South-East Asia Regional Conference on Epidemiology

248 | <strong>South</strong>-<strong>East</strong> <strong>Asia</strong> <strong>Regional</strong> <strong>Conference</strong> on Epidemiology
Relation between climate extremes and cholera outbreak patterns
Toxigenic V. cholerae O1 or O139, the pathogens that cause cholera, are autochthonous to aquatic
environs and are intricately associated with the microflora and fauna of aquatic environs, particularly
zooplankton. V. cholerae utilizes chitin present in certain types of zooplankton as nutrients. Cholera
has the ability to spread as a worldwide pandemic and currently the seventh pandemic of cholera,
which started in 1961, is ongoing. WHO, in 2008, registered a total of 190 130 cases with 5143
deaths, with a case-fatality rate (CFR) of 2.7%. This shows an increase in the number of cases by
7.6% as well as an increase by 27% in the number of deaths compared to the previous year (16). In
some <strong>South</strong> <strong>Asia</strong>n countries it is endemic, in some parts like Africa and <strong>South</strong> America, sporadic
outbreaks occur, which is also a common feature in the endemic areas (Fig. 1). Past studies have
shown how cholera varies with seasons (17, 18). The outbreak pattern of cholera varies with season,
mainly with heavy rainfall leading to water logging and lack of potable water, and the socioeconomic,
environmental and climatic factors are all intermingled. Besides the rainfall, two remote drivers of
inter-annual climate variability, the El Niño–<strong>South</strong>ern Oscillation (ENSO), sea surface temperatures
(SSTs) and chlorophyll A in the Bay of Bengal are supposed to influence cholera in Bangladesh and
India (Fig. 2) (19). A recent review revealed that cholera in India is grossly underreported in the last
decade (1997-2006), both in terms of disease burden and case-fatality rates, which highlights the
magnitude of the problem (20).
Fig.1: Monthly outbreaks of cholera by regions (19)
There are evidences of an increased role of inter-annual climate variability on the temporal
dynamics of cholera, based on the time-series analyses of the relationship between ENSO and cholera
prevalence. A change in remote ENSO modulation alone can only partially serve to substantiate the
differences observed in cholera. For the recent cholera series, and during specific time intervals
corresponding to local maxima in ENSO, this climate phenomenon accounts for over 70% of the
disease variance (21-22).