Introduction In Nepal, dengue fever has spread at an alarming rate, with nationwide outbreaks experienced in 2019 and 2022. Findings of a study showed that the projected increase in temperatures due to climate change could result in higher cases of vector-borne diseases in the short term (2011–2040) and medium term (2041–2070). The recent Intergovernmental Panel on Climate Change Sixth Assessment Report (IPCC-AR6) projected that vector-borne disease risks will increase under all levels of global warming in the absence of additional adaptation by countries. As climate change impacts temperatures and weather patterns across the globe, more regions are becoming suitable for vectors of disease, such as mosquitoes. Increased rainfall, for example, can create more pools of still water that are breeding grounds for vectors, while warmer temperatures can extend the transmission seasons of vector-borne diseases. Improving water supply, sanitation, hygiene, and solid waste management (WASH) services can deliver known health benefits, such as lessening diarrhoeal disease and increasing hygiene practices, as well as help build resilience to vector-borne diseases. Investments in WASH have the potential to reduce such health risks. In Nepal, dengue and malaria can be brought down by 41% and 17% in urban areas, respectively. Climate Change, Vector-Borne Diseases, and WASH A study was conducted to explore the qualitative and quantitative relationships between vector-borne diseases, climate change, and WASH in Nepal. It sought to understand whether investments in WASH could potentially build resilience to these diseases and climate change. Quantitative relationships between climate variables and diseases were derived from a review of literature. These were combined with climate projections specific for the country. Two different climate change scenarios were applied to estimate increases in four different vector-borne diseases in Nepal in the short term (2011–2040) and medium term (2041–2070). The first represented a “middle of the road” scenario (SSP2-4.5) that would lead to global temperature increases of approximately 2.1°C to 3.5°C by the end of the century. The second one (SSP5-8.5) represented a “fossil-fueled,” high-growth scenario with limited progress on mitigation that could lead to global temperature rises of around 3.3°C to 5.7°C by the end of the century. Under all climate change projections, hospitalization due to visceral leishmaniasis (also known as kala-azar) and Japanese encephalitis would slightly increase. However, the increases would be high to very high for new cases of malaria and dengue fever. Figure 1: Maximum Increases in 4 Vector-Borne Diseases in Nepal under 2 Climate Scenarios for the Short Term (2011–2040) and Medium Term (2041–2070) VBD = vector-borne disease. Source: Deltares. In the short term (2011–2040), projected consequences of climate change were similar under the two scenarios, with malaria rising by 13% to 15% and dengue by 10% to 11%, following projected 0.5°C to 0.6°C increases in mean and maximum temperatures. In the medium term (2041–2070), mean and maximum temperatures would increase by 1.4°C under the first scenario and up to 1.9°C under the second scenario. This could raise the number of malaria cases by 35% to 48% and that of dengue fever by 26% to 36%. Increases in minimum temperature were projected to have an even stronger impact on vector-borne diseases. Increases in minimum (night) temperatures can speed up larval development and extend the transmission season. For malaria, the forecast higher minimum temperature of 0.9°C to 1.0°C in the short term (2011–2040) could push the number of new cases up by 23% to 27% in the next 20–25 years, unless the dedicated elimination program in Nepal reaches its target and eradicates malaria from the country by 2025. Table 1: Projected Increases in Malaria and Dengue Fever Based on the Two Climate Scenarios Variable Short term SSP 2-4.5 Expected consequence 2011–2040 Mean temperature +0.60°C +15% new malaria cases Maximum temperature +0.55°C +10% new dengue cases Minimum temperature +0.86°C +38% new dengue cases 2 months later +23% new malaria cases Variable Short term SSP 5-8.5 Expected consequence 2011–2040 Mean temperature +0.50°C +13% new malaria cases Maximum temperature +0.60°C +11% new dengue cases Minimum temperature +0.99°C +44% new dengue cases 2 months later +27% new malaria cases Variable Short term SSP 2-4.5 Expected consequence 2041–2070 Mean temperature +1.40°C +35% new malaria cases Maximum temperature +1.37°C +26% new dengue cases Minimum temperature +1.76°C +77% new dengue cases 2 months later +48% new malaria cases Variable Short term SSP 5-8.5 Expected consequence 2041–2070 Mean temperature +1.90°C +48% new malaria cases Maximum temperature +1.92°C +36% new dengue cases Minimum temperature +2.56°C +113% new dengue cases 2 months later +69% new malaria cases Note: These are projected increases related to changes in temperature under two climate scenarios for the short term (2011–2040) and medium term (2041–2070). Source: Deltares. For dengue, the projections were downright alarming. The current infection rate in Nepal is about 10 times as high as that of malaria. The country also experienced dengue fever outbreaks in 2019 and in 2022. Between September and December 2022, more than 50,000 dengue cases and over 60 deaths were recorded. In the short term (2011–2040), increases in minimum temperature by 0.9°C to 1.0°C might create 38% to 44% more dengue cases with a time lag effect of 2 months. This means that the increase in cases would happen 2 months after the minimum temperature has increased. In the medium term (2041–2070), the minimum temperature was projected to increase by 1.8°C and 2.6°C under the “middle of the road” and “fossil-fueled” scenarios, respectively, with possibly 77% and 113% more lagged cases of dengue fever. The actual impact might be worse as these projections suggested gradual trends of average values only. The forecast excluded the devastating impact of outbreaks when thousands of people get infected and fall ill within a few months or even weeks. Potential Impact of WASH Investments Investments in WASH have the potential to reduce vector-borne diseases by improving the environment of impoverished homes and eliminating mosquito-breeding sites. Continuous water supply at the household level will eliminate the need for water storage, and effective household management of solid waste will reduce the number of containers that fill up with rainwater and become mosquito-breeding sites. Better drinking water, sanitation, and waste management at the household level have the potential to reduce disease risk. Nepal’s efforts to reach the Sustainable Development Goals (SDGs) in water and sanitation and improve waste management can reduce malaria by 17% and dengue by 41% in urban areas. This is based on the quantitative relations between changes in WASH services and diseases as derived from various studies and subsequently applied to the country setting. Table 2: Impact of Potential WASH Interventions at Household Level on Disease Risk in Urban Areas Intervention (increase in population served) Effect on VBD disease risk Drinking water supply +37.1% Tap water -7% malaria cases -29% dengue cases Sanitation +20.4% Septic tanks, sewer connection -10% malaria cases Solid waste disposal +30% Safe management -12% dengue cases VBD = vector-borne disease. Source: Deltares. Implications The Nepal National Adaptation Plan includes, among its priorities, improvements in water availability and quality, sustainability of sanitation and hygiene behaviours, and enhanced health benefits, such as reducing vector-borne diseases. Safe and reliable water supply—ideally continuous supply at the household level—drastically reduces the need for water storage, thereby removing important mosquito habitats. Figure 2: Potential Compensation of Expected Dengue Cases with Investments in Urban Water Supply and Solid Waste Management at Household Level Note: These are projected cases under two climate change scenarios (SSP2-4.5 and SSP5-8.5) for the short term (2011–2040) and the medium term (2041–2070). Source: Deltares. The combined effect of the planned increase in water connections and solid waste management in households could reduce dengue cases by more than 40%. Meanwhile, malaria cases could be reduced by a sixth through improved water supply and sanitation. These reductions could largely compensate for the adverse effect of climate change in the short term. Other vector-borne diseases, as well as diarrhoeal diseases, might also be lessened. Figure 3: Possible Actions to Respond to Climate Change and Their Effects on Vector-Borne Diseases in Nepal Source: Deltares. WASH interventions have the potential to at least partially compensate for the negative impact of climate change on vector-borne diseases. The proposed Integrated Water Supply and Sewerage Management Project of the Government of Nepal and the Asian Development Bank aims to improve water supply, sanitation, and drainage systems in secondary and small towns. It would be supported by hygiene awareness and behavioural change campaigns. The successful implementation of this project could substantially reduce dengue fever, malaria, and other vector-borne diseases. Resources B. K. Acharya, et al. 2018. Present and Future of Dengue Fever in Nepal: Mapping Climatic Suitability by Ecological Niche Model. International Journal of Environmental Research and Public Health. 15 (2). B. Majuru, M. Suhrcke, and P. R. Hunter. 2016. How Do Households Respond to Unreliable Water Supplies? A Systematic Review. International Journal of Environmental Research and Public Health. 13 (12). B. Neupane, K. R.Rijal, and M. R. Banjara. 2014. Knowledge and Prevention Measures Against Dengue in Southern Nepal. Journal of Coastal Life Medicine. July. 10–14. G.J. Bhandari, et al. 2013. Climate Change and Malaria in Jhapa District of Nepal: Emerging Evidences from Nepal. Journal of Health Management, 15 (1). M. Dhimal, B. Ahrens, and U. Kuch. 2015. Climate Change and Spatiotemporal Distributions of Vector-borne Diseases in Nepal–A Systematic Synthesis of Literature. PLoS ONE, 10 (6), 1–31. M. Dhimal, et al. 2021. Impact of Climate Change on Health and Well-being of People in Hindu Kush Himalayan Region: A Narrative Review. Frontiers in Physiology, 12 (August). M. Dhimal et al. 2014. Spatio-temporal Distribution of Malaria and Its Association with Climatic Factors and Vector-Control Interventions in Two High-Risk Districts of Nepal. Malaria Journal, 13 (1). 1–14. N. Suwannapong et al. 2014. Effect of Community Participation on Household Environment to Mitigate Dengue Transmission in Thailand. Tropical Biomedicine, 31 (1), 149–158. O. Telle et al. 2021. Social and Environmental Risk Factors for Dengue in Delhi City: A Retrospective Study. PLoS Neglected Tropical Diseases, 15 (2), 1–17. R. K. Sharma et al. 2021. Socio-economic Determinants of Malaria in Tribal Dominated Mandla District Enrolled in Malaria Elimination Demonstration Project in Madhya Pradesh. Malaria Journal, 20 (1), 1–13. R. Tuladhar et al. 2019. Climatic Factors Influencing Dengue Incidence in an Epidemic Area of Nepal. BMC Research Notes, 12 (1), 1–7. S. L. Shrestha. 2018. Negative Binomial Model in Linking Water-borne and Vector-borne Disease Hospitalizations with Climate Sensitive Variables in Nepal. Nepalese Journal of Statistics. 2. 11–26. V. Mishra, U. Bhatia, and A.D. Tiwari 2020. Bias-corrected climate projections for South Asia from Coupled Model Intercomparison Project-6. Sci Data 7, 338. Ask the Experts Eline Boelee Expert Advisor, Water and Health, Deltares Eline Boelee is an expert in water-health interlinkages at Deltares, with experience in health impacts of water resources development; agricultural and domestic water use and management; environmental control of water-related diseases; and multiple-use water services. Previously, she had her own company, Water Health, and worked at the International Water Management Institute. Alessio Giardino Senior Water Specialist (Climate Change), Climate Change, Resilience, and Environment Cluster, Climate Change and Sustainable Development Department, Asian Development Bank Alessio Giardino is a senior climate adaptation and disaster risk management specialist with over 19 years’ professional experience in the water sector. His current focus at ADB is on leading knowledge and innovation work to inform the design of resilient investments in the water sector. Prior to ADB, he worked at Deltares. He holds an MSc in Environmental Engineering (cum laude) from the University of Turin in Italy and a PhD in Coastal Engineering from KU Leuven in Belgium. Follow Alessio Giardino on Alexandra Conroy Urban Development Specialist, Water and Urban Development Sector Office, Sectors Group Alexandra Conroy is a water and wastewater engineer with over 12 years of experience in utility operations, water sanitation and hygiene, development finance and research. She joined ADB’s Pacific Department in 2015, delivering water sector projects involving climate financing in small island developing states. She currently focuses on urban operations in India and Nepal. She holds a master’s degree in Management and Engineering of Environment and Energy, a bachelor’s degree in Chemical Engineering, and a graduate certificate in Public Health. Asian Development Bank (ADB) The Asian Development Bank is committed to achieving a prosperous, inclusive, resilient, and sustainable Asia and the Pacific, while sustaining its efforts to eradicate extreme poverty. Established in 1966, it is owned by 68 members—49 from the region. Its main instruments for helping its developing member countries are policy dialogue, loans, equity investments, guarantees, grants, and technical assistance. Follow Asian Development Bank (ADB) on Leave your question or comment in the section below: View the discussion thread.