South-East Asia Regional Conference on Epidemiology

<strong>South</strong>-<strong>East</strong> <strong>Asia</strong> <strong>Regional</strong> <strong>Conference</strong> on Epidemiology | 249
Fig. 2: Epidemiological and environmental dynamics for (A) Kolkata and (B) Bangladesh. Cholera cases,
Cholorophyll A, SST and rainfall are shown in black, green, red and blue lines, respectively25
(reproduced with permission from the publisher of The National Academy of Sciences of the USA).
Evidence shows that there is existence of refractory periods during which climate-driven increases
in transmission do not result in large outbreaks. Once the interplay of climate forcing and disease
dynamics is taken into account, a clear evidence emerges for a role of climate variability in the
transmission of cholera. A meta-analysis of 32 years of cholera data from WHO was attempted to see
the association between cholera cases and different environmental factors like rainfall, sea-surface
temperature, humidity and altitude (21), which show a great variability in cholera incidences.
Additional variables like human, socioeconomic and demographic factors are also responsible
for cholera outbreaks (23). Environmental factors like increased temperature, salinity, pH, iron
concentration, chitin level resulting from zooplankton bloom, etc., all result in an increased growth
of V. cholerae and facilitate its survival in the environment (24). Inverse correlation between
environmental phage concentration (post-flood and post-monsoon periods) and epidemics has also
been postulated recently (25).
Effect of temperature variability
It has been predicted that over the next 100 years there will be a 1.4 °C to 5.8 °C rise in the mean
temperatures, and this rise may cause an increase in the prevalence and endemicity of cholera and may
affect a larger geographical region if public health measures are not implemented. V. cholerae O1 and
other pathogenic vibrios grow well at higher temperatures; so, any rise in temperature will favourably
affect the pathogen. As predicted with higher temperatures and melting of glaciers, the sea level may
rise and this may cause incursion of salt water, resulting in more conducive conditions and increased