Climate change behind prolonged dengue season

Climate change also changes the pattern of humidity in weather, which has changed the adaptation pattern of Aedes mosquitoes and also changed the behaviour of Aedes mosquitoes. This is why the seasonal dynamics of Aedes mosquitoes have changed.

— Prof Kabirul Bashar Entomologist at JU

In the first 22 days in November, the country has already witnessed over 123 deaths and 23,009 dengue cases, showing an alarming trend this year.

Experts identified climate change as one of the main factors behind this prolonged dengue season this year, which is also the picture of the last few years.

According to the Directorate General of Health Services (DGHS), November 2022 saw 274 dengue deaths and 40,716 cases, while December recorded 83 deaths and 9,288 cases. The figures for 2021 and 2022 were significantly lower, with 27 deaths and 5,024 cases in 2021, and 113 deaths and 19,334 cases in 2022.

This year, dengue has claimed 438 lives, with total cases reaching 84,826 so far.

According to the LANCET Countdown on Health and Climate Change data sheet 2023, from 2018 to 2022, the average summer temperatures that people were exposed to were 0.9 degrees Celsius higher than the 1986-2005 baseline average.

The conditions are suitable for the spread of malaria for more than half of the year, and the basic reproduction number (an indication of how many other people each infected person will make sick) for dengue has been above 4 since at least the 1950s, it said.

Entomologist Manzur A Chowdhury noted that the warm, humid conditions, which typically taper off by November, persist this year.

"Temperatures between 16°C and 30°C are ideal for Aedes mosquitoes, and biting tendencies increase in such conditions," he said.

He warned that infected individuals, not the mosquitoes themselves, primarily spread the virus to new areas. "Aedes mosquitoes can be infected from a dengue patient one day prior to fever symptoms appearing and up to the day of recovery," he explained.

Manzur emphasised preventing patient movement and using mosquito nets to curtail transmission. He also called for targeted anti-mosquito drives around patients' homes when case numbers are low.

Prof Kabirul Bashar, an entomologist at Jahangirnagar University, stressed that climate change is altering Aedes mosquitoes' behaviour.

"Rain patterns have shifted, with rain now observed even in late October and November. This, combined with changing humidity levels, has led to year-round mosquito breeding," he said.

"Climate change also changes the pattern of humidity in weather, which has changed the adaptation pattern of Aedes mosquitoes and also changed the behaviour of Aedes mosquitoes. This is why the seasonal dynamics of Aedes mosquitoes have changed," he added.

Aedes mosquitoes, once limited to summer, are now active throughout the year, he said, adding that their biting habits have also changed, with bites occurring not just in the morning and evening but also at night.

These behavioural changes are happening due to climate change, he said.

Prof Bashar added that factors like water storage during winter and unplanned urbanisation exacerbate the problem, providing breeding grounds for mosquitoes.