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

evealed a correlation between cholera incidence and rainfall and land
surface temperatures. In Djibouti, Somalia, Kenya, Mozambique, and
Tanzania, cholera epidemics have been correlated with flooding as well as
sea surface temperatures.
Global climate drivers such as the El Nino Southern Oscillation and the
Indian Ocean Dipole appear to be similarly linked to the incidence of
cholera. The El Nino Southern Oscillation — in which warm waters build up
in the central and eastern Pacific and the Indian Ocean — creates extreme
weather conditions, including floods. El Nino cycles have occurred for
centuries, but some scientists believe they will become more frequent and
will intensify as the world warms.
Scientists from the University of Michigan and the University of Barcelona
analyzed 70 years of data on cholera prevalence in Bangladesh, finding an
association between cholera incidence and increasingly intense El Nino
events that began in 1980. Statistical modeling by scientists at the
University of Santiago de Compostela in Spain found a correlation between
the pattern of pathogenic vibrio infections in Peru with El Nino events, a
relationship experts suspect may be responsible for a massive cholera
outbreak in Peru that occurred during El Nino events in 1991.
Predicted changes to the climate thanks to global warming may intensify
these effects. A 2003 WHO study warned that predicted warming of African
lakes, such as Lake Tanganyika, may increase the risk of cholera
transmission among local people, and that countries such as Tanzania,
Kenya, Guinea-Bissau, Chad, Somalia, Peru, Nicaragua, and Honduras —
which suffered major cholera outbreaks after heavy rains in 1997 — may
face more cholera epidemics as the climate changes.
Climate change-driven storms, flooding, and heavy rainfall may lead to the
spread of cholera in other ways, too. “Extreme weather events can lead to
a breakdown in sanitation, sewage treatment plants, water treatment
systems,” says Colwell. “And indeed, because the bacteria are part of the
natural environment, we could again begin to see epidemics of cholera in
the U.S. and in Europe that we haven’t seen in almost a hundred years.”
Climatic and hydrological changes lead to cholera outbreaks by creating
the ecological conditions in which cholera vibrios thrive. While Vibrio
cholerae has been found free-living in aquatic environments, and clinging
to fish, insects, and waterfowl, they are most often found in association
with widely dispersed zooplankton called copepods. The vibrios attach to
the carapace and guts of copepods, where they replicate and ultimately
cover the surface of the female copepod’s egg sack.
Warm, nutrient-rich water helps promote cholera vibrio growth because it
leads to phytoplankton blooms, and within a week or two, corresponding
spikes in copepods and other zooplankton that feed on phytoplankton.
Droughts can promote the growth of cholera vibrios by increasing salinity
in local waters, which helps Vibrio cholerae attach to copepods, while
floods help distribute the bacteria more widely. Climatic and
environmental changes may alter the local species composition of
copepods, some of which may play a bigger role in hosting cholera vibrio
than others, says Colwell’s collaborator, the microbiologist Anwar Huq.
The scientific debate over the relative role of the environment in shaping
cholera recently flared into political controversy in the wake of the
devastating outbreak in Haiti. Enraged rioters in Haiti blamed the United
Nations’ Nepalese peacekeepers and leaky sewage pipes at their camps for
contaminating the country with cholera.