2.1. Conditions conducive for Vibrio cholerae proliferation

The proliferation of V. cholerae, the bacterium that causes cholera, is influenced by a combination of climatic and environmental factors, including temperature, salinity, rainfall and nutrient availability [22]. The organism thrives in warm aquatic environments, with optimal growth typically observed in water temperatures between (25–30) °C, while growth declines above 30°C [23]. In most developing regions, particularly in tropical and subtropical zones, surface water temperatures commonly fall within or exceed this range during much of the year, creating favorable conditions for V. cholerae. Average river and lake temperatures in East Africa, Bay of Bengal and parts of the coastal West Africa reach 26–30 °C, supporting sustained bacterial survival [20,24]. In South Asia, elevated coastal and estuarine water temperatures during pre-monsoon months have been associated with increased cholera incidence in Bangladesh and India, while similar warming of inland water bodies has been observed in East Africa’s Great Lakes region [25,26]. Additionally, elevated temperatures above climatological averages, low river flows and heavy rainfall leading to flooding create favorable conditions for V. cholerae where there is poor WASH infrastructure [27]. In Bangladesh, SST fluctuations in the Bay of Bengal play a significant role in shaping seasonal cholera outbreaks, with warmer SSTs fostering the proliferation of V. cholerae reservoirs [19,21]. Also, the magnitude of cholera in Matlab showed that it is temperature driven, agreeing with the notion that V. cholerae thrives in warm waters [28].

Salinity is another key determinant of V. cholerae proliferation besides temperature [29]. The bacterium thrives in brackish waters with moderate salinity levels, typically between 5–25ppt [30]. Such conditions are widespread in estuarine environments across developing coastal regions including the Ganges – Brahmaputra (Bangladesh), the Niger Delta (Nigeria) and the Rufiji Delta (Tanzania) [31]. These areas experience regular mixing of fresh and saline waters, producing environments ideal for plankton blooms that serve as V. cholerae reservoirs. While moderate salinity enhances bacterial growth, extremely high or low salinity inhibits its survival [27,30,32]. Elevated SSTs increase seawater evaporation, thereby increasing salt concentration in sea water and in turn increasing salinity [18,19]; providing suitable conditions for V. cholerae as long as the salinity has not exceeded the optimum levels. Additionally, elevated SSTs promote plankton blooms, enhancing bacterial persistence and transmission [18,19]. During unfavorable climatic conditions, V. cholerae associates with planktonic hosts to enhance survival.

Coastal and estuarine regions are particularly susceptible to climate-induced changes in salinity and sea surface temperature. Rising SSTs in the Bay of Bengal have been associated with increased plankton blooms, providing ideal breeding environments for V. cholerae [33]. Likewise, elevated sea surface height and coastal flooding events contaminate fresh water supplies in low-lying settlements such as Beira (Mozambique)and Chittagong (Bangladesh), reinforcing the link between climatic stressors and cholera risk [19,31]. Contrasting this view, cholera outbreaks in coastal cities in Africa are mostly imported and they need to be better understood [34]. Conversely, drought reduces access to safe drinking water and increases salinity in shallow wells and rivers forcing populations to rely on contaminated sources, thus increasing infection risk [17]. Thus, regions with compromised WASH infrastructure and experience such climate extremes create cyclical outbreaks of cholera [27].

Overall, the environmental and climatic profiles common to many developing countries include high air and water temperatures, fluctuating salinity, nutrient rich surface waters, and variable rainfall regimes that create ideal conditions for V. cholerae proliferation, particularly where WASH infrastructure is weak. These interacting factors underpin the persistent vulnerability of sub-Saharan Africa and South Asia to recurrent cholera epidemics.

2.2. Spatial variability and trends in climatic factors related to cholera

Given the range of influences on cholera, this section focuses on temperature and rainfall as the dominant climatic factors shaping cholera dynamics across the regions under study. The relationship between climate variability and cholera outbreaks, reveals significant spatial heterogeneity. While temperature and rainfall are the most frequently identified climatic drivers of cholera in both sub-Saharan Africa and South Asia, their impact varies across regions [23,25,35,36]. Temperature primarily acts by influencing water balance and hydrological conditions through processes such as increased evapotranspiration and reduced river discharge, which in turn affects water quality and availability [37]. Rainfall on the other hand directly alters surface runoff, flooding and contamination pathways [16]. Temperature emerges as the dominant factor in some areas, while in others rainfall plays a stronger role [16,21,26,35,36,38]. In Tanzania, a one-degree Celsius rise in air temperature has been linked to a 15–29% increase in cholera risk, whereas in Zanzibar and Ethiopia, rainfall intensity more strongly predicts outbreaks [26,36]. This distinction highlights the need to consider both thermal and hydrological stressors as interacting, rather than isolated, climate influences.

Rainfall-driven cholera patterns are predominantly evident in West Africa, where most outbreaks occur during the peak rainy months of September and October [39]. Zanzibar, however, demonstrated that both rainfall and temperature significantly increase the occurrence and extent of cholera outbreaks [26]. In East Africa, bimodal rainfall regimes coincide with biannual cholera peaks aligned with rainfall maxima, especially in Ethiopia and Zanzibar [39]. Contrarywise, in parts of Southern Africa such as Zimbabwe and Mozambique, temperature has a stronger delayed influence, with high temperatures (> 24 °C) associated with outstanding cholera risk after 5–6months [16,35]. Similarly, in Dhaka (Bangladesh), elevated air temperature and SST anomalies correlates positively with increased cholera cases [25,28]. These findings suggest that rainfall tends to exert short-term, immediate effects by increasing contamination while temperature operates over longer timescales by modifying the environmental suitability for V. cholerae. Overall, the reviewed studies indicate that temperature generally exhibits the longest lag periods up to several months, while rainfall effects are more immediate. This temporal distinction has major implications for developing climate-based early-warning systems. Compound and cascading effects between these climatic drivers also emerge in several regions [34,40–43]. When heatwaves coincide with heavy rainfall, the combined stress on water systems amplifies the magnitude of the outbreak, as observed in Bangladesh and coastal East Africa [40]. Such simultaneous extremes underline the need to move beyond single-driver analyses toward integrated models of climatic influence on cholera. While sea-surface temperature is an important climatic variable, its role in promoting bacterial growth in coastal environments has been discussed earlier under section 2.1.

Seasonal variability further modulates the spatial variability and trends and climatic trend influencing cholera outbreaks. In tropical regions, cholera incidence follows annual rainfall cycles, peaking in West Africa and revealing bimodal peaks in East Africa and South Asia [39,40,44]. These repeated cycles strengthen the role of temperature and rainfall as short-term, predictable drivers of cholera; offering a fundamental basis for seasonal forecasting and preparedness planning.

In summary, spatial variability in climatic influences demonstrates that cholera is driven by different processes across regions. Temperature-driven cholera risk is prevalent in inland and semi-arid areas where warming and evaporation reduce water availability, whereas rainfall-driven risk dominates in flood - prone, high density regions. Spatial heterogeneity extends to lag times and not just drivers, the same climatic variable has different temporal signatures across space which is rarely emphasized in earlier literature. Thus, spatial variability is both geographical and temporal. The review brings out clear inland-coastal differentiation in cholera ecology, clarifying that coastal and inland cholera systems operate under different environmental controls reinforcing spatial differentiation. Additionally, most African coastal outbreaks are imported rather than locally amplified, distinguishing sub-Saharan Africa from South Asia. Findings also show non-linearity in the effects of climate shocks on cholera outcome across regions. Understanding these spatial disparities, alongside lag times, is crucial for tailoring cholera forecasting and preparedness models to regional climatic certainties.

The summary of spatial differences and lag times are shown in Table 1. High temperatures are associated with increased cholera risk with longer lag periods compared with heavy rainfall [16,26,35,40]. Additional climate related factors that increase cholera risk especially in coastal regions include relative humidity, Normalized Difference Vegetation Index (NDVI) [16], and Sea Surface Height (SSH) [19]. The summary of how each climatic factor influences cholera alongside lag times are on Table 2.

Download:

• PNG
  larger image
• TIFF
  original image

Table 1. Variability of climatic factors’ influence on cholera - variable lag times.

https://doi.org/10.1371/journal.pclm.0000840.t001

Download:

• PNG
  larger image
• TIFF
  original image

Table 2. Summary of the influence of environmental factors on cholera outbreaks in Sub Saharan Africa and South Asia.

https://doi.org/10.1371/journal.pclm.0000840.t002

2.3. Interconnectedness of cholera drivers: Understanding complexity, compounding, and cascading effects in developing regions

This subsection is being presented as the integrative component of the discussion, to synthesize how climatic and non-climatic factors interact to shape cholera dynamics in developing regions. Poverty and socioeconomic inequality are key non-climatic factors exacerbating cholera outbreaks in developing countries. While poverty and inequality increase cholera risk, climate change and variability also play significant roles with increased extreme weather (floods) and climate (droughts) events associated with it [16,17]. The increasing frequency and intensity of droughts and floods globally heighten cholera risks, both in incidence and prevalence [16,17,21]. On the other hand, drought-induced crop failure leads to food insecurity and malnutrition, weakening immune systems, and increasing susceptibility to cholera [8,17]. Cholera risk is particularly high in densely populated areas due to close human contact, limited as well as shared water sources, which facilitate the rapid spread of V. cholerae, the bacteria causing cholera [43].

Thus, cholera outbreaks are rarely a result of a single factor. Rather, they emerge from the interconnectedness of climatic and socio-environmental processes that operate across scales. Climatic factors such as temperature, rainfall and large-scale oscillations like ENSO influence water availability and bacterial proliferation while non-climatic factors such as poor WASH infrastructure, population displacement and weak governance shape exposure and vulnerability [10,27,35]. These drivers do not act independently but interact in complex ways, creating feedbacks that amplify epidemic potential. The concept of compounding effects refers to the co-occurrence of multiple stressors within a given period. For instance, floods following prolonged droughts can overwhelm already fragile WASH systems as observed in Mozambique and Malawi during El niño years [52]. Malawi’s 2022 cholera outbreak, exacerbated by Cyclone Freddy, illustrates the compounding effect of climate and health crises as the country experiences poor health care systems [41]. Such compound events magnify transmission pathways by concurrently increasing bacterial proliferation and human exposure. Similarly, cascading effects arise when one event triggers or intensifies another, creating a sequence of interlinked risks. Heavy rainfall may cause flooding that disrupts sanitation infrastructure, leading to contamination of water sources, which in turn triggers cholera outbreaks that strain public health systems [52].

The interaction of these factors underscores the systemic complexity of cholera epidemiology. Climate extremes interact with chronic social vulnerabilities, producing non-linear outbreak responses [1,41,43]. In developing regions, socio-economic barriers have cascading effects on cholera risk. These could be enhanced by weak institutional capacity, delay in medical treatment, intensifying the disease’s severity and spread [34,43]. From a systems perspective, cholera can be viewed as the outcome of intersecting hazard, exposure and vulnerability pathways rather than a linear climate disease relationship. Integrating this system thinking approach can improve understanding of why similar climatic shocks yield different outcomes across countries and communities.

Evidence from the reviewed literature indicates that multiple climatic hazards often overlap spatially and temporally, producing higher than expected outbreak intensities [53]. East African region and Bangladesh are good examples where concurrent flooding and heat stress have been linked to unusually severe cholera seasons, while post drought floods in Zimbabwe have triggered secondary outbreaks [16,26,35,40]. Thus, interventions targeting single factors such as improving water supply or vaccination are inadequate unless embedded within broader resilience frameworks that address the interconnected nature of cholera risks. Recognizing the compounding and cascading nature of cholera drivers emphasizes the need for integrated early-warning systems that combine meteorological forecasting, hydrological modelling and reduce the reactive nature of cholera response in climate sensitive regions.

In summary, this part of the review shows spatial clustering of compound and cascading risks. This moves beyond single-driver explanation by showing that compound (simultaneous) and cascading (sequential) climate risks are spatially patterned. The spatial clustering of compound risks is an advance over earlier fragmented studies. Additionally, the review paper demonstrates that spatial variability fundamentally limits the transferability of cholera models across regions, necessitating region specific early warning systems. Furthermore, the same climatic extreme produces different outcomes depending on spatial context, due to interaction with WASH, population density and governance. This supports a systems interpretation of spatial variability not simple climate forcing.