# Projecting long-term excess risks of major infectious diseases associated with future extreme weather events in Thailand

## TL;DR

The study projects how extreme weather events linked to climate change will affect the spread of infectious diseases in Thailand, showing mixed outcomes for dengue and influenza.

## Contribution

The paper introduces a novel analytical framework to model and project the impacts of extreme heat and precipitation on infectious disease incidence in Thailand under future climate scenarios.

## Key findings

- Extreme heat historically increased most infectious disease cases except malaria and leptospirosis.
- Dengue risk is projected to decline in most future scenarios but rise in Northern and Central Thailand under moderate warming.
- Influenza risk is expected to increase in dry conditions but decrease with heavy rainfall in certain regions.

## Abstract

Climate change is postulated to impact infectious disease transmission, yet few studies have characterised the excess risks of infectious diseases associated with extreme weather events. To address this, we conducted a study estimating and projecting the impacts of extreme heat and precipitation on the incidence of major infectious diseases in Thailand. We developed, fitted and validated an analytical framework to model province-level disease cases and their relationship with extreme weather indicators based on historical data. We used generalised additive models to delineate the relationship between monthly extreme heat days, standardised precipitation index and incidence rates of seven infectious diseases (dengue, malaria, Japanese encephalitis, melioidosis, leptospirosis, pneumonia, influenza) across Thailand’s provinces. Disease-specific models were fitted to historical surveillance data and used to project future disease incidence across 4 Shared Socioeconomic Pathways (SSP) based on MIROC6 climate projections. Historically, extreme heat was associated with an increase in all infectious disease incidences except malaria and leptospirosis. We projected that dengue risk declines in most future climate change scenarios, except SSP245 where extreme heat drives a significant rise in Northern and Central Thailand from 2021–2060. Nationwide dengue risk is expected to decrease by 24.9% (95%CI:9.68%,40.0%) during future periods of extreme weather from 2061–2080 compared to historical baselines. Influenced by heat and dry weather in Northeastern and Central regions, influenza risk is expected to increase under SSP245 in 2021–2060, then decrease with extreme precipitation. Influenza risk in Nakhon Ratchasima is expected to increase by 36.8% (95%CI:9.83%,63.8%) in 2021–2040 under SSP245. Localised public health interventions are necessary to address climate change impacts.

Climate change is increasingly recognised as having a significant influence on infectious disease spread, yet our understanding of how extreme weather conditions specifically affect disease risks remains limited. This study investigated how extreme heat and rainfall affect the incidence of nine major respiratory, food-borne, and vector-borne diseases across 77 provinces in Thailand. Using historical surveillance and climate data, we developed statistical models to describe how disease incidence responds to prolonged periods of high temperatures and fluctuations in rainfall.

We found that extended extreme heat typically led to inclines in most infectious diseases, including influenza, which increased in hot and dry conditions. Looking ahead, our models project that dengue incidence will generally fall under most future climate scenarios, with the exception of moderate warming, where cases are expected to notably increase in Northern and Central Thailand through to 2060. By the late 21st century, dengue risk during extreme weather events is projected to decline substantially compared to historical levels. In contrast, influenza is likely to rise initially with dry spells before decreasing with heavier rainfall. As the frequency and intensity of climate extremes escalate, our findings highlight the need to integrate climate resilience into local public health responses before risks further intensify.