Climate and cholera: an increasingly important link
By Nancy Ross-Flanigan / News and Information Services
The link between climate and cholera, a serious health problem
in many parts of the world, has become stronger in recent decades,
say researchers from U-M, the University of Barcelona and the International
Center for Diarrhoeal Disease Research in Bangladesh.
Their research has been published in the on-line version of the
Proceedings of the National Academy of Sciences.
In a previous study published in the journal Science, the researchers
found evidence that El Nino-Southern Oscillation (ENSO), a major
source of climate variability from year to year, influences cycles
of cholera. In that work, they looked only at climate and disease
data from Bangladesh for the past two decades.
In the new research, they compared those results with data from
Bangladesh for the periods 1893-1920 and 1920-1940 to see whether
the coupling between climate variability and cholera cycles has
become stronger in recent decades. Their examination of the data,
which relied on a suite of techniques called time series analysis,
suggests that it has.
What is new in this work is not showing that ENSO plays
a role in the variability of cholera, but that the role of ENSO
has intensified, says Mercedes Pascual, an assistant professor
in the department of Ecology and Evolutionary Biology at U-M. In
addition, the link is strongest following ENSO events, with cholera
increasing after warm events and decreasing after cold events. In
the years between events, the climate-cholera connection breaks
This on-off relationship may result from a threshold above
which the influence of climate on disease dynamics is activated,
the researchers say.
Cholera, an intestinal infection with symptoms that may include
diarrhea, vomiting and leg cramps, is caused by the bacterium Vibrio
cholerae. People usually get the disease by eating or drinking contaminated
food or water.
The greater role of ENSO in cholera dynamics probably reflects
known changes in ENSO itself, the researchers believe. Since the
late 1970s, there has been a tendency toward warmer ENSO events,
in conjunction with global warming. Because the disease-causing
bacterium lives in brackish water and thrives in warm temperatures,
it may be particularly sensitive to climate patterns. People also
may be more likely to come in contact with contaminated water in
Other diseases, such as malaria and dengue, may be affected similarly
by climate variability, Pascual says. But because other factors,
such as patterns of immunity, also lead to cycles in disease dynamics,
Pascual and her colleagues are working on methods to sort out the
relative roles of climate and intrinsic factors such as temporary
Scientists who study climate change predict that ENSO will become
stronger and more variable in coming years under a global warming
scenario, so understanding how its connection to human disease changes
will be increasingly important, Pascual says.