There was a time when we could drink surface water freely without fear for
our health, but the rules of the game have changed. Giardia, considered the most
common human parasite in the US, causes debilitating and irritating diarrhea.
There are reported cases of Hepatitis A, Enteric viruses, E. Coli, and
Shigella infection in the US, but these are less common and are usually
associated with accidental ingestion of "recreational waters." Boaters who take
an accidental spill and suck in a few pints of the river are among the most
susceptible. Although Giardia is responsible for only 2% of all diarrhea cases
among North American travelers to "underdeveloped" countries, it is responsible
for a much larger percentage of cases in North America. Drinking water taken
from streams on backcountry trips is the most likely source of problems. There
are some simple precautions we can take to protect ourselves and the groups with
which we work.
You are more likely to get a giardia infection than a bacterial infection for
several reasons:
- Giardia cysts can survive without a host for two to three months. Bacteria
usually perish after three to five weeks outside a host.
- It is harder to kill Giardia.
- Giardia is carried by many animals who in turn carry it to water supplies.
The cysts' tenacity is rewarded when they find their way into your
gastrointestinal tract (Gl). They are often successful. Between 1964 and 1984,
24,000 reported cases of giardiasis in the US were linked to water
contamination, and the prevalence of Giardia is growing. After an incubation
period of four to 22 days, active Giardia protozoa develop and symptoms begin.
Not everyone exposed to the cysts becomes ill: however, as few as 10-25 cysts
can cause infection. It is not clear why some people become ill and others do
not, but Gl tract chemistry and specialization of Giardia to different hosts
hold some clues.
Some people can "clear" the infection within 40 days without treatment. There
is evidence that they then develop a tolerance. Others who become ill and do
self-clear (or may not be willing to wait and see if symptoms pass) need
treatment. In the U.S., two drugs are available to treat Giardia: Quinacine and
Flagyl.
- Quinacine is good at killing Giardia, but has caused toxic psychosis in a
handful of cases.
- Fiagyl, which is not approved for this use, is a known carcinogen. It is
85-90% effective and is much better tolerated.
The best treatment is Tinidazole. It has a 90-95% cure rate. Unfortunately,
it is only available outside the U.S. Many travelers buy it for their medical
kits while traveling abroad.
Disinfection
Keeping active cysts out of you is the best strategy for avoiding infection.
Following these precautions should help.
1) Drink only uncontaminated water.
The only way to know that your water is uncontaminated is to drink tap water,
boil water directly from snow, or collect rain water. All other means make some
assumption about water quality.
2) Have a resistance to giardiasis.
Unless you know you are resistant, you are assuming. You may carry the cyst;
7.4% of the US population are carriers.
3) Treat your water.
Enteric pathogens, like giardiasis, can be disinfected in the field by three
methods: heat, filtration, and chemical treatment. Heat kills all organisms, but
the amount of time and temperature required varies. The time used to bring the
water to a boil works toward disinfection. Bringing the water to boil is enough
to kill most micro-organisms. Bacterial spores and some viruses are quite hardy.
The old recommendation of "20 minutes at a rolling boil" is necessary for
complete sterilization. Boiling points vary with altitude but do not generally
alter disinfection procedures.
Filtration removes particulates (sediment and debris) and some
micro-organisms. Particulates in the water make it taste bad and give it a
strange texture. More importantly, particulates bind active chlorine or iodine,
making them unavailable for disinfection. The chief advantage of filtration is
the removal of such debris. It can be used as a disinfection method, but the
pore size of the filter will determine its effectiveness. Before purchasing and
using a filter, know its limitations.
If you opt to treat your water with chemicals, there are many products on the
market relying on either chlorine or iodine as a disinfectant. Potable Agua, for
example, releases iodine when dissolved in water.
Many filter manufacturers recommend using filtration in conjunction with
chemical disinfection. PUR water purifiers combine filtration and iodine
treatment in one unit. An optional attachable carbon cartridge removes
unpleasant iodine tastes.
If you opt to treat your water with chemicals, there are many products on the
market relying on either chlorine or iodine as a disinfectant. Potable Agua, for
example, releases iodine when dissolved in water.
To understand how chemical disinfection works, we need to understand the
Disinfection Constant. Each microorganism has a Disinfection Constant (DC) which
is the product of two variables: the concentration of the chemical used in
disinfection and the time of contact necessary to kill it at a given temperature
and pH.
Thus: DC= Concentration x Time
8mg of iodine per liter of water is a common concentration, and is usually
provided in one fresh tablet of iodine.
Because of the relationship between time and concentration, we can cut the
amount of iodine in half if we double the contact time. The standard procedure
for Giardia control is to use two iodine tablets (or 14mg. per liter). Waiting
20 minutes before drinking the water ensures a DC of 280 (20min x 14mg.).
Alternatively, we could use one tablet (about 7mg) double the time and have the
same DC (40 min x 7mg. = 280). Two tablets are necessary only if you want to
drink the water in 20 minutes. Four are necessary if you want to drink cloudy
water in 20 minutes. This is due to something called the halogen demand.
Water and the suspended particulates in it may actually bind some of the
iodine so that it is unavailable for disinfection. This is called the halogen
demand. For clear water it is approximately 1 mg. per liter, but for cloudy
water it may be as much as 5-6 mg. per liter. While a tablet of iodine provides
8 mg. of iodine if the water is cloudy, most of this may be "used up" before any
disinfection occurs. That is why the second tablet is needed. Filtration, even
with a crude filter, can reduce the halogen demand.
Iodine tablets begin to lose their potency on contact with air, especially in
warm, humid environments. "In studies to document their stability, tablets
placed in an open dish at 1 40°F. Iost 40% of their iodine in seven days. At
room temperature and 100% humidity, the tablets lost 33% of their iodine in four
days " Medicine for Mountaineering, p. 88.
That seems extremely fast. Once opened, it is a downhill slide. Al1 opened
bottles should be discarded at the end of each season. Even sealed bottles have
a limited shelf-life. The small bottle tucked into your fanny pack will be
worthless next summer.
Toxicity of iodine is not an issue except for people with unstable thyroid
disease or known iodine allergy. Pregnant women should not use iodine for more
than a month. If these populations are on your trip, you may want to use
alternative methods of water treatment.
Take-Home Lessons On Disinfection
-Double the contact time, halve the concentration. Double the concentration,
halve the time.
-Iodine tablets quickly lose potency with exposure to air, heat, and
moisture.
-Warmer water means faster reaction time.
-If you want a margin of safety, wait longer instead of adding more iodine.
-Taste problems can be eliminated by using lower concentrations and allowing
for longer contact times. Add drink mixes to mask unwanted taste after
disinfection is complete.
-Use two water bottles, plan ahead, and apply the concentration x time
formula when you treat your water to reduce iodine consumption.
W. Nicholas Beer is a graduate student at the University of Washington. He
spends his summers as a chief instructor for Pacific Crest Outward Bound
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