Ancient killer. Modern weapons.
Cholera is an intestinal disease that has been killing since Old Testament times. The Vibrio cholerae bacterium that causes deadly cholera epidemics is carried by tiny water-borne animals. After a long period of relative inactivity, epidemics in South America and Bangladesh indicate that cholera has returned to the march.

Bangladesh copepod
Research on samples from sites such as this one in Bangladesh [ L E F T ] helps track the spread of cholera. A microscopic view of a female, cholera-carrying copepod [ R I G H T ]. Photos courtesy of Rita Colwell, and NSF / Frontiers.

Now cholera is meeting a modern foe -- satellites that detect sea temperatures and chlorophyll, the green molecule that converts sunlight into simple sugars.

Satellites. Chlorophyll. Cholera. We'll get to the connection in a moment. But first, recall cholera's pivotal role in the history of public health. In 1854, pioneering anesthesiologist John Snow traced a cholera outbreak to a polluted well in London. The sanitary measures that Snow advocated halted further infections. (However, cholera researcher Rita Colwell of the University of Maryland says Snow did not, contrary to myth, remove the pump handle).

Cholera disturbs the balance of sodium and potassium in the body, causing severe diarrhea and dehydration. It's fairly easy to treat with antibiotics and oral rehydration therapy (clean water with certain proportions of salt and sugar added).

If the fluids are not replaced quickly, cholera can kill in 48 hours.

Far better than treatment is prevention -- which mainly consists of purifying drinking water. But how to tell when an epidemic is likely and preventive measures are most urgent? By studying environmental conditions.

In hot water
Over the past few years, scientists have noticed a link between cholera and ocean temperatures. Epidemics in Latin America seem worst in El Niño years, when the nearby Pacific Ocean is unusually warm. The same relationship holds in Bangladesh. "There's a direct correlation between water temperatures and the number of cholera cases in Bangladesh," says Anwarul Huq, a cholera researcher at the University of Maryland Biotechnology Institute.

This Global Composite SST image from 1992 shows the Bay of Bengal in early summer when the water is the hottest. Courtesy of NASA

hot water This is where satellites enter the picture. It's easy enough to measure sea-surface temperature via satellite. But there's more. The cholera bacterium spends a good part of its life attached to a floating animal called a copepod. Copepods feed on free-floating plants, which contain chlorophyll. According to Huq, "There is a direct relationship between chlorophyll levels in the ocean and Vibrio cholerae."

Satellites, it turns out, can measure chlorophyll rather easily. Thus the goal of Huq and Colwell's research is to interpret satellite chlorophyll and temperature data to "predict the organism pressure," Huq says. That prediction would become an early warning to public health authorities that cholera was again on the march.

Promising predictions
Satellites can add another piece of data to the mix by measuring sea surface levels. When the sea rises, it pushes water higher into the flat, intensely populated delta of Bangladesh. That "increases the chance that more people will be coming into contact with water, and makes a greater chance for an epidemic," says Huq, who is a native of Bangladesh.

Thus predictions based in large part on satellite data could help local people and health officials prepare for an epidemic -- before it began.

From there, the response may shift to the low-tech direction of appropriate technology. Colwell, a microbiologist who has had a life-long fascination with cholera and recently became the first biologist to head the National Science Foundation, says filtering water through a few layers of sari, a locally available cloth, removes more than 99 percent of cholera bacteria.

The simple filtering works for the same reason that chlorophyll tracking does: Most Vibrio cholerae are attached to copepods.

You can see a red tide from the air as well. What is a red tide, and why should I care?


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