Despite the complex interplay of multiple factors that influence the incidence of dengue, meteorological parameters play a vital role in influencing the timing and magnitude of dengue epidemics [16, 45]. With the limited success achieved during controlling dengue epidemics, recognition of vulnerable communities and evaluating the degree of vulnerability to dengue due to climate change is of paramount importance, especially in developing countries like Sri Lanka. This would also enable the implementation of proactive programmes to reduce existing vulnerabilities and to improve the resilience capacity of the vulnerable communities, guaranteeing the success of dengue epidemic management [16 – 17].
Exposure Index (EI)
The EI considers the climate related hazardous events or trends and their direct physical impacts that impose a risk on dengue [30]. Monthly cumulative rainfall, average temperature and mean relative humidity retained as the climate related parameters in the EI, along with reported dengue cases, BI and PI as the direct physical impacts of climate variables. The rainfall events indicated a positive impact on the abundance of Aedes vectors by increasing the abundance of potential vector breeding sites either by replenishing water levels or formation of new breeding sites [21, 23], and modifying the relative humidity to favourable levels for mosquito survival and longevity [26]. However, extreme rainfall events followed by flooding may flush the Aedes larvae from their breeding sites resulting in a negative impact on the vector abundance [45]. Therefore, rainfall plays a key role in governing the population dynamics of Aedes vectors mosquitoes, allowing it to be considered as risk factor for increasing the exposure of dengue.
Relative humidity is another vital factor, which directly enhance the feeding frequency, inter sexual attractions and oviposition rates of Aedes mosquitoes [25-27]. Further, the adult longevity and survival success after being infected by DENV have also been found to increase under high humid conditions [26, 46] leading to a wide geographical dispersion of dengue [19]. In addition, higher levels of humidity have shown elevations in the duplication process of dengue fever, increasing the chance of DENV transmission [26, 47-48].
On the other hand, temperature also cause favourable impacts on the incidence of dengue epidemics in several ways such as increasing the survival rate, accelerating the maturity rate and by shortening the EIP of DENV [21, 26, 48]. The average EIP of DENV was twelve days at 30 oC, which may be shortened to seven days at 32 to 35 oC, resulting in higher transmission rates [49-50]. Aedes larvae can survive at 34 oC water temperature, while the adults are capable of surviving even at 40 oC atmospheric temperature. Therefore, minimum temperature has been recognized as the limiting factor of Aedes population growth in many regions [19, 46]. Hence, global warming would favour higher levels of vector breeding and increase the abundance of Aedes mosquitoes leading to elevated risk levels of dengue. In addition, increased temperature due to global warming may increase the DENV transmission rates, which in turn increase the vulnerability of communities to dengue infection [45].
Despite the limitations and lapses in the entomological and epidemiological databases in Sri Lanka, the BI, PI and the number of reported dengue cases are the only reflective parameters of the direct impacts of climate variability on dengue [17]. Similar to many countries in the world, BI and PI are the most representative stegomyia indices that reflect the dynamics of dengue vector populations in Sri Lanka with an adequate accuracy [17, 51-53]. All vector controlling activities conducted by local VCE, are often guided by the BI, PI and the reported dengue cases, especially in timing the control efforts and in prioritizing the areas for resource allocation [17]. The current vulnerability assessment has recognized all these parameters under exposure, due to their capability of representing the direct physical impacts of climate variability on dengue within the studied MOH areas.
Sensitivity Index (SI)
The attributes that make the communities residing in Colombo and Kandy districts vulnerable to dengue under climate change, were considered under the SI [30]. Total population, percentage of males and females, percentage of population belonging to the age group of 21 to 40 years and above 60 years were selected as demographic parameters that reflect the sensitivity of local communities to dengue. As emphasized by previous studies, total population has often been recognized as a risk factor that increase the dengue risk. Among different age groups, only the proportion of population that is belonging to the age groups of 21 to 40 and > 60 years, were specifically recognized as groups with a relatively higher susceptibility to dengue by the PCA, which was further verified by the demographics of reported dengue patients [18]. Spending a relatively higher time duration at public places (such as work places, schools and public transport stations etc.) with elevated chances of being exposed to the bites of dengue vectors are potential reasons for the higher susceptibility of the people belonging to the age group of 20 to 40 years [18]. On the other hand, relatively lower immune strength to resist the DENV could be the contributing factor behind the high risk indicated by the elderly groups (> 60 years) in the considered communities [54].
Percentage of households practicing composting, disposing waste to the Municipal Council or Pradeshiya Sabha and burning waste also remained under sensitivity after the PCA. Properly planned urbanization and waste disposal services are key features that reduce the risk of dengue incidence in many countries [55-56]. Maintaining solid waste for a long time, often more than seven days, was found to enhance the breeding of Aedes mosquitoes and thereby increase the transmission of dengue [55]. Composting, collection of waste by the Municipality or Urban Council and burning of waste would essentially reduce the number of disposed containers available for oviposition of Aedes mosquitoes, which contribute to the source reduction of breeding sites [55]. Therefore, above factors heavily contribute to the reduction of the existing vulnerability of local communities to dengue in the study areas.
Meanwhile, the extent of land covered by built-up environment and the forests were also included under the SI. The high prevalence of built-up environment (urban environment) is a critical risk factor associated with the incidence of dengue outbreaks in many developing countries including Sri Lanka [18, 57-58]. On the other hand, forest areas could also provide ideal breeding (leaf axils and tree holes) and resting grounds, especially for Ae. albopictus, the secondary vector of dengue [57, 59].
Adaptive Capacity Index (AC)
The knowledge possessed by the communities of potential technologies and methods for dengue control, institutional policies and resources owned by VCE for dengue management, that could be utilized to reduce the risk imposed by dengue, are considered as the AC of the community [43]. Telecommunication facilities are widely used as an effective tool for awareness raising and community mobilization in the fight against dengue [59-61]. Often, television and radio have signified their vital importance in knowledge dissemination (regarding vector control, symptoms and patient management of dengue), while allowing the VCE to motivate the local communities to contribute toward community involved dengue control activities [61-62]. Therefore, the percentage of households with access to television and radios is a vital parameter that would enhance the adaptive capacity of local communities, reducing the vulnerability to dengue.
Population with no formal education or people with poor literacy, often act as a significant risk factor for dengue, since their awareness on the general vector management aspects and dengue control aspects remain limited, leading to high prevalence rates of dengue [61-64]. On the contrary, people with a formal education level such as GCE Ordinary Level (O/L) would be highly effective in community driven vector control activities conducted within the country, since they tend to share a higher level of knowledge on dengue along with positive attitudes toward supporting the VCE in the fight against dengue [18, 65]. The number of medical officers and PHI for 1000 residents are two of the vital indicators, which directly influence the health-related vulnerabilities of a community. In this case, both factors were retained among the indicators used for adaptive capacity, due to their paramount importance in the patient management aspects of dengue. Therefore, the availability of more medical officers and PHI would directly contribute to an elevated level of adaptive capacity, reducing the vulnerability of local communities to dengue.
Vulnerability Index (VI)
The relatively higher EI and SI values in the Colombo district (0.71-0.89 and 0.38-0.86, respectively) than in Kandy (0.19-0.79 and 0.15 to 0.77) could be the reason behind the spatial disparity of the composite VI. When the AC values are considered, the MOH areas in Kandy denoted a relatively higher AC (0.68 – 0.16) level than Colombo (0.66-0.28). This also plays a significant role in relatively higher VI levels for the district of Colombo. The MOH areas in Colombo are characterized by high rate of urbanization, poorly planned infrastructure facilities (especially waste disposal), high population densities, prevalence of notable levels of slums and shanties and high percentage of built-up environments, which could have caused elevated SI levels. Meanwhile, the relatively higher temperature, high levels of BI and PI could lead to relatively higher EI levels in Colombo [18, 55, 65]. On the other hand, the district of Kandy is characterized by relatively low level of BI and PI, degree of urbanization, population density and built environment [17, 54]. Variations in the degree of urbanization and the environmental characteristics among these two districts could be recognized as the influencing factors for spatial dissimilarities in VI [18, 54]. In both districts the urban centers, namely CMC and KMC MOH areas denoted notable levels of vulnerabilities, further supporting the above claim.
On the other hand, previous studies in these two districts suggest that the local communities in Kandy have relatively higher level of awareness on dengue, with better attitudes towards dengue control [18]. Further, the environmental management and dengue preventive practices have also been better among the local communities in Kandy, than in Colombo [18, 54]. This can also play a critical role in influencing the climate change induced vulnerability of local communities to dengue, as such parameters may cause direct and indirect impacts on EI, SI and AC. Meanwhile, a relatively higher spatial variation in VI was observed within the Kandy district, which may also be attributed to the diverse nature in the degree of urbanization (rural to urban), land use and socio-economic conditions described above under individual indices [18].
Proper identification of the risk factors that directly characterize the risk imposed by dengue outbreaks, understanding the relationship of those factors with dengue outbreak incidence and evaluation of the vulnerability of local communities to dengue, are key requirements in understanding the actual burden of dengue on the country [16]. Further, recognition of the most vulnerable localities and their potential risk factors would immensely assist the VCE in implementing proactive programs to reduce existing vulnerabilities and enhancing resilience capacity of the vulnerable communities, to ensure the success of dengue epidemic management [16-17]. In addition, the findings would enable the VCE to remain prepared for the expected variations in dengue risk caused by the changing climate.