A new University
of Michigan computer model of disease
transmission in space and time can predict cholera outbreaks in Bangladesh up
to 11 months in advance, providing an early warning system that could help
public health officials there.
The new forecast model
applies specifically to the capital city of Dhaka and incorporates data on both
year-to-year climate variability and the spatial location of cholera cases at
the district level. This allowed the researchers to study both local variation
in disease transmission and response to climate factors within the megacity of
14 million people.
U-M theoretical ecologists
Mercedes Pascual and Aaron King, along with former U-M postdoctoral researcher
Robert Reiner and other colleagues, found evidence for a climate-sensitive
urban core in Dhaka that acts to propagate
cholera risk to the rest of the city. By including those findings in their
model, the researchers were able to increase its accuracy and extend its
forecasting ability far beyond previous disease models for the city.
Earlier models had
prediction lead times of a month or less—too short to be of use in an early
warning systems. The longer lead time of the new model will help inform
decisions about treatment preparedness, vaccination, and other disease-prevention
strategies.
"What is new here is
that we have analyzed the data in space and time by considering the cholera
cases at the level of districts within the city," said Pascual, the
Rosemary Grant Collegiate Professor of Ecology and Evolutionary Biology. "Previous
analyses here and in other places have aggregated the cases at the level of the
whole city.
"This enables us to
provide early warnings that are useful because they can help hospitals prepare
for the effective treatment of large numbers of people," she said.
The research team's latest
findings are online in the Proceedings of
the National Academy of Sciences.
In research done over the
past decade, Pascual and her colleagues have found evidence that a phenomenon
known as the El Niño-Southern Oscillation, a major source of climate
variability from year to year, influences cycles of cholera in Bangladesh.
Outbreaks increase after warm ENSO events and decrease following cold ENSO
episodes.
The team also showed that
the coupling between ENSO climate variability and cholera outbreaks has become
stronger in recent decades, compared to the first part of the 20th century.
Cholera, a serious health
problem in many parts of the world, results from a bacterial infection. The
bacterium takes up residence in the intestines, causing vomiting and diarrhea,
which can lead to severe dehydration and death if patients are not promptly
treated.
Sanitary conditions and
access to clean water are the main determinants of disease risk for cholera and
other fecal-oral diseases. Those risk factors, in turn, are linked to the
intensity of annual monsoon flooding, which varies greatly from place to place
in Dhaka.
After analyzing cholera
case data collected over many years by the International Centre for Diarrheal
Disease Research in Dhaka, the U-M-led research team found evidence for two
distinct regions within Dhaka: One comprising
the older, central districts and another in the newer urban periphery.
Cholera attack rates in the
core districts are higher than in the periphery. The core districts are largely
those with the highest population density, the highest number of the poorest
quality housing, and the greatest reliance on municipal tap water, as opposed
to wells.
The core districts are also
most sensitive to ENSO-related climate variability, including flooding. The
researchers found that flooding correlates strongly with the mid-monsoon-season
cholera level in the core regions of the city, but is essentially uncorrelated
within the peripheral regions.
And the flooding-induced
breakdown of sanitary conditions is likely the principal mediator of climate's
effect on the post-monsoon peak in cholera cases in the core districts of Dhaka, the authors wrote.
"We can infer that the
spatial heterogeneity we have identified reflects differences in sanitary and
socioeconomic conditions that affect susceptibility to the disease,"
Pascual said.
This differs from the
current emphasis placed in the literature on the ecology of the pathogen, the
bacterium Vibrio cholerae, in its environmental aquatic reservoir, she said.
"Our results
underscore instead the importance of well-known risk factors for the
small-scale transmission of diarrheal diseases, including cholera, that have
been somewhat neglected in the recent ecological/climate literature on cholera,
and support a role of these same factors in the response of the disease to
climate forcing," she said.
Pascual said she is working
with Bangladesh researchers
to implement the new warning system in Dhaka.
SOURCE
