Sheet n°271 - July 2007
Climatic variations influence
the emergence of cholera in
Africa
holera is an acute
intestinal infection
caused by the bacterium Vibrio cholerae.
It is rife mainly in the
tropical regions where
it affects 100 000 people per year. Previous
studies in Bangladesh
and South America have
shown the existence of
a relation between climatic variations and the
emergence of cholera.
However, no work had
ever been done in Africa
where the public health
situation is cause for the
most concern. In studies
aiming to understand
better the emergence
and persistence of cholera on this continent,
IRD and CNRS researchers showed the strong
correlation that exists
between outbreaks and
the different parameters
linked to climate changes in West Africa. This
research (1) should provide the scientists with
the bases for creating a
predictive model which
could improve early warning of risks by anticipating the emergence
of foci of infection and
therefore facilitate the
deployment of protection measures for the
populations affected.
© IRD/Cheikh Sokhna
C
Poor drainage of waste waters can be then source of cholera epidemics.
Cholera is an infectious disease caused
by a bacterium, the bacillus Vibrio cholerae. In 2004, 101 383 cases, including 95
000 solely for the African continent, and 2 345
deaths were reported to the World Health
Organization. Global climate change has for
several years been contributing greatly to the
spread of cholera through associated increase
in the frequency of torrential rain, floods and
periods of drought. It is now established that
the spread of zooplankton which harbours
the Vibrio cholerae bacterium follows that of
phytoplankton, whose growth is directly related to climate variations. However, a host of
factors act on the climatic conditions and they
are difficult to study. Certain parameters vary
depending on the regions of the world whereas others act on the global scale. The interactions between the climate and emergence
of cholera must therefore be studied region
by region. Research has been ongoing in
Bangladesh and also in South America for
many years, but up to now few studies have
been conducted in Africa. Yet it is on that
continent that the public health situation is
giving the most cause for concern.
A study published by research scientists of
the Laboratoire de Génétique et Evolution
des Maladies Infectieuses (GEMI), mixed
research unit IRD/CNRS (2), is the first to
yield evidence of the correlations between
the outbreak of cholera epidemics and climatic data in 5 West African countries (Togo,
Ivory Coast, Ghana, Benin and Nigeria). The
research team set up an epidemiological
database founded on cases recorded by
the WHO over a 20-year period, between
1975 and 1995, in each of these countries.
Comparison of these figures with parameters
of local and global climate variations showed
the factors particularly involved to be the
volume of rainfall and the Indian Oscillation
Index (IOI), an indicator of the global climate variability constructed from variations
in atmospheric pressure in the Indian Ocean.
Values of this index lower than –1 showed
an association with a warm event such as
an increase in sea surface temperature. In
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CONTACTS :
JEAN-FRANÇOIS GUÉGAN,
Laboratoire de Génétique
et Evolution des Maladies
Infectieuses (GEMI)
UMR 2724 CNRS/IRD et UR
165 IRD 33.
+33 (0)4 67 41 62 05
jf.guegan@mpl.ird.fr
GUILLAUME CONSTANTIN DE MAGNY,
Université du Maryland,
College Park, MD, USA.
magny@umiacs.umd.edu
MEDIA RELATIONS
+33 (0)1 48 03 75 19 ;
presse.ird@paris.ird.fr
INDIGO, IRD IMAGE LIBRARY
+33 (0)1 48 03 78 99 ;
indigo@paris.ird.fr
www.ird.fr/indigo
IRD AUDIOVISUAL
+33 (0)1 48 02 56 24 ;
audiovisuel@bondy.ird.fr
www.audiovisuel.ird.fr/
REFERENCES :
GUILLAUME CONSTANTIN DE MAGNY
ET AL. "Regional-scale climatevariability synchrony of cholera epidemics in West Africa”
BMC Infectious Diseases,
http://www.biomedcentral.
com/1471-2334/7/20
GUILLAUME CONSTANTIN DE MAGNY
ET AL.
"Cholera Threat to Human in
Ghana Is Influenced by Both
Global and Regional Climatic
Variability"
EcoHealth, 3, 223-231, 2007
KEY-WORDS :
CHOLERA, CLIMATE VARIATIONS,
WAVELET ANALYSES,
AFRICA
WEST
© IRD
variations or variations in rainfall volume and
emergence of foci of infection can persist for
several years. These results agree with those
obtained previously in Bangladesh and South
America.
Over the coming years, the results of this
research work should contribute to the creation of an early warning system that takes
climatic parameters into account for prediction of cholera epidemic dynamics. That
should facilitate the organization of prevention actions, such as drinking water filtration
schemes, and the planning of care provision
for people by supplying medical kits and
rehydration kits. This kind of approach could
also be applied to the understanding and
prevention of other climate-sensitive illnesses
such as malaria, dengue and other vectorborne diseases.
S. Sapolin / G. Fléchet - IRD
Translation : Nicholas Flay
(1) This work benefited from the financial
support from the “Gestion et Impacts du
Changement Climatique (GICC)” programme
of the French Ministère de l’Ecologie et du
Développement Durable and of the Centre
National d’Etudes Spatiales.
(2) The study was conducted jointly with
Bernard Cazelles of the CNRS (UMR 7625
and UR IRD GEODES) and Michel Petit, IRD
researcher at US ESPACE, Montpellier.
© NASA
Sheet n°271 - July 2007
For further information
contrast, values above +1 coincided with cold
events.
The annual rainfall regime and the IOI act
on the aquatic environment in which Vibrio
cholerae develops (estuaries, sea shores,
river beds and so on). In the wild, the cholera
bacillus lives in contact with small aquatic
crustaceans, copepods, are a component of
the zooplankton. These microscopic animals,
which constitute the principal reservoir of the
bacterium, feed on phytoplankton. They therefore have a tendency to congregate in the
zones where the density of microscopic algae
is highest. This relationship is fundamentally
important. It provides a means to monitor the
areas rich in plankton by remote sensing, and
therefore to detect from space the potential
reservoirs for Vibrio around the coasts.
Outbreaks of cholera appear irregular and, in
order better to understand the epidemic pattern dynamics, the GEMI researchers used
an adapted statistical tool that favours a
wavelet analysis method. This novel process
allows comparison of the frequencies of epidemic outbreaks with a range of climatic or
environmental parameters (climate variability
index, volume of precipitation, phytoplankton
concentration near the coasts). This approach
also takes into account the random variation
of frequencies of emergence of epidemic
foci.
The research team thus successfully linked the number of new cases of cholera
first to the global climate variability index
and then to monthly rainfall readings
between 1989 and 1994. For that period,
a frequency of epidemic occurrence of 2 to
3 years was indicated for the countries studied, except for the Ivory Coast. A significant
correlation was also observed between the
IOI and the annual rainfall regime for these
same four countries. Furthermore, analysis of
the interannual variability of rainfall between
1975 and 1996 indicated the existence of a 3
to 5–year long cycle in the appearance of the
disease for the whole of the areas covered
by the study. The IOI and volume of precipitation are therefore two climatic variables
that appear to be strongly correlated with the
appearance of epidemic foci of cholera. The
latter usually develop during seasonal periods
but their rhythm period can also exceed the
annual cycle (between 2 to 5 years). In other
words, indirect relationships between climatic
Remote-sensing visualization of the increase in density of phytoplankton off the West African coasts
Grégory Fléchet, coordinateur
Délégation à l’information et à la communication
Tél. : +33(0)1 48 03 76 07 - fax : +33(0)1 40 36 24 55 - fichesactu@ird.fr