TODAY'S ARTICLE: Climate Change and Cholera

Climate Change and Cholera

Research from many fields is uncovering important connections.

Sandra J. Ackerman

When Americans worry about public health, cholera is rarely a part of the conversation. In striking contrast to this outlook, however, the recent annual meeting of the American Geophysical Union (AGU) featured much discussion of the disease. From geophysicists to hydrologists to environmental statisticians, researchers steeped in the data of climate change see the potential for cholera to increase as sea levels and water temperatures rise.

Fortunately, the technology that is documenting this gloomy picture also offers vital tools for containing the threat, by allowing scientists to predict the timing and location of cholera outbreaks with growing confidence. “This is a very exciting time in the intersection of climate, hydrology, and epidemiology,” says Rita Colwell, Distinguished University Professor at both the University of Maryland–College Park and the Johns Hopkins University. Colwell, a former director of the National Science Foundation as well as a past-president of Sigma Xi, took part in several presentations at the AGU conference on the coordination of climate data with public health studies.

After the meeting, Colwell condensed all this information into what she calls her elevator speech: “It is now possible to relate vector-borne infectious disease patterns directly to climate.” Cholera proves this point clearly, she says: “In Bangladesh, for example, we see consistent spring and fall peaks of disease over a number of years. These peaks correspond with low river discharge in the spring, which allows bacteria-laden tidal seawater to wash in, and high river discharge in the fall, leading to the cross-contamination of water supplies.”

Cholera is a scourge mainly in tropical regions. The infectious agent is the bacterium Vibrio cholerae, a commensal of copepods that thrives in water along the coast and in large bodies of water inland. According to estimates from the World Health Organization, V. cholerae infects three to five million people per year, causing diarrhea that can range from mild to very severe. In up to 80 percent of cases the illness can be treated successfully with oral rehydration solution. But for the remaining 20 percent, dehydration resulting from severe diarrhea can be fatal, killing at least 100,000 people a year globally.

The conditions that lend themselves to cholera are well known: hot weather and above-average rainfall in the context of poor infrastructure and crowding. Says Colwell: “The pattern fit Haiti in 2010. First came the earthquake, then the hottest summer in 60 years, then a hurricane that dumped an enormous amount of rain in a short time, and finally—because of the earthquake—thousands of people displaced, with major crowding in refugee camps and the breakdown of a strained, already-poor infrastructure. These conditions correlate with those historically in India and other regions suffering from cholera outbreaks.”

The main host of V. cholerae is a zooplankton species, known as the copepod.Warmer waters, particularly those rich with nutrients, support thriving populations of phytoplankton, and this abundance leads to a bloom of zooplankton. Most Vibrio species have the ability to break down chitin (the material that makes up the shell of crustaceans); thus these bacteria serve an important function in the ecosystem, recycling vast amounts of carbon. On the one hand, in scientific terms, the role of Vibrio in the carbon cycle is interesting, says Colwell, because it ties a major human disease directly to the environment. On the other hand, in terms of human suffering, it is tragic. “The risk of cholera is permanently with us,” she says. “It can be very effectively prevented, as in the United States and Europe, and treated and cured, but it can never be eradicated, because the host for the agent of cholera is an integral part of the environment.”

Nevertheless, the patterns that scientists can now discern from satellite sensors make it possible to foresee outbreaks of cholera in time to reduce their toll considerably. Says Colwell: “We have developed an algorithm to convert the time scale of the phytoplankton bloom in the water to the bloom in zooplankton, and to convert that to an increase in local populations of Vibrio.Now we’re moving to greater resolution and a finer scale: to predict not on the order just of large swaths of coastline but down to specific lakes.”

The ability to predict when cholera will strike has great practical value, because it allows people in vulnerable areas a warning to prepare for an outbreak. In Colwell’s words, “We’ve been able in recent years to use sea surface temperature, sea surface height, and, most recently, salinity, to predict likely outbreaks of cholera in a given region, because all these variables affect the timing, location, and severity of an outbreak.”

Although cholera is caused by one species of Vibrio bacteria, the disease appears in two different outbreak patterns: endemic, in coastal areas directly influenced by tidal cycles, and epidemic, occurring more in inland areas such as Delhi, which can go years without many cases and then experience a huge outbreak. “We have isolated strains of V. choleraeother than the epidemic strain, serotype 01 that accounts for most of the major epidemics,”says Colwell. “Because V. cholerae as a species is highly subject to lateral gene transfer, the genes that code for toxin and other properties related to pathogenicity can easily be transferred to a strain that is more prevalent in the environment but causes less severe symptoms, and vice versa. We can’t get at this by, say, targeting ‘the gene that causes infection’ or ‘the gene that makes the disease lethal.’ It’s a more complex problem.”

For now, in countries from Haiti to Mozambique, health officials are starting to see benefits from an early warning system where there was none before. A few weeks’ notice can allow local authorities to post “boil-water alerts” and gather life-saving supplies of rehydration solution and safe drinking water, and to protect the public in affected areas. —Sandra J. Ackerman

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