Effects of climate change and variability on community vulnerability and exposure to dengue in South East Asia

Background. Climate change is a global concern that can affect health indirectly through impacts on disease vectors. Dengue and other arboviruses are climate sensitive diseases caused by viruses transmitted by Aedes mosquitoes. Assessing the impact of climate change on dengue requires knowledge on the effects on vectors, virus, and the vulnerability of human populations and their capacity to respond to new conditions. In Southeast Asia, frequent dengue outbreaks occur with varying severity. Objectives. 1) Identify and quantify populations at risk of dengue by comparative vulnerability mapping. 2) Determine socio-economic and environmental risk factors. 3) Assess strength of relationships between entomological indices and variation in dengue incidence. 4) Determine associations between climate-caused changes in seasonality and dengue incidence. 5) Estimate and predict current and future dengue risk based on common climate change emission scenarios. Methods. During 2018-2020, we collected data on fever occurrence, entomology, local weather patterns, socioeconomics and knowledge, attitudes, and practices (KAP) about climate change and dengue in 8 urban and rural study sites in Laos and Thailand. A seroconversion study was done between May and November 2019 in all sites. RNA metaviromes in wild-caught Ae. aegypti were determined by shotgun metagenomic sequencing. Official dengue statistics, climate, and land use/land cover were collected from 2002 to 2019. Predictions of dengue trends were assessed for different Representative Concentration Pathways (RCP) and Shared Socioeconomic Pathways (SSP). Results. 1) Vulnerability to dengue was higher in urban centers and in plantation and forest areas compared to other areas. 2) Householders and government officials in both countries had low knowledge and practice scores about climate change and dengue. 3) Mean DENV IgG antibody seroprevalence was 91% in May and 93% in Nov. 2019, with a total of 33 seroconversions (3%) of which 26 were in Laos and 21 in rural areas. Dengue seroconversion was significantly associated with young age (<15 years old), female gender, province, and duration of living in the current residence. The odds of seroconversion were higher at high ambient temperatures, low daily temperature ranges, and low humidity. 4) A total of 2,733 adult male and female Aedes mosquitoes were collected in 2019, consisting of Ae. aegypti (97.2%) and Ae. albopictus (2.8%). Entomological indices were higher in Thailand than in Laos but were higher in urban sites compared to rural sites. Higher abundance of adult female Ae. aegypti was significantly associated with low education levels, crowded households, indoor bathrooms, unscreened windows, poor premise conditions, surrounding house density, and high number of water containers. 5) A 1°C weekly increase of mean temperature increased dengue risk by 28-29% in Laos and by 15-22% in Thailand. A 1 mm increase in weekly rainfall up to 60 mm increased dengue risk by 1.8-3.2% across the 4 provinces. 6) Increases in rubber plantation area during the last 2 decades combined with variations in climate variables potentially extend the geographic limits of dengue by providing improved ecological and climate conditions for vectors. 7) In all future climate scenarios Savannakhet province will be most affected with spikes in dengue case numbers and possible outbreaks, especially under RCP6.0 and RCP8.5 scenarios combined with higher socioeconomic challenges (SSP3 and SSP5). Temperatures in Thailand will likely increase by at least 1°C, but dengue will remain the same as during the retrospective period probably due to low population growth. 8) The mosquito virome contains many more viruses than just pathogenic arboviruses. We found Rhabdoviridae, Geminiviridae, Baculoviridae, Ascoviridae, and others. The genome of a novel Phasi Charoen-like virus (PCLV) was highly prevalent in the majority of the mosquito samples. Discussion. KAP findings indicate that integrated awareness programs should be carried out to increase KAP levels regarding climate change adaptation, mitigation and dengue prevention to improve the health and welfare of people in these two countries. The fact that >90% of people have antibodies against dengue indicate a very high exposure and risk. Continued and strengthened dengue control should be prioritized, especially targeting vulnerable segments of society, such as youth <15 years old and females. Control measures will also help prevent transmission of other arboviruses e.g., Zika. As climate variables – especially high temperatures and low daily temperature ranges – have profound effects on dengue, disease control authorities should be aware of increased future risks and implement and strengthen risk reduction and mitigation strategies. The project expanded knowledge of the Aedes mosquito virome which contributes to the surveillance of arboviruses and of emergence of pathogens with pandemic potential.

In the grant application we stated that the expected outcomes could be used to mitigate and prevent dengue and arboviruses transmission and outbreaks. As most diseases – including arboviral diseases – are multifactorial, it is important to understand the multiple risk factors, determinants, barriers, and drivers and to develop transdisciplinary approaches to vector and disease control. The knowledge gained through this project will help to build adaptation and mitigation capacity in the study countries and elsewhere. Future predictions will help prioritize segments of a given community for action and will provide aid in designing adaptation strategies for whole communities or regions. The identified project user groups, i.e. communities, local government officials, and the research community were involved in the project in different ways. Household and community members provided information and supported disease surveillance activities during the regular weekly visits at households. Although a before-and-after study was not carried out, we believe this constant engagement with project staff and project activities improved their knowledge, action and support of vector and disease control efforts. Local governments secured continued support by anchoring the activities locally. Local health authorities and hospitals helped with data collection and compilation. Local government support improved impementation of surveillance tools, resulting in improved knowledge dissemination and capacity building. However, the Covid-19 pandemic probably influenced the uptake of results by communities and local governments as minds and actions were diverted to other more pressing activities. The use and impact of project results also improved knowledge and capacity building of students and scientists.

Climate change is currently one of the most important emerging global concerns. It affects health directly by exposure to climatic extremes and indirectly through impacts on water quality and quantity, temperature, social infrastructure or through direct effects on secondary organisms, such as disease vectors. Assessing potential health impacts of climate change and climate variability requires understanding of the vulnerability of a population and its capacity to respond to new conditions. Laos and Thailand, the two study countries, are both vulnerable, in various ways, to the direct effects of climate change, such as floods and droughts and several indirect effects that increase their vulnerability. Dengue, Zika and chikungunya are arboviruses transmitted by mosquito vectors. Higher temperatures affect mosquito and virus development, and rainfall may increase mosquito proliferation. So far, no comprehensive models measures climate-induced vulnerability, vector ecology, and socio-economic conditions, with disease dynamics and their impact on dengue incidence. Dengue is often mapped on a global scale, but its distribution is often driven by local spatiotemporal patterns influenced by fine-scale, socio-economic, environmental, virological, and demographic factors. In this project, disease surveillance, mosquito infestation, meteorology, socioeconomics, knowledge, attitudes and practices, and land cover will be combined with future climatic scenarios and population growth trends to predict potential changes in dengue risk factors and community vulnerabilities in border areas of these two countries. Assessing these factors are instrumental in developing adaptation strategies at local and regional levels. Outputs will add new knowledge on climate-induced vulnerability and its impact on dengue transmission. Results obtained for dengue, may also be applicable to Zika and chikungunya.