Rising temperatures responsible for dengue outbreak?

The dengue outbreak in Bangladesh has an effective connection with rising temperature and changing patterns of rainfall, says a recent study on the mosquito-borne viral disease.

According to the study, the average temperature increased by 0.49°C from the first decade (2000-2010) to the second one (2011-2022), while rainfall decreased by 314 mm. But there was a rise in unusual rainfall in the pre- and post-monsoon period.

An increase in temperature gears up the growth and transmission of virus in mosquitoes and enables them to digest more blood, while the unusual pattern of the monsoon favours mosquito breeding. This is what happened in Bangladesh.

The research, titled “Two decades of endemic dengue in Bangladesh (2000–2022): Trends, seasonality, and impact of temperature and rainfall patterns on transmission dynamics,” was published in the US Journal of Medical Entomology recently.

Researchers and public health specialists said pesticides are not being applied in a proper way due to rising rainfall before and after the monsoon. They underscored the need for changes in mosquito prevention plans and activities.

Dr Mushtuq Husain, a noted public health specialist, told Prothom Alo that the study showed a link between the dengue outbreak and the weather with necessary data. However, in addition to environmental factors, the prevailing poor health system is also largely responsible for the dengue menace.

Over the past 22 years, a total of 244,246 dengue cases were reported, while the average annual number of cases is 10,619. The number of dengue cases increased eightfold during the second decade, with 2,216 cases during 2000-2011 and 18,321 during 2012-2022. Also, the deaths doubled from 21 to 46 between the two decades, while the overall case-fatality ratio decreased to one-third from 0.69 to 0.24.

According to the study, the average annual temperature rose by 0.49°C from 26.35°C recorded in the first decade to 26.84°C in the recent decade.

Najmul Haider, a researcher with specialisation in mosquito-borne diseases at Keele University, said the average increase of 0.49°C in temperature added approximately 4,292 degree-hours of equivalent heat per year, when only 349-degree-hour equivalent heat is needed for the dengue virus to complete transmission and incubation.

Therefore, the addition of 0.49 °C temperature shortens the incubation period and thus increases the rate of dengue virus transmission. An eightfold increase in dengue cases is a possible indication of the impact of increases in temperature in the country.

'Degree hour' is an index unit for temperature measurement. If one stays at 10°C for one hour or at 1°C for 10 hours, he is exposed to 10 degree-hours of heat. According to researchers, the dengue virus starts growing in Aedes mosquitoes at a temperature of 17°C. It takes an average of eight days to complete incubation at the temperature in Bangladesh.

Touhid Uddin Ahmed, former chief scientific officer of the Institute of Epidemiology, Disease Control and Research (IEDCR), said mosquito breeding increases if the temperature prolongs. There are some other factors at play. There is no survey on mosquitoes throughout the year, while the effectiveness of pesticides is not being monitored regularly.

The annual rainfall declined by 314 mm between the 2 decades, from 2078.6 to 1764.5 mm, of which 308 mm decreased during the monsoon (July–October) season. It prolonged the period for mosquito breeding as well as dengue infection.

More than 77 per cent of the total dengue infections were reported between April and July, while it was only 16 per cent during the August-March period.

Najumul Haider said the breeding of mosquitoes has increased due to the rise in rain before and after the monsoon. But the dengue eradication activities remain limited within the monsoon season, ignoring the unusual breeding before and after the season.

It was feared in the study that there might be a rise in dengue and other mosquito-borne diseases, along with unusual rainfall.

A significant disclosure of the study is the monthly rise in dengue cases. Analysing the data of previous years, it was found that the number of dengue patients always increased between April and July, compared to the previous months.

Professor Saiful Islam, director of the Institute of Water and Flood Management at Bangladesh University of Engineering and Technology (BUET), said the outbreak of bacterial diseases is rising in tropical countries. There is a connection of the dengue outbreak with rising temperature and changing rainfall patterns. It deserves more research work.

Last year, dengue deaths and infections surpassed the records of the previous 23 years. The study offered some suggestions to prevent dengue in the coming days. These include integrated and comprehensive mosquito control programmes, with the participation of locals. Besides, mosquitoes and infected ones should be monitored regularly.

Dr Mushtuq Husain said policymakers should consider the changes in disease and the environment with great importance. The study provides a message to bring changes into the activities.