liter near Long Lake, southwest of Bishop. Samples taken in the Mt. Whitney area varied
from 0 (most sites) to 0.013 (Lone Pine Creek at Trail Camp) per liter. The concentration was 0.003 per
liter in Lone Pine Creek at Whitney Portal.
Recall that San Francisco water can contain a concentration approaching 0.12 cysts per liter,26 a
figure now seen to be higher than that measured anywhere in the Sierra. San Francisco city officials go
to great lengths to assure their citizens that the water is safe to drink, and if true?as it most assuredly
must be?this comparison alone is quite revealing.
Even Los Angeles Aqueduct water, with only 0.03 cysts per liter,27 has a higher concentration of
Giardia than all but two of the 69 Sierra sites examined.
Taking the highest concentration measured in the Sierra (0.108), we can make some calculations.
The probability? of finding 10 or more cysts in a liter of water?to have at least a one-third chance of
contracting giardiasis?is about 10-17. Ten cysts in 10 liters of water, about 10-7. In fact, one would
have to drink over 89 liters to have a 50 percent probability of ingesting 10 or more cysts.
A word of caution: The concentration is never uniform, as suggested by the ?lake incident? above.
Another reason for caution: 1984 was quite a while ago, and areas of the Sierra may be differently
contaminated now: some perhaps more, some perhaps less. Also, while so much attention is being
given to Giardia, there are worse organisms to worry about such as Campylobacter, Cryptosporidium,
E. coli, and the others mentioned earlier.
In an informative study,41 investigators contacted thousands of visitors to one of the high-use sites
during the summers of 1988 through 1990. Water samples taken on 10 different dates at each of three
locations exhibited Giardia cyst concentrations between 0 and 0.062 (average 0.009) per liter. A goal
was to enlist volunteers who were cyst-negative before their trip, verified by stool analysis, and then
determine what fraction were cyst carriers after the trip. Unfortunately, stool collection is not a
particularly enjoyable task, and only 41 people agreed to participate. Of these, two acquired Giardia
cysts during their trip, but neither came down with symptoms. Six of the others exhibited post-visit
intestinal symptoms, but none tested positive for Giardia (interestingly, all six had filtered their water).
In sum, no cases of laboratory-confirmed symptomatic giardiasis were found.
The water that wilderness travelers are apt to drink, assuming that they use a little care, seems
almost universally safe as far as Giardia is concerned. The study referred to earlier,2 in which the
researchers concluded that the risk of contracting giardiasis in the wilderness is similar to that of a shark
attack, is telling. What they did find is that Giardia and other intestinal bugs are for the most part
? These calculations involve use of a tool called the Poisson distribution.
spread by direct fecal-oral or food-borne transmission, not by contaminated drinking water. Since
personal hygiene often takes a backseat when camping, the possibility of contracting giardiasis from
someone in your own party?someone who is asymptomatic, probably?is real. Recalling that up to 7
percent of Americans, or 1 in 14, are infected, it is not surprising that wilderness visitors can indeed
come home with a case of giardiasis contracted not from the water?but from one of their friends.
This theme, that reduced attention to personal hygiene is an important factor for contracting
giardiasis in the wilderness, is becoming more frequent in the literature.2, 10, 16, 41, 42
Outside of the Sierra, Giardia cysts in concentrations ?as high as four per gallon?? have been
detected in untreated water in northeastern and western states.43 But even with this concentration, one
would have to consume over nine liters of water to have a 50 percent chance of ingesting 10 or more
Indeed, there may be as much unwarranted hysteria surrounding Giardia in wilderness water in
these other areas as there is for the Sierra. For example, an oft-cited report describing acquisition of the
disease by 65 percent of a group of students hiking in the Uinta Mountains of Utah44 is now viewed with
considerable skepticism. Specifically, the attack rate was far beyond that usually seen with watercontracted
giardiasis, no cysts were identified in the suspect water, there was no association between
water consumption rates and the likelihood of the disease, and the authors categorically discounted
food-borne or fecal-oral spread, stating that it had never been reported (correct at the time).2
I started visiting the Sierra Nevada in the early 1950s and have spent much of my free time there. I
have never treated the water, and I have never had symptoms of giardiasis as a consequence of my
visits. My many similarly active friends and acquaintances also drink the water with no ill effects. But
because of other organisms possibly present, we are always careful to ?drink smart?:
• Drink from large fast-flowing streams whenever possible, preferably those entering from the
side rather than those paralleling the trail.
• Drinking water from a lake is best advised at the inlet, with the next best place at the outlet?
especially if it is fast-flowing.
• Few Giardia cysts survive harsh Sierra winters. Contamination begins essentially anew each
year, so springtime water is safer than summer or fall.
• Water at higher elevations is safer than lower, partly because of reduced human and animal
presence up high, and partly because water flowing to lower elevations picks up more
contaminants the more distance it travels.
• The colder the water is, the more likely it is freshly melted, meaning less opportunity for
• Because filtration of water through soil removes Giardia cysts, deep well water is considered
safe.7 By implication, springs in the wilderness should be, too.
• One would think that, after a heavy snow year when streams run full and long, some kind of
?flushing out? effect of lakes and streams must be occurring. Conversely, it makes sense to
be more cautious in dry years.
• Avoid water that likely could have passed through an area subject to heavy human or animal
• If it doesn?t look good?it?s cloudy or has surface foam?treat it or don?t drink it.
? Author?s words.
If in doubt, treat it?but how? While useful in many instances, chlorine is not in general effective
for Giardia disinfection, which is why swimming pools are primary sources for the disease. The best
filters work, although they are costly, heavy, and bulky, and many are somewhat awkward to use.8, 45
Boiling is usually inconvenient, but if you are preparing hot water for meals anyway, you may as
well take advantage. Giardia cysts are highly susceptible to heat, and simply bringing water to 150? F.
for five minutes, 176? for a minute, or 190? momentarily, will kill them.8, 10 But boiling for a few
minutes at altitude is usually recommended because of the other organisms that may be present. At
10,000 feet elevation, water boils at 194?; at 14,000 feet, 187?.
Iodine is probably the best treatment choice, being inexpensive, convenient, and safe. Iodine is
effective against most bacteria and viruses, too?and over a wide range of temperatures. (But
Cryptosporidium may be resistant to iodine.) A popular system uses iodine crystals in a saturated water
solution. Methods exist to mask or remove the iodine taste.
Here are some of the Giardia cyst concentrations discussed in various places above. Units are cysts
~1000 Typical swimming pool contamination
~100 Giardiasis is plausible**
~10 Minimum needed to contract giardiasis**
~1 Some wilderness water outside California
0.12 Some San Francisco water
0.108 Worst Sierra Nevada water
0.030 Los Angeles Aqueduct water
0.013 Mt. Whitney at Trail Camp
0.003 Mt. Whitney at Whitney Portal
In a recent letter46 the Supervisor of the Inyo National Forest told me: ?As to whether or not
Giardia exists in the Sierra, we are not in a position to state a fact one way or the other.? This is a
significant admission. So why do they persist in informing everyone that giardiasis is a potential hazard
when visiting the Sierra Nevada?
First: They know that some waters might be contaminated by something, and Giardia is the
organism on people?s minds so needs no elaboration. Contaminated water is certainly possible at lower
elevations and in some locales. Noting in particular that novice hikers cannot be expected to make
correct choices of which sources may be safe to drink, they point out that a conservative approach is to
treat all water.
Second: If a person believes, albeit incorrectly, that they contracted giardiasis from Sierra Nevada
water, they cannot accuse the Forest Service of not warning them. Potential confrontations are therefore
Unfortunately, this approach results in an incorrect perception of overall water quality in the Sierra
by the general public, tainting the image of this pristine wilderness. It also means that if someone
contracts a gastrointestinal illness after a visit, they will be more apt to blame the water, having been
?forewarned? that all water is suspect. And so the myth is perpetuated.
** If one liter is consumed.
Untreated Sierra Nevada water is, almost everywhere, safe to drink?if you ?drink smart.? If you
don?t ?drink smart? you may ingest diarrhea-causing organisms. But it almost certainly won?t be
Because up to 1 in 14 of us carries the Giardia parasite, we all need to do what we can to keep the
water pure. Defecate away from water, and bury it or carry it out.
Camp cooks in particular need to pay special attention to cleanliness. Wash hands thoroughly,
especially before handling utensils and preparing meals. If you contract giardiasis in the backcountry,
blame your friends?not the water.
High Sierra water has far too few Giardia cysts for you to contract an infestation from it. Even if
you go somewhere where the concentration is high, you probably won?t get giardiasis. If you do get
giardiasis, you probably won?t have any symptoms. If you have symptoms, they will probably go away
by themselves in a week or so. If they don?t or you develop serious persistent symptoms, you should
seek medical treatment. Finally, those contracting giardiasis may develop immunity to it, thus lowering
the likelihood that they will get it again.
There is certainly no reason for anxiety about giardiasis. Fewer than 1 percent of those who have an
infestation, or about 5 percent of those with symptoms, need medical help.
Our wilderness managers are in a position to educate the outdoor public about the real culprit in the
Giardia lamblia story: inadequate human hygiene. When they realize that Sierra Nevada water has
fewer Giardia cysts than, for example, the municipal water supply of the city of San Francisco, maybe
they will turn their attention to it.
The thrust of the following observation is long overdue:
?Given the casual approach to personal hygiene that characterizes most backpacking
treks, hand washing is likely to be a much more useful preventative strategy than water
?? This simple expedient, strictly enforced in health care, child care, and
food service settings, is rarely mentioned in wilderness education materials.?2
About the author
Bob Rockwell is an active mountaineer who made his first trip into the Sierra Nevada in 1952 to
climb Mt. Whitney, and he repeats this climb several times annually. He has a bachelor?s degree in
Physics from UC-Berkeley, and a PhD in Aeronautical and Astronautical Engineering (Biomechanics)
from Stanford. In the course of making over a thousand ascents of hundreds of individual Sierra Nevada
mountains, he has never filtered or otherwise treated the water and he has never contracted symptoms of
giardiasis. Retired since 1990, he is now able to fully indulge in his favorite pastime and spends more
time there, never treating the water, than ever before.
1 Wilkerson, James A., MD: Medicine for Mountaineering and Other Wilderness Activities. The
Mountaineers, 4th edition, 1992
2 Welch, Thomas R. and Welch, Timothy P.: Giardiasis as a Threat to Backpackers in the United
States: A Survey of State Health Departments. Wilderness and Environmental Medicine, Vol. 6, 1995
3 Rockwell, Bob: Giardiasis: Let?s Be Rational About It. Summit Magazine, Nov.-Dec. 1987
4 Rockwell, Bob: Giardia Lamblia and Giardiasis, with Implications for Sierra Nevada Visitors.
California Mountaineering Club Newsletter, Vol. 7 no. 2, April 1996
?? Emphasis is in the original.
5 Rockwell, Bob: Giardia Update. California Mountaineering Club Newsletter, Vol. 8 no. 2, April 1997
6 Juranek, Dennis D.: Giardiasis. Centers for Disease Control and Prevention, 1990
7 Swartz, Morton N., MD: Intestinal Protozoan Infections. Scientific American Medicine, 1994
8 Kerasote, Ted: Great Outdoors; Drops to Drink. Audubon, July 1986
9 Microbial Risk Assessment for Reclaimed Water. EOA, Inc., Final Report. Prepared in Association
with the University of California School of Public Health, Oakland, CA. May 1995
10 Backer, Howard D.: Giardiasis: An Elusive Cause of Gastrointestinal Distress. The Physician and
Sportsmedicine, Vol. 28 no. 7, July 2000
11 DeReigner D. P., et al: Viability of Giardia Cysts Suspended in Lake, River, and Tap Water. Applied
Environmental Microbiology, Vol. 55 no. 5, 1989
12 Olson, M. E: Human and Animal Pathogens in Manure. Conference on Livestock Options for the
Future. Winnepeg, Manitoba, June 2001
13 Olsen, M. E., et al: Survival of Giardia Cysts and Cryptosporidium Oocysts in Water, Soil and Cattle
Feces. American Association of Veterinary Parasitologists, 43rd Annual Meeting, 1998
14 Fact Sheet on Enteric Protozoa and the Food Industry. National Food Processors Institute, October
15 Moser, Penny Ward: Danger in Diaperland. Health, September/October 1991
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