"A Global Thirst for Safe Water: The Case of Cholera"
Dr. Rita R. Colwell
National Science Foundation
2002 Abel Wolman Distinguished Lecture
National Academy of Sciences
January 25, 2002
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Good afternoon to everyone, and thank you, Dr. Luthy,
for a gracious introduction. I'd like to thank everyone
from the Water Science and Technology Board who helped
organize this year's Abel Wolman Distinguished Lecture.
A very special thank-you to Stephen Parker and Anita
Hall for their dedicated work in bringing us all together
for this event.
It is an honor and a pleasure to be here today at the
Academy, especially since our topic--"the global thirst
for safe water"-- presents a compelling challenge.
Today we commemorate Abel Wolman, and as part of that
I would also like to recognize the work of his son,
"Reds" Wolman, another water luminary in his own right
and a friend whom I've known for many years. Reds
is a fellow member of the National Academy of Sciences
and one of my predecessors at this podium in giving
this lecture named for his father.
I note that the first person to give this lecture
was Luna Leopold, sometimes called the godfather of
hydrology. However, I want reach back for a moment
to his father--Aldo Leopold.
Aldo Leopold, a great environmentalist and an eloquent
writer, borrowed a metaphor from one of the tall tales
about the mythical lumberjack, Paul Bunyan. This was
the "Round River" of early Wisconsin, the river that
flowed around, ceaselessly, into itself.
Aldo Leopold extended the meaning of the Round River
to the "never-ending circuit" of energy through life
and the environment.
That imaginative notion is evocative today, because
it neatly conveys the central thesis of how we need
to view water on a planetary scale.
The cycling of water and our global interconnections
mean that all of us are "living downstream"--everyone
on this planet. Thus, we need to step back and take
a clear, dispassionate look at the world's supply
of water, using all available tools, from the nanoscale
to the global scale.
Just as Abel Wolman brought new insight to developing
a dependable method for treating drinking water, so
in the 21st century we have to develop an integrated
approach across the disciplines to understand the
complexity of water issues worldwide.
I'm going to move from the broad picture to focus on
one specific aspect of that complex challenge: my
own life's work on water and infectious disease, highlighting
the case of cholera.
I'll conclude with a survey of how we can use new ways
of thinking, such as complexity, as well as new tools--from
genomics to mathematics to NSF's proposed nationwide
ecological observing network--to deal with the new
realities of our "downstream" world community.
where fresh water is]
I use this illustration--not because it is news to
most of you--but to draw the "bigger picture." Water
is indeed everywhere, but there are very few drops
left for us to drink. Earth is the blue planet, but
only 2.5% of our water is fresh. Of that mere 2.5%,
glaciers and ice caps comprise two-thirds. Of the
remaining third of this freshwater (again, a third
of that 2.5% that is fresh water), 20% lies in remote,
We are left with 80%--and of that one-third of 2.5%,
three-quarters is not usable, because it comes in
the form of monsoons and floods. To sum up, according
to the World Water Council, human beings have access
to less than eight-tenths of one percent of the total
water on this blue planet.
Where there is scarcity, there is contention. Bodies
of water can unite and divide the nations that
depend on them.
For example, approximately 260 rivers around the world
flow through two or more nations, leading to dual
national claims for the water. But some of these rivers
no longer complete their flow to the sea.
The Yellow River did not reach the sea for more than
an aggregate of seven months in 1997. Closer to home,
the "mightiest river" of the American southwest, the
Colorado, now terminates in a "bone-dry" delta in
A Mexican biologist, quoted in the Washington Post
this month, said flatly, "The Colorado ceases to exist
here"--in Mexico. Water management has become, more
than ever before, a fundamental element in national
security, for every nation.
Arguably, although only one war in history was fought
directly over water--this was 4500 years ago
on the Tigris River--the thirst for water on our planet
has never been greater, and it is growing. We can
predict wars over water in the not so distant future.
population growth rate; three scenarios]
In the words of a recent World Water Council study,
"the arithmetic of water in coming decades does not
add up," in view of the expected growth in the world's
Global population reached 6.1 billion in mid-2000.
This graph shows the estimated and projected populations
of the world from 1950 to 2050, according to three
different scenarios: low, medium, and high.
By 2050, world population should reach between 7.9
billion--the low scenario, and 10.9. billion--the
population growth rates]
Where are the people now and where will they be? Populations
of more developed regions, shown in lighter colors,
grey and white, are expected to change little over
the next half-century.
The aggregate population of the less developed regions,
however--shown in red and yellow, is expected to rise
from 4.9 billion in 2000 to 8.2 billion in 2050. Moreover,
the fastest growth will be in the least developed
where the water is]
This map looks at the world through a water lens. Blues
and greens show regions with sufficient ground and
surface water to sustain their populations, while
red and yellow denote regions without adequate water.
Most of Africa and southwest Asia have serious water
deficits. Comparing the previous map--population growth--to
water supplies, we see that the water equation is
out of balance, and getting dramatically worse.
Already in the year 2000, 1.1 billion people lacked
safe water. More than twice as many--2.4 billion--lacked
Water use is expected to increase by 40% over the next
two decades. Of the new demand, 17% will be needed
to grow food.
For example, it requires 1000 tons of water to produce
one ton of wheat, and 2000 tons of water to produce
one ton of rice. Agriculture is using more water,
and industry's use of water is also increasing--at
a much faster rate, in fact, than that of agriculture.
treated by effective treatment plants]
This graph slices the facts another way but the conclusion
is the same. We see a dramatic gap between the treatment
of wastewater in developing nations, compared to North
America and Europe.
From these data flows a stark underlying truth--this
discrepancy is related to global security. We recently
lost Sir Peter Blake, world sailor and environmentalist,
but his wisdom about water lingers.
As he put it, "There are more refugees escaping poor
water quality today than fleeing war." His observation
highlights a key issue: as serious as the gap in water
quantity is, it is only one factor in the equation.
Equally critical is water quality.
[Access to safe
drinking water VS under age 5 mortality rate: countries
We know that water quality and public health are intimately
linked. This graph shows how that differs between
developing and developed nations. On the left, in
blue, we see the nations with best access to safe
water. On the right, in yellow, we see the comparatively
high mortality rates in developing nations without
access to safe water.
Water security scholar Aaron T. Wolf writes, "While
water quantity has been the major issue of the 20th
century, water quality has been neglected to the point
of catastrophe." That is particularly true in the
case of waterborne infectious diseases.
Chicago, circa 1900]
While we face vast and complex challenges on this front
today, the history of drinking water quality in the
United States holds some valuable lessons. It was
only a century ago that many Americans first began
having access to safe drinking water. At the end of
the 19th century, one out of ten U.S. infants
died of water-borne typhoid or dysentery before they
turned one year of age.
Typhoid epidemics were common. In Chicago, for example,
citizens died by the thousands -- victims of Lake
Michigan water polluted with raw sewage.
River filtration plant, Fairmount Waterworks, Philadelphia]
After the Civil War, filtration was introduced to remove
offensive particles and to improve the aesthetic quality
of the water. Here we see Fairmount Waterworks on
the Schuylkill River in Philadelphia. At first, however,
filtration was not given credit for reducing disease.
[Graph of U.S.
typhoid cases, 1890-1940]
That changed after a typhoid epidemic hit the Hudson
River Valley in the 1890s. It soon became clear that
communities that filtered water had fewer typhoid
cases than those that did not. When filtration was
introduced, the number of cases plummeted.
In 1908, Chicago and the Jersey Water Works became
the first utilities in the country to use chlorine
The precipitous decline of typhoid cases, coupled with
the rise in water treatment plants, dramatically illustrates
the impact of Abel Wolman.
By the 1920s and 30s, filtration and chlorination had
virtually eliminated epidemics of major waterborne
diseases from the American landscape.
Part of Wolman's genius was his ability to synthesize
research from disparate fields to fashion a holistic
solution. My own journey in science has brought me
to the realization that an integrative approach is
critical to scientific progress.
My research, which has helped to link cholera outbreaks
to climate, has rested on findings from several disciplines--fields
that at first might not appear to be at all related.
My voyage of discovery, the study of how factors combine
to cause cholera, began more than 30 years ago. Our
team gradually accumulated the evidence that a disease
like cholera does not exist in a vacuum.
In endemic regions, cholera appears seasonally. Early
studies, summarized by Pollitzer in his compendium,
noted that cholera outbreaks showed several seasonal
peaks, although definitive data were not yet obtained.
As we now know, environmental, seasonal and climate
factors influence the populations of the larger host
organism for cholera, the copepod, which we saw in
the video. It peaks in abundance in spring and fall.
Add in economic and social factors of poverty, poor
sanitation, and unsafe drinking water, and we begin
to see how this microorganism sets off the vast societal
traumas of cholera pandemics.
We explore the problem on different scales. We study
the relationship among bacteria, its copepod host,
and many other ecological and social factors. On a
microscopic level, we look at molecular factors related
to the toxin genes in vibrios.
My team now comprises collaborators working in the
coastal areas of Southeast Asia; others in Central
and South America, where cholera reemerged in the
last decade; and researchers from the National Aeronautics
and Space Administration (NASA).
All are needed to elucidate ancient questions--such
as from where does cholera arise, how does it spread
in a given community, and what can be done to prevent
it--especially in developing countries without central
It is an enormous challenge. The latest numbers of
cholera cases from the World Health Organization are
for the year 2000: 137,071 cases and almost 5,000
But note that those figures don't include Bangladesh,
Pakistan, both North and South Korea, and several
other countries, whose aggregate numbers may equal
those reported for the rest of the world, shown in
To give an idea of possible worldwide totals, especially
in years of epidemics, a representative year is 1991,
when over 200,000 cases were counted in three months
in Bangladesh alone.
During the epidemic from August through October, a
thousand cholera victims entered the cholera hospital
in Dhaka, Bangladesh every day during the peak month
of that year.
spread of cholera, 1961-91]
The map summarizes the epidemic years of the seventh
pandemic, which began in the Celebes in 1961. There
is now consideration that we may be in an eighth pandemic,
since a new serotype of V. cholerae--0139--has
emerged as an epidemic form.
[Map of cholera
spread from old medical textbook]
This map, tracing the path of cholera's spread, is
taken from an 1875 medical textbook. One sees little
change today in the areas where cholera is endemic.
But cholera is much older than that. A disease similar
to cholera was first recorded in Sanskrit writings
in what is now India about 2,500 years ago. But almost
everything we know about it is very recent.
[John Snow map:
cholera cases in London, 1854]
By the mid-1800s, British physician John Snow plotted
deaths from cholera in a London neighborhood related
to water pump locations. This was the first epidemiological
study ever done.
pump and death]
According to the mythology, Snow removed the handle
of the water pump in the affected area of London,
and the epidemic abated.
Italian scientist Filippo Pacini--then a young medical
student--was the first to describe the cholera bacterium
in 1854. However, he was ignored because at that time
the germ theory of disease was not accepted.
German physician Robert Koch redefined the bacterium
as the causative agent of cholera and isolated what
he called "Vibrio comma" --because of its curved
shape--in pure culture in 1883.
But it wasn't until 1959, more than 140 years after
the beginning of the first pandemic, that an Indian
scientist, Sambhunath De, discovered the Vibrio
The production of this toxin by the bacterium changes
the permeability of the cell membrane, enabling secretion
of massive amounts of water and electrolytes into
the lumen of the intestine.
Bay cholera sampling sites]
In the 1970s, my colleagues and I realized that the
ocean itself is a reservoir for V. cholerae,
including V. cholerae 01, when we identified
the organism in water samples from the Chesapeake
Earlier detection methods for V. cholerae 01
were developed strictly for testing clinical samples.
In environmental samples, V. cholerae 01 is
more difficult to detect.
The organisms may be more dispersed in the water and
they may be dormant--not actively metabolizing.
Here we see a copepod close up--the minute relative
of shrimp which forms part of the zooplankton populations.
This microscopic animal lives in salt or brackish
waters and travels with currents and tides.
Copepods harbor both dormant, nutrient-deprived, and
culturable vibrio. The bacteria can survive as an
inactive, spore-like form in the guts and on the surfaces
of the copepods between the epidemics.
This copepod is a female whose egg case is covered
with vibrios. The vibrio can be cultured somewhat
more easily in the summer months.
[graph: V. cholerae
numbers on copepods]
In fact, a single copepod can harbor as many as 10,000
Vibrio cholerae cells. Most recently, we have
used genetic techniques--PCR and gene probes--to detect
Vibrio directly from environmental samples,
confirming earlier immunofluorescent detection results
employing monoclonal antibodies.
temporal patterns of cholera]
Moving to the larger picture, our most recent studies
have used remote sensing imagery to gather data on
sea surface temperature and sea surface height, discovering
in the process a clear correlation between sea surface
temperature and the periodicity of cholera outbreaks
in Bangladesh and in South America.
Heating of surface waters, especially off a tropical
or subtropical coast, results in an increase in phytoplankton.
Through remote sensing, we can now determine when
the phytoplankton bloom is occurring. The phytoplankton,
in turn, provide food for zooplankton, such as copepods,
which then increase.
SST and SSH]
We know that cholera epidemics are seasonal. Using
remote sensing imagery, we recently discovered that,
in areas of Bangladesh, cholera outbreaks occur shortly
after sea surface temperature and sea surface height
are at their zenith. This usually occurs twice a year,
in spring and fall.
You see here the annual ebb and flow of cholera cases
from October 1992 to November 1995.
[SST in Bay
Here, we see the waters warm in spring, are denoted
by green and yellow, even red colors--a higher temperature,
and then the monsoon cooling. The ocean warms once
again in the fall, with some red visible at the mouth
of the Ganges River. This cycle repeats year after
year in the Bay of Bengal.
Let's move to the Western Hemisphere. The sea surface
temperature there is influenced by the El Nino-Southern
Oscillation, or "ENSO" for short. El Nino is the anomalous
warming of the ocean surface that occurs periodically
off the west coast of South America.
After a century without a major outbreak of cholera,
a massive Vibrio cholerae epidemic occurred
in the Western Hemisphere in the El Nino year of 1991,
starting in Peru and spreading across South America.
We'll now see two videos side-by-side, showing sea
temperature off South America in two contrasting years:
one with an El Nino and one without. The videos, please.
[SST off South
America in contrasting years]
On the right we see sea surface temperature fluctuating
over an El Nino year, 1997-98. On the left, we see
a year without El Nino, 1995-96. The comparison clearly
illustrates the signature of El Nino.
Look especially at the coastal waters off Peru and
Mexico, near the equator, and see how that area differs
between the two years, notably in July through December.
Incidentally, there is yet another dimension to the
spread of cholera. We recently published a brief communication
in the journal Nature on how ships can transport
pathogens in their ballast tanks.
It turns out that such ballast harbors significantly
large numbers of copepods--nearly the entire plankton
population of ballast water can be copepods. They
are hardy creatures--they don't need sunlight and
they survive well in the hold of a ship during a long
Some cholera outbreaks associated with shellfish may
have originated with discharged ballast water. But
these cases are episodic and not responsible for the
endemic, annual cholera epidemics of Bangladesh or
the massive epidemic of Latin America.
Although the explanation of cholera outbreaks is global
in scale and includes many factors, the solutions
can be surprisingly simple--and these can be implemented
on a very local scale.
Cholera transmission is easily controlled by providing
people with clean, uncontaminated water for drinking
and bathing. As we saw in the film excerpt from Bangladesh,
even at the most basic level, we have found that filtering
water through several layers of sari cloth may be
enough to prevent ingestion of infectious levels of
V. cholerae by removing the particulate matter,
including the zooplankton (copepods).
[cholera cases: sari vs. non-sari]
A sari cloth, readily available even in the poorest
household, can be folded eight to ten times. This
creates a 20-micron mesh filter, as we determined
by electron microscopy. Here we see the results of
testing sari cloth as a filter to remove plankton,
which are much larger than 20 microns. Its use reduces
the number of cholera cases by half or more.
As was shown 150 years ago, filtration is effective
but now we know why, at least in the case of
cholera: It removes the bacteria associated with plankton
and it can be applied on a household-by-household
As my group's research with Vibrio cholerae
has shown, what appears to be a simple tightly circumscribed
biological problem-a bacterium that infects people--can
have ramifications and interrelationships on a global
Our work has touched upon remote sensing, biodiversity,
and even genomics, since we were partners in the sequencing
of the Vibrio cholerae genome.
Our most recent results underscore the absolute importance
of including social scientists in a project.
In our study villages, for instance, children may drink
from a tube well but bathe in a canal or pond where
clothes from cholera-infected individuals are washed
and where sewage is directly discharged. Or villagers
using sari filters nonetheless contract cholera on
visits to infected households that don't filter water.
Moving back into the laboratory, genomics has given
us a powerful new tool to learn even more about cholera,
at the molecular scale. In the year 2000, the genomes
of the two chromosomes possessed by V. cholerae
The discovery of two chromosomes was interesting,
because all bacteria were presumed to have a single
The toxin genes reside on the large chromosome. The
sequencing data indeed confirm that V. cholerae
is a versatile organism, able to live in several habitats,
as well as to infect the human gastrointestinal tract.
[list of worst
water-borne infectious diseases]
Each of the worst water-borne diseases presumably has
its own complex ecology, and each needs to be studied
in its own ecological context.
Complexity science, not available to Wolman and his
compatriots, gives us a wonderful tool to help us
trace the interconnections of epidemiology and ecology.
We need this approach at a time when strange diseases
are emerging or reemerging from unexpected quarters.
This map gives a global snapshot of infectious disease
outbreaks in 2001 alone. We are learning of new mechanisms
of survival and transmission.
Dr. Gerba at the University of Arizona, for instance,
has found that enteric, or intestinal, viruses survive
for very long periods of time in the depths of the
sea. We have demonstrated the same for enteric bacteria
in studies carried out in the Atlantic Ocean.
The outbreak of Cryptosporidium in Milwaukee,
Wisconsin in 1993 --we see the organism here--illustrated
the fact that even citizens of developed countries
are "downstream dwellers."
Cryptosporidium is all too familiar in developing
countries. In northeastern Brazil, 90% of the children
Another story--the strange case of Helicobacter
pylori: In 1984, researcher Barry Marshall suspected
a bacterial link to ulcers, and identified the culprit:
Helicobacter pylori, and showed a link to blood
In work done with my colleague, Dr. Shahamat at the
University of Maryland, we have shown that Helicobacter
pylori will persist in a dormant form in fresh
water for up to a year or more. Subsequent research
in Peru showed water transmission of H. pylori.
tube wells illustration]
Complexity science, with its ability to integrate events
taking place across the scales of space and time,
gives us hope that we can trace the many facets of
emerging and reemerging diseases.
Many of you may be aware of another story of water
and Bangladesh. This is not directly a story of my
own research, but it is one that cries out for the
perspective of complexity.
Bangladesh is currently "grappling with the largest
mass poisoning in history," according to the World
Estimates range from 20 million to 77 million people
subject to chronic poisoning by arsenic in their drinking
How could this have come about? In pursuit of safe
drinking water, more than four million tubewells,
deep wells, have been installed in Bangladesh over
the past two decades.
The object was worthy--to supply microbiologically
uncontaminated groundwater, but the result has been
unwitting poisoning. Not only that; in our own work
we have shown that in times of floods, the tubewells
can become contaminated with cholera bacteria.
Now, new sources of drinking water must be found in
Bangladesh. The population is turning back to surface
water, which makes our results even more timely. Here
is a clear case of a water problem in which it was
critical to map all the variables and their interconnections.
Our challenge today is to apply all our tools of vision,
from genomics to computers to satellites, to embrace
and understand complex systems.
I use this array of images, from the small scale to
the immense, to convey the connections within this
complexity, as a perspective on water today. I call
this approach "biocomplexity." For several years,
the U.S. National Science Foundation has supported
biocomplexity studies, in which interdisciplinary
teams examine a wide array of interactions.
Recently NSF proposed a network called "NEON," shown
here very schematically, without pinpointing specific
sites. This is the planned National Ecological Observatory
Network--an array of sites across the country furnished
with cutting-edge sensor technologies.
Here's an imaginative concept of a NEON site fully
instrumented (with apologies to the artist Rousseau).
Networks such as NEON require state-of-the-art sensors
of every stripe. Such a site will measure dozens of
variables in organisms and their physical surroundings.
I think of this as a biological "early warning system."
All the sites would be linked by high-capacity computer
lines, and the entire system would track environmental
change from the microbiological to the global scales.
We can imagine how a network such as NEON could also
serve to monitor disruptions by an attack of bioterrorism.
[women and children
at water source in Bangladesh]
Our progress in understanding environmental interactions
is closely linked to new insights in the social sciences.
The puzzle of cholera outbreaks in Bangladesh contains
many variables that only a social scientist can elucidate.
Only by incorporating the social sciences can we ground
our theories in reality, and move our solutions into
I recall our studies in Bangladesh once again--how
we were solemnly told that men would never accept
drinking water that had been filtered through the
garment of a woman. This turned out not to
be a problem--as it happened, men had already been
using sari cloth routinely to strain flies from their
U.S. water facilities: Hoover Dam under guard, a water
Back here in the United States, September 11 has transformed
our nation's complacent view of water security. Now
we are paying attention once more to our neglected
public health programs, and to the security of our
water treatment plants and dams.
Ultimately, water security is possible only if it embraces
everyone living downstream. Like the "round river"
I evoked earlier, the dimensions of water security
overflow our borders to include the entire planet.
It has been said that we all live "by the grace of
water." I grew up, in fact, by the water, just
a block from this lighthouse in Beverly Cove, Massachusetts.
The lure of the sea led me to my life's work in microbiology,
which I have discussed today. This attachment to the
sea also led to many hours serving as crew in sailing
regattas with my husband.
I recall one of those sailing yarns now. It is said
that one stormy night, the captain of a warship peered
through the gloom--and he spotted a light, barely
visible in the fog.
With a loudspeaker on the ship, the captain bellowed,
"Unknown vessel, change your course to starboard!"
A voice across the water replied, "You must
tack to starboard!"
Angrily the captain shouted back, "This is the ship's
captain you are speaking to. You are in our path.
Tack to starboard immediately!"
Again, a voice in the mist: "This is seaman first class
Jones. Change course to starboard immediately!"
The captain bellowed his ultimatum: "This is a warship
of the U.S. Navy. Tack to starboard or be destroyed!"
The voice replied, "Sir, this is a lighthouse!"
Sometimes a course change is vital. In regard to global
water, I would suggest that now is one such juncture.
A lighthouse is a symbol of vigilance on our shores,
so let's start at home. Today we can spend hundreds
of thousands of dollars testing our water for heavy
metals, but we monitor microorganisms only in the
most elementary way. We now have the molecular tools--PCR,
gene probes, microchips--to do highly sophisticated
and accurate detection of pathogens in water.
We need to move beyond the fecal coliform test to full
understanding of the microbial populations and the
microbial ecology of our drinking water.
Nanotechnology, along with other initiatives by the
National Science Foundation, offer the promise of
great progress in this area. The bioterrorism threat
lends even greater urgency to protecting our drinking
water nationwide using the best science and best technology
It's time for us to move water science into the twenty-first
I would like to thank my staff, postdoctoral fellows,
and students in the audience here today. I acknowledge
with thanks their excellent hard work. And I'd like
to recognize Lynn Simarski and Curt Suplee from the
National Science Foundation, and Kirk, Josh, Deena,
and others from NSF who worked diligently to perfect
the graphics for this talk.