April 2011 - Centre for Civil Society - University of KwaZulu-Natal

While cholera epidemics are caused by multiple factors, of which
environmental influences are just one, Colwell and other experts are
closely monitoring the potential impact of global warming on the diarrheal
disease. “Although there is no clear understanding of the exact nature of
the relationship between cholera and climate,” says Tufts University
cholera expert Shafiqul Islam, “if climate change leads to more extremes,
it will have an impact on cholera.” In fact, it may already have. Over the
past 30 years, El Nino events in the Bay of Bengal — characterized, in part,
by warmer sea surface temperatures — have increased, paralleling a rise in
cholera cases in Bangladesh. The World Health Organization calls it “one of
the first pieces of evidence that warming trends are affecting human
infectious diseases.”
The growing understanding of the role that climate plays in cholera
outbreaks has sparked disagreement between environmental scientists,
such as Colwell, and the medical community. While many cholera
researchers concur that these environment influences play an important
role in the incidence of cholera, especially in places such as Bangladesh
and India, most hail from medical fields, which continue to emphasize the
role of human activity in the spread of the disease. Scientists such as
Matthew Waldor, an infectious disease expert from Harvard University,
worry that characterizing cholera outbreaks as a result of environmental
influences undermines efforts to prevent the disease using vaccines and
other methods.
“There is an inevitability to the environmentalist arguments about
cholera,” he says. “If cholera travels by the environment... then it is not
preventable.” Whereas linking cholera outbreaks to human activity, he
says, reinforces the indisputable — and uncontested — truth that
preventing cholera requires changing human activity.
And yet, new research on cholera’s links to the environment may help
minimize cholera’s damage in other ways. The macro-environmental
factors that drive cholera can be tracked using remote sensing data and
other forecasting methods, opening up the possibility of an early warning
system for cholera. Islam is among several scientists developing methods
to use remote sensing data to track the coastal plankton blooms that
presage cholera outbreaks. Satellite data on chlorophyll concentrations, a
discernible proxy for zooplankton levels, can successfully predict cholera
incidence, too. NOAA recently funded the development of a system to
predict levels of Vibrio cholerae in the Chesapeake Bay using data on the
bay’s changing salinity and surface temperature.
In fact, cholera surveillance teams from the World Health Organization
(WHO) have already used El Nino forecasting to help prepare communities
for potential cholera outbreaks, including one in Mozambique in 1997,
predicted by the WHO based on forecasts of El Nino-related drought in
southeast Africa.
What is clear is that populations of Vibrio cholerae in coastal waters,
estuaries, and bays rise and fall in association with a range of
environmental factors. In Peru, levels of cholera vibrio bacteria have been
linked to the temperature of local rivers; in Italy, to the surface
temperatures of estuaries along the Adriatic coast. In Mexico, the
abundance of cholera vibrios in lagoon oysters rise as seas warm. In the
Chesapeake Bay, Vibrio cholerae levels increase during the summer, as
water temperatures spike. In Bangladesh, cholera risk increases by two to
four times in the six weeks following a 5-degree C (9-degree F) spike in the
water temperature. Likewise, in Ghana, an analysis of 20 years of data