You are hereGiardia Lamblia and Giardiasis

Giardia Lamblia and Giardiasis

With Particular Attention to the Sierra Nevada

By Robert L. Rockwell June 4, 2003 
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Figure 1.† Trophozoite Emerging from Cyst[1] Figure 2.† Trophozoite Undergoing Division1


Ask the average outdoors person about Giardia lamblia,
and they have certainly heard about it.† Almost always, however, they are considerably
misinformed about both the organismís significance in wilder≠ness water and
the seriousness of the disease, giardiasis, if contracted.

The amount of information easily found on the subject is
voluminous.† Unfortunately, almost all of it is flawed in important aspects,
being unsubstantiated, anecdotal or speculative.† Official informational publications
put out by the United States government are far from 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 high mountains of the Sierra Nevada, but it almost
certainly wonít be from the water.† So drink freely and confidently.† Proper
personal hygiene is far more impor≠tant in avoiding giardiasis than treating
the water.

First, an excerpt
written by a highly regarded wilderness physician:

    ìIn 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 ëhazardous.íî [2]

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 people:

    ìNeither 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 extraor≠dinarily rare event to which the
    public and press have seemingly devoted inappropriate attention.î [3]

I first explored this subject in 1987[4] and again in 1996[5],
with an update in 1997 name="_ednref6" title="">[6].† The emphasis has always been to
waters of the High SierraóìHighî meaning elevations of 8,000 or 9,000 feet and
aboveóbut much of the material applies to wilderness water at lower elevations
and beyond the Sierra.

Since 1997 a wealth of additional information resulted in
a follow-on paper[7]
that was not published in the usual sense, but was made available to several
mountaineering and hiking organizations.† As even newer data became available
I incorporated it, keeping the same title but amending the date.† These various
versions were picked up by a number of additional websites,[i]
and have migrated further.†

In 2002, having seen the paper on one of these websites,
an editor at National Geographic Adventure magazine contacted me for details.†
The staff at NGA then independently examined the information and research, and
wrote their own article.[8]
That article verified the findings of this paper.

From the beginning, the conclusions have always been that
ìthe Giardia problemî in the High Sierra and elsewhere is grossly exaggerated,
and that virtually all of the few cases of giardiasis subsequent to wilderness
visits are wrongly blamed on the water.† After incorporating the most recent
information, those prior conclusions are not only still valid but also considerably

Just who is this little guy, anyway? [ii],
[9], [10],
[11], [12],

The parasite
Giardia lamblia, now known
also as G. intestinalis or G. duodenalis, was first observed in
1681 by Anton van Leeuwenhoek, inventor of the microscope.† It was named in
1915 for two scientists who had studied it:† Prof. A. Giard in Paris and Dr.
F. Lambl in Prague.† There are species of Giardia other than G. lamblia
(e.g., G. muris) that infect small rodents, amphibians, birds and fishes,
but they arenít passed on to humans and most other mammals. href="#_edn14" name="_ednref14" title="">[14]† This paper deals exclusively with G.

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, so 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 ìhatchî into trophozoites due to stomach acid action and digestive
enzymes, and the cycle repeats.

The trophozoite
is 9 ñ 15 microns long, 5
- 15 microns wide, and 2 - 4 microns thick.† Unlike the cyst,
it cannot live for long outside a host.

Cysts are 8
- 12 microns long by 6 - 9 microns in diameter, so a million
could fit under a fingernail.† Cysts can survive for as long as 2 to 3 months
in cold water, but they cannot tolerate drying or freezing.12, 13, [15],
[16], [17],

They are also destroyed by UV radiation, heat and biocides such as bleach.15

A significant infestation can leave millions of trophozoites
stuck tight to the intestinal lining.† There, they cripple the gutís ability
to secrete enzymes and absorb food, especially fats, thereby producing the diseaseís
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.13, [19]

There are three ways that giardiasis, the disease caused
by Giardia infections, 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 pathways, but in a suspect period ranging from typically one
week earlier to four weeks earlier.

The bad news:† Giardia lamblia is everywhere 2, 10, 11, 12,
13, 14, [20], [21],
[22], [23],
[24], [25],

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 U.
S. cities.† In fact, in the 1930s and 1940s, before regulated municipal water
treatment plants, we were all ingesting Giardia all the time.[27]

Giardia lamblia is the most commonly diagnosed intestinal
parasite in North America.31† It is the
most frequently identified cause of diarrheal outbreaks associated with drinking
water in this country.† To be classified a disease outbreak, the US Centers
for Disease Control and Prevention (CDC) must have implicated the source, and
there must be multiple cases.† For 1999 ñ 2000, there were 39 drinking water
disease outbreaks (all causes) in the US, involving 2068 persons. [28]
This is an average of 53 persons per outbreak, but sometimes a single outbreak
can be devastating:† In 1993, Cryptosporidium managed to get through
Milwaukeeís water treatment system and sickened 403,000 people.
name="_Ref41626580"> name="_ednref29" title="">[29]† We take drinking water for granted
in this country, but it is far from being completely trustworthy.

Figure 3.† Number of Drinking Water Disease Outbreaks, by Year and Cause28
(AGI = Acute Gastrointestinal Illness of
unknown cause)

Estimates vary,
but fully 20 percent of the worldís population have giardiasis, and 4 to 7 percent
of Americans, most without any symptoms at all.13, 15, 16, [30]† The CDC estimates that
as many as 2,500,000 cases occur in the US, or about one for every 100 personsóevery

Infestation rates of 50 percent of the children in day care
centers across the country have been noted, with many being asymptomatic. href="#_edn32" name="_ednref32" title="">[32]† 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 ìfecal accidentî
in a community swimming pool, and nine swimmers came down with the disease.[33]
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
a day for a human.24

Municipal water utilities must use filters to remove the
organism.† San Francisco city water, coming primarily from the Hetch Hetchy
watershed in Yosemite National Park, tested positive for Giardia about
23 percent of the time in 2000, although at very low levels:† fewer than 0.12
cysts per liter[iii]. name="_Ref527265631">† 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.[34]

The city of Fairfield, 45 miles northeast of San Francisco,
stated in 2001, ìGiardia cysts were detected three times at levels of
0.19, 0.21 and 0.50 cysts per liter.† At these levels, the source water is considered
an insignificant risk for Giardia[35]

The Los Angeles Aqueduct, which transports water to that
city from the eastern slopes of the Sierra Nevadaówhere many wilderness visitors
obtain their water directlyóaverages an even lower 0.03 cysts per liter.[36]

Of course, there
are contaminants other than Giardia to worry about, and most water districts
treat their water before distribution.

Drinking 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 didnít have giardiasis, but one of
her children didówithout any symptoms.19† A similar
situation occurred in New Jersey, with the salad preparer testing positive for
Giardia along with her child and pet rabbit.[37]

There is accumulating data that Giardia is one of
the most common parasites of companion animals throughout the world.† The problem
is greater in multi-pet homes, due to the ease of infection from one pet to
anotherómore so if the pets come in contact with animals outside the home.14† And, because
of the generally close and frequent contact with their pets, it is easy for
infections to flow to household members.

Commercially prepared food can sometimes be tainted if it
is moist and not cooked.† For example, one outbreak of giardiasis was traced
to Giardia in canned salmon. href="#_edn38" name="_ednref38" title="">[38]

Contaminated food may be an unlikely source for the general
population in this country but, for wilderness visitors, it may be the most
common one.† Put another way:† If the water is cleanóa topic explored in this
paperófood-borne and direct fecal-oral routes are the only pathways.

On a climbing expedition to Tibet in 1993, 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.† Most of the food they preparedópotatoes, rice, cauliflower,
cabbage, onionsócame from Nepal.† We were continually assured that the cooks
were practic≠ing 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 ìbeaver feverî because
of a presumed link to those water-dwelling animals known to be carriers.† However,
it now appears 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 either case, beavers can and do contract giardiasis.†
Being water-dwellers, they are able to contaminate water more directly than
an animal that defecates on the ground.

Table 1.† Prevalence of Enteric Pathogens in Humans, Cattle, Pigs and Poultry16

Other animals that can harbor Giardia lamblia are
bighorn sheep, cats, cattle, coyotes, deer, dogs, elk, muskrats, pet rabbits,
pigs, raccoons and squirrels.† But naturally occurring infec≠tions have not
been found in most wild animals including badgers, bears, bobcats, ferrets,
lynxes, marmots, moose, porcupines, skunks and wild rabbits.† Horses and domestic
sheep were once thought to be Giardia-free, but more recent studies have
shown that they can be infected.15, [39],

Most strains
of Giardia lamblia in animals can cause human infections, but some (e.g.,
in pigs) are apparently unlikely to do so.16

If ìItís everywhere!î
why is it not more of a problem?

The good news:† Most of the time, the concentration of Giardia cysts is very low 2, 9, 11

Outside of places where ìfecal accidentsî 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 will cause no harm,
and in fact may be useful in acquiring immunity to the disease.

How many cysts does it take to get the disease?† Theoretically,
only one.† But there are no documented cases of giardiasis being contracted
from such low levels.8
Volunteer studies have shown that 10 or more are required to have a reasonable
probability of it, with about one-third of persons ingesting 10 ñ 25 cysts getting
detectable cysts in their stools.9,
10, 11,
13, name="_ednref41" title="">[41], [42]

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 greater.

Table 2.† Survival of Animal Fecal Pathogens in the Environment16

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.[43]† Using
a simple cat hole is not exactly a good approxima≠tion to the sewage plant process,
but this points out the wisdom of burying it.† On the other hand, cysts perish
in a day or less on dry surfaces or when frozen,15, 16
so leaving it exposed to air makes some sense when burial is not feasibleóespecially
when below-freezing temperatures are expected.

Since cysts that ìwinter overî in the Sierra Nevada are
either in liquid water for considerably more than 2 to 3 months, or exposed
to freezing temperatures, fewóif anyósurvive the harsh Sierra winters.† So,
except for pollution by winter visitors and non-hibernating animals, Giardia
contamination in the high country must begin essentially anew each spring.

The viability of Giardia cysts found in water is
commonly assumed to be high, but monitoring experiments suggest otherwise.†
Subsequent to a drinking water outbreak in Ontario, Canada, in 1994, approximately
half of the cysts found were dead. href="#_edn44" name="_ednref44" title="">[44]

More good news:† If you get a Giardia infection, you are unlikely to have symptoms
2, 9, 10, 21, 22, [45], [46]

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 ìrotten eggî burps,
horrific 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, but fever is uncommon.† Studies have shown that giardiasis can be
suspected when the illness lasts seven or more days with at least two of the
above symptoms.10

However, most infected individuals have no symptoms at all.†
In a 1977 incident carefully studied by the CDC, disruption in the Berlin, New
Hampshireís 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 useful statistic is that
drinking heavily contaminated water resulted in symptoms of giardiasis
in only 1 case in 9. 2, 8, [47]

If you have symptoms it may not be giardiasis 2, 10, 19, 22, [48]

Many people claim that they ìgot itî 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.31)† Depending
on the situation, other likely offenders are Campylobacter, Cryptosporidium,
Salmonella, Shigella, Yersinia, Aeromonas, Clostridia,
and some strains of Escherichia coli, with the last being the most common
cause of travelerís 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.29

The severity of cryptosporidio≠sis depends on the condition
of the hostís immune system.† In immu≠nologically 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
(up to 25), voluminous (up to 25 liters) daily bowel movements, serious weight
loss, and cyst shedding often persist for months.

The diarrhea being blamed on Giardia from that Sierra
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 perpetrators 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 wilderness 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 that lasts less than a week, untreated, is probably not from giardiasis.

Almost always, giardiasis goes away without treatment 2, 9, 10, 19, 20, 21, 45,
[49], [50]

If you are unlucky enough to get giardiasis with symptoms,
the symptoms will probably be gone 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 habitsódefecating
100 feet from water, burying or packing out feces and toilet paper, washing
before handling food, etc.óis an excellent idea.

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.† Fortu≠nately,
this occurs in less 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 US Food and Drug Administration 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.10, 31

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.9

More on immunity 51

The FDA, observing that giardiasis is more prevalent in children than adults, has long suggested that
many individuals seem to have a lasting immunity after infection.52† Furthermore, citizens of cities and
countries where the parasite is numerous have few if any problems with their own water, which also
points to acquired immunity.

If immunity can be acquired, might a vaccine be developed to counter Giardia?† The subject has been
explored successfully, and such a vaccine is now commercially available for dogs and cats.14,
† Effective agents for the vaccine
have been prepared from antigens on the surface of trophozoites, as well as
from whole trophozoites themselves.† Human response to these antigens has been
studied and found promising.[54]

Giardiasis has been called a disease of ìsomes.î† Some people
do not contract it even from heavily contaminated sources.† Some infestations
vanish with no treatment at all.† Some people become asymp≠tomatic carriers.†
There is some evidence that some people acquire a natural immunity to some strains.†
And some strains seem more virulent than others. 16,

SoÖwhat about the Sierra
Nevada? 2,
9, 10,
11, 20,

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.[55], [56]† Forty-two
of the stream sites were considered ìhigh-useî (high probabil≠ity of human fecal
contamination), and 27 were ìlow-use.î† 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,
south≠west 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, 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 trueóas it most
assuredly must beóthis comparison is quite revealing.

If San Francisco water is reasonably pure, Los Angeles Aqueduct
water is even more so, with only 0.03 cysts per liter.36
But 0.03 cysts per liter is a higher concentration of Giardia than all
but two of the 69 Sierra sites examined in 1984.† Los Angeles Aqueduct water
is collected from the streams of the eastern slopes of the Sierra:† the very
streams some people worry about when visiting those wilderness areas.† It is
interesting to conjecture how individual water sources in the eastern High Sierra
can be seriously contaminated, if those sources are at the same time providing
almost perfectly pure water for the aqueduct.

Taking the highest concentration measured in the Sierra
(0.108), we can make some calculations.† The probability[iv]
of finding 10 or more cysts in a liter of wateróto have at least a one-third
chance of contract≠ing giardiasisóis 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:† 1984 was quite a while ago, and areas
of the Sierra may be differ≠ently contaminated now:† some perhaps more and some
perhaps less, depending on visitor population statistics.

Another reason for caution:† A cyst does not just materialize,
here and there, now and then.† It was deposited by a mammal at a specific location
in the space of a few seconds, along with many of its companions.† They were
then dispersed by the dynamic actions of water and wind, with some being killed
by freezing, drying or aging, to eventually reach these low concentrations.†
The person, who fills his canteen too close to that location and too soon after
the deposit, is unlucky indeed.† Fortunately, the fact that people rarely contract
giardiasis after wilderness visits means that this is not at all common.† Later,
some tips on ìdrinking smartî will be given to further reduce this already-minuscule

And another:† While so much attention is being given to
Giardia, there are more serious organisms to worry about such as Campylobacter,
Cryptosporidia, E. coli and the others mentioned earlier.† But
are these likely to be found either?† While it is easy to conjecture about such
organisms in High Sierra water, and many people do, searching for evidence of
it is fruitless.† Each year the CDC documents many disease outbreaks involving
recreational water, but almost all occur from swimming pools, hot tubs and water
parks.† Some do occur from lakes, streams and rivers, but these are all at elevations
of a few thousand feet or lower.† If people are contracting these diseases from
water sources like the High Sierra, or measuring contaminants in them, they
certainly arenít reporting it (see, for example, References 28
and 29).

A valid claim that giardiasis (or any disease) was contracted
from a specific water source requires affirmative answers to each of the following
questions:† (1) Was the presence of Giardia confirmed in the laboratory?†
(2) Was the person Giardia-free prior to drinking it?† (3) Did the suspect
water source contain Giardia cysts in sufficient numbers to cause the
disease?† The answer to question (1) is easily obtained, and that to question
(2) can usually, but not always, be presumed.† In the case of wilderness water,
however, the answer to question (3) is virtually never pursued.†

In an informative study, 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 later determine what fraction
were cyst carriers after the trip.† Unfortu≠nately, 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. name="_Ref528281379"> name="_ednref57" title="">[57]

Beyond the Sierra

Outside of the Sierra, Giardia cysts in concentrations
ìas high as four per gallonî [v]
have been detected in untreated water in northeastern and western states.[58]
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 cysts.

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 Utah href="#_edn59" name="_ednref59" title="">[59] is now viewed with
considerable skepticism.† Specifically, the attack rate of 65% was far beyond
that usually seen with water-contracted 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 categori≠cally discounted
food-borne or fecal-oral spread, stating that it had never been reported (which
was correct at the time).3

Summary figures

Here are some of the figures
discussed in various places above.† Units are cysts per liter.

†††††Concentration ††††††††††††† Comment
~1000 Typical swimming pool contamination
~100 Giardiasis is plausible [vi]
~10 Minimum needed to contract giardiasisvi
~1 Some wilderness water outside California
0 .12 Some San Francisco water
0 .108 Worst Sierra Nevada water
0 .030 Some Los Angeles Aqueduct water
0 .013 Mt. Whitney at Trail Camp
0 .003 Mt. Whitney at Whitney Portal

Table 3.† Giardia Cyst Concentrations Discussed in This Report


If not from the water, from where?

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,3
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 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 partyósomeone who is asymptomatic, probablyóis
real.† Recalling that up to 7 percent of Americans, or up to 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 waterÖbut 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.3, 13, 20, 57, [60]

Personal observations

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, in
the High Sierra and elsewhere, with no ill effects.† But we are always careful
to ìdrink smartî:†

  • Drink from large streams whenever possible, preferably those entering
    from the side rather than those paralleling the trail.†

  • Water in fast-flowing streams is safer because any contaminants
    present at any location are swept downstream, being quickly displaced by presumably
    clean water from above.[vii]

  • Water at higher elevations
    is safer, partly because of reduced human and animal presence, and partly because
    water flowing to lower elevations has a chance to pick up more contaminants
    the farther it travels.

  • Taking water from a lake is best advised at the inlet, with the
    next best place at the outlet.† Inlet water has a tendency to flow somewhat
    directly to the outlet, undergoing little mixing with the lake water as a whole.

  • Few Giardia cysts survive harsh Sierra winters.† Contamina≠tion
    begins essentially anew each year, so springtime water is safer than summer
    or fall.

  • The colder the water is, the more likely it is freshly melted,
    meaning less opportunity for contamina≠tion.†

  • Because filtration of water through soil removes Giardia
    cysts, deep well water is considered safe.10
    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 ìflushing outî 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 use.†

  • If it doesnít look
    goodóitís cloudy or has surface foamótreat it or donít drink it.

If in doubt, treat itóbut how?† While useful in many instances,
chlorine is not very 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.11, [61]

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.11,

But boiling for a few minutes 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∞; so longer boiling times are recommended at altitude.

Iodine is perhaps the best treatment choice, being inexpensive,
convenient, and safe.† Iodine is effective against most bacteria and viruses,
tooóand 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.

Does it matter what the organism is, if you are going to
treat the water anyway?† Filters effective for Giardia (a protozoan)
are not always effective for Campylobacter (a bacterium).† Chlorine may
work against Campylobacter, but most of the time is ineffective against
Giardia.† You need to know your enemy.

Advice for visitors

On the subject of drinking water safety in the Sierra Nevada,
we are told,† ìAn intestinal disorder called giardiasis may be contracted from
drinking untreated ënaturalí water.  This disorder is caused by a microscopic
organism, Giardia lamblia, the cystic form of which is often found in
mountain streams and lakes.  Such waters may look, smell and taste good,
but you should be aware of possible danger.î name="_Ref41729565"> name="_ednref62" title="">[62]† We are instructed to filter or boil
all drinking water.62, [63]

Many people make the leap to a belief, approaching paranoia,
that every water source is seriously contaminated.† I have seen day hikers to
Mt. Whitney carrying 3 gallons of water from the grocery store.† I have seen
people filtering water for washing their dishes.† If their filter breaks down
on a hike, some will endure thirst in their rush to camp to boil water, passing
pristine streams along the way.† They do not trust fresh snow, and they certainly
do not trust the trickles coming from it.†

In 2001 I wrote to the Inyo National Forest office, asking
for evidence that the water quality could be as questionable as they suggest.†
The Forest Supervisor wrote back: †ìAs to whether or not Giardia exists
in the Sierra, we are not in a position to state a fact one way or the other.î[64]
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 can be contaminated by
something, and Giardia is the organism on peopleís minds so needs no
elaboration.† Contaminated water, with Giardia or otherwise, is certainly
possible at lower elevations and in some locales.† Noting that novice hikers
in particular cannot be expected to make correct choices of which sources may
be safe to drink, they suggest treating all water.†

Second:† If a person believes, albeit incorrectly, that
they contracted giardiasis from Sierra Nevada water, they cannot claim they
werenít informed.† Potential confrontations are therefore avoided.

Third:† It is the CDCís Division of Parasitic Diseases that
advises the national park and forest managers in devising and revising those
agenciesí warnings and recommendations.8† It is
not surprising that this office would take a very conservative stance.

Unfortunately, the result is 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
ìforewarnedî that all water is suspect.† And so the egregious myth is perpetuated.

But what do rangers say off the record, and what practices
do they themselves follow?† Here are three data points:

  • At a National Forest display booth recently, I approached a wilderness
    ranger I was acquainted with, one who had spent a lot of time in the Mt. Whitney
    district.† The rangerís initial greeting was ìYou are right on about the water.†
    Itís just that, as an agency, we canít say it.† I havenít filtered water up
    there for years.î [65]

  • After reading the then-current version of this paper, a National
    Park ranger wrote me ìI've been a backcountry ranger in Sequoia and Kings Parks
    for 30 years, and your article matches my experience.† My wife and I have never
    treated the water.î† He went on to say that almost none of the backcountry professionals
    heís worked with over the years do, either. href="#_edn66" name="_ednref66" title="">[66]

  • One of my long-time climbing companions recently retired from
    the Inyo National Forest after 34 years, much of it as a wilderness ranger.†
    My friend and I have always followed the same protocols regarding the water.

Untreated Sierra Nevada water is, almost everywhere, safe
to drinkóif you ìdrink smart.î† If you donít ìdrink smartî you may ingest diarrhea-causing
organisms.† But they almost certainly wonít be Giardia.†

Still, 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.†

In summary:† High Sierra water has too few Giardia
cysts to pose a genuine risk.† Even if you drink water elsewhere where the concentration
is high, you probably wonít get giardiasis.† If you do get giardiasis, you probably
wonít have any symptoms.† If you have symptoms, they will probably go away by
them≠selves in a week or so.† If they donít 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.†
Less than 1 percent of those who have an infestation, or about 5 percent of
those with symptoms, needs medical help.

Closing thoughts

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 acknowledge 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:

    the casual approach to personal hygiene that characterizes most backpacking
    treks, hand washing is likely to be a much more useful preventative strategy
    (for Giardia) than water disinfection!
    title="">[viii]† This simple expedient,
    strictly enforced in health care, child care, and food service settings, is
    rarely mentioned in wilderness education materials.î 3

We are cautioned, ìWilderness water might be contaminated,
so you should always treat it.î† Given the well-documented instances when municipal
water treatment systems have failed or been contaminated, the same warning could
apply to drinking waterówith much more validity.† But the government does not
say, ìDrinking water might be contaminated, so you should always treat it.î†
Nor should they.† We fill our glass from the tap without concern, well aware
that on rare occasions our confidence is breached.† We accept the minuscule

Yet a few Giardia cysts are found in some wilderness
waters, with no evidence that it is even remotely a problem, and an all-encompassing
warning is issued.†

How much more useful, candid and factual information would

About the author

The author 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 bachelorís degree in Physics from UC-Berkeley,
and a PhD in Aeronautical and Astronautical Engineering from Stanford University.†
In the course of making well 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 up
there, drinking freely out of the lakes and streams, than ever before.


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Unpublished note, 18 October 2001

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[18] National Food
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[35] City of Fairfield, California:
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[45] Mayo Clinic Family Health
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.† Journal
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[62] Inyo National Forest web
site: href="">†
Modified March 26, 2003

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Sequoia National Forests
.† Brochure handed out with 2003 wilderness permits.
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[i] Examples: href="">,, href="">htm and href="">

When a reference number appears after the title of a section, such as here,
that reference has been used repeatedly within the section.† When a reference
number appears embedded in a section, information from that reference has
been used for that specific statement or concept.†

[iii] 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.

[iv] These calculations involve use of a mathematical
tool called the Poisson distribution.

[v] Quoted from the original.

[vi] If one liter is consumed.

[vii] ìIn rivers, the water that you touch is
the last of what has passed and the first of that which comes; so with present
time.îóLeonardo da Vinci

[viii] Emphasis is in the original.