ST Explains
Wetter and warmer: How does climate change impact dengue transmission?
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The warmer and wetter climate impacts the replication of the dengue virus and the mosquitoes which carry it.
ST PHOTO: LIM YAOHUI
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SINGAPORE – 2024 was one of Singapore’s wettest and warmest years. Total annual rainfall was 8.1 per cent above the long-term average, and the year tied with 2019 and 2016 for the warmest years on record
And from Jan 10 to 13, 2025, the Republic was battered by incessant rain second monsoon surge
According to Singapore’s third national climate change study, such stormy weather is set to increase in frequency and intensity.
Unfortunately, our changing climate creates conditions ripe for the reproduction of mosquitoes and transmission of the diseases they carry. For instance, the torrential rain creates more stale water bodies which the dengue vector, the Aedes aegypti mosquito, may breed in. More than 13,600 cases of dengue
Dengue cases in Singapore have been on the rise since the 1990s, with the largest recorded outbreaks occurring in 2020 and 2022, said the National Environment Agency in response to queries from The Straits Times.
Globally, the link between climate change and dengue incidence is becoming ever more salient, with case numbers rising exponentially. In 2023, more than five million cases were reported worldwide, according to the World Health Organisation (WHO).
As the planet warms, that number has more than doubled, with more than 14 million dengue cases recorded worldwide in 2024, as reported by the European Centre for Disease Prevention and Control.
ST Explains how climate change affects dengue incidence.
1. How changing weather patterns affect dengue spread
With climate change, the world is both warmer and wetter.
Traditionally, dengue disproportionately impacts tropical and sub-tropical regions around the world, which have hot and humid climates primed for mosquitoes to thrive.
But with climate change, dengue cases are rising even in temperate regions such as North America and Europe, which are now seeing shorter and warmer winters.
Mosquitoes are increasingly inhabiting environments at higher altitudes and latitudes, expanding their geographical range in a phenomenon called climate-driven range shifts.
In 2023, 130 dengue cases were reported in the European Economic Area, as compared with 71 in 2022, reported the European health agency. This was a marked increase from 2020 and 2021, when a total of 73 cases were reported in the region
“Climate change extends the periods in which mosquitoes can thrive in temperate regions – so what used to be a three- to five-month period when there could be transmission is now longer,” said Dr Ooi Eng Eong, a professor specialising in emerging infectious diseases at Duke-NUS Medical School.
With every 1 deg C of warming, the earth’s atmosphere can store and release 7 per cent more water, according to the US Department of Agriculture. This means that we are likely to see more frequent and heavy rainfall in the future.
More rain also creates more stale water bodies and receptacles that mosquitoes may breed in. Three of the four stages of the mosquito life cycle take place in water – eggs are laid in pools of stagnant water, which hatch into larvae and pupae which develop within them.
When asked how climate change may impact dengue trends in Singapore, Dr Ooi said: “With warmer temperatures and more unseasonal rains, peak dengue season may stretch for longer during the year, rather than just between May and September, when it usually peaks in Singapore.
“Perhaps it becomes spread out over a longer period – that’s one possibility.”
Another driver of dengue is how humans move as a result of our changing climate.
With climate change, there is an increased risk of extreme weather events, which can displace communities, said Dr Kimberly Fornace, an associate professor from the Saw Swee Hock School of Public Health at NUS.
After a natural disaster, people may move into makeshift homes, which lack proper water or sanitation facilities. Improper water storage practices and increased population density are both risk factors for dengue, said the WHO.
2. How climate change affects virus replication
Aside from creating more viable habitats for mosquitoes to breed, climate change also speeds up the physical maturation of the mosquito, while also enabling the virus to develop faster within it, said Dr Fornace.
The dengue virus is a very simple organism – it has only 10 genes. It thus infects both mosquitoes and humans, which have tens of thousands of genes, and utilises their replication machinery to make new copies of itself.
For the virus to grow, it needs to interact with other proteins and factors in the host cell – a process which requires energy.
When the temperature is warmer, molecules move more, and are more likely to meet and react with each other. With global warming, bodily chemical processes thus speed up, resulting in more efficient replication of the dengue virus.
After the mosquito takes its blood meal, the virus infects its midgut and travels to its salivary glands over about eight to 12 days. The time the virus takes to become infectious within its mosquito host is also temperature-dependent, said Dr Fornace.
“This means that it takes a shorter time for mosquitoes to become infectious after getting the virus,” she added.
3. Challenges to suppressing dengue in Singapore
Singapore’s vector control programme, which seeks to control population numbers of the dengue vector, was officially introduced in 1970.
According to Dr Ooi, the Ministry of Health (MOH) focused on trying to control Aedes aegypti because among mosquito-borne infections, dengue was the most common cause of death in children, overtaking even malaria.
In its early years, the programme was very effective.
Dr Ooi said: “Between 1975 and 1990, Singapore had no outbreaks, even when the rest of South-east Asia had outbreaks once every three or four years.
“But since the 1990s we’ve had repeated outbreaks once every five to eight years.”
This is because with successful vector control, overall dengue transmission is very low. Singapore’s herd immunity levels thus kept dropping, resulting in a low population immunity to the disease.
“Over time, paradoxically, your population becomes more vulnerable to dengue,” said Dr Ooi.
However, dengue vaccines are notoriously difficult to perfect
Two vaccines – Dengvaxia and Qdenga – currently exist on the market. According to MOH, only the former is approved for use in Singapore. It is also recommended only for those aged 12 to 45 years who have had a previous dengue infection.
When asked what might be the best way forward, both Dr Fornace and Dr Ooi suggested a multi-pronged solution.
Dr Fornace said: “You need good diagnosis and treatment.
“Ideally, you kind of have some sort of vaccination strategy, outreach and public awareness, alongside vector control.”
Dr Ooi also stressed the importance of continuing with vector control efforts, while fortifying Singapore’s dengue vaccination strategy.
