Early intervention to control mosquitoes could reduce the burden of dengue fever incidence in Bangladesh

Dengueis currently a worrying and important public health challenge for Bangladesh. The first official dengue virus outbreak in Bangladesh was reported in 2000, and since then, dengue has become endemic in the country posing a significant health challenge. Over the past few years, the number of dengue cases has been steadily increasing with significant seasonaland regional variations. Climate change, including changes in precipitation, temperature, and humidity, as well as rapid unplanned urbanization, were identified as strong predictors of an ecological imbalance that has led to an increase in dengue cases in Bangladesh.

A new studypublished in the medical Preprint, MedRxiv, compared the dengue virus infection, deaths, case-fatality ratio, and meteorological parameters between the first and the recent decade (2000-2010 vs. 2011-2022) and to understand the trends, seasonality, and impact of change of temperature and rainfall pattern on transmission dynamics of dengue in Bangladesh¹.

Over the past 23 years, a total of 244,246 dengue cases were reported including 849 deaths (Case fatality ratio [CFR] =0.34%). Themean annual number of dengue cases increased eight-fold during the second decade, with 2216 cases during 2000-2010 vs. 18,321 cases during 2011-2022. The mean annual deaths have doubled (21 vs. 46). The annual temperature increased by 0.49 °C, rainfall decreased by 314 mm and the monsoon has changed its season, which has put the dengue epidemic in Bangladesh at greater risk¹.

The increase of 0.49 °C temperature adds approximately 4292-degree-hours equivalent heat per year in the country. This additional heat would favor mosquito-borne disease transmission. For dengue virus transmission, approximately 305-degree-hours equivalent heat is needed to accomplish the extrinsic incubation period in Aedes mosquitoes at 26° C. Thus, the additional 0.49°C temperature will add to the burden of more than 14 generations of infectious mosquitoes in the environment of Bangladesh. An 8-fold increase in dengue cases is an indication of such changes in temperature in the country. Our model identified a significant role of monthly meantemperature with an additional 1 °C temperature increasing the monthly cases by 26%.

Rainfall facilitates mosquito breeding and plays an important role in mosquito-borne disease transmission. Although the research showed a 15% reduction in annual rainfall in the recent decade from the immediate past decade, it also detected an increase in unusually high rainfall in pre-and-post monsoon season allowing an extended season for mosquitoes and other arthropod vectors. The regression model of the research showed that both the first and the second lagged month’s rainfall increased monthly dengue cases by 8% and 17%, respectively. These findings are consistent with earlier studies in Bangladesh that showed that peak dengue cases occurred two months after the peak rainfall. Similar findings were reported in Vietnam with the dengue incidence being associated with both first and second-lagged months. Thus, dengue virus transmission season is extending in Bangladesh. This has been reflected in the years 2019 and 2022. The 2019 outbreak was characterized by early rainfall of 120 mm in February compared to a monthly mean of 20 mm precipitation. The 2022 outbreak was characterized by the late onset of rainfall with 297 mm of rainfall in October compared to a monthly mean of 156 mm, and thus prolongation of vector transmission season.

Bangladesh’s dengue season is characterized by hot and wet periods running between June to August. This is the period with the highest amount of rainfall in the country facilitating Aedes mosquito breeding in the country. The monthly mean growth factor (the comparative number of dengue cases in two successive months)remains above 1 for April – June indicatingthat for each of these months, the incidence of dengue cases will surpass the current month. Thus, the studysuggests starting vector control intervention in April in Bangladesh.

Globally and regionally in South and Southeast Asia, dengue cases are increasing. DENV infection increased by more than 46% between 2015 and 2019 in the region. In 2023, up until 31 May, a total of 1515,460 DENV infections were recorded in Brazil with 387 deaths. In Malaysia, a total of 43,619 DENV infections have been recorded by 21 May 2023. We found an increasing trend of DENV infection in Bangladesh. This increasing trend was much stiffer after the serotype DENV-3 was introduced in the country in 2018. This increased trend is possibly linked with climate change in the region attributed to increased temperature and unusual rainfall, urbanization, population growth, inadequate water supply and storage practice, poor sewer, and waste management system, rise in global commerce and tourism.

So, what are the solutions?

Regular destruction of mosquito developmental habitats and increasing surveillance for detecting active cases should be prioritized for controlling DENV infection in Bangladesh. Policymakers need to design an Aedes-borne disease management plan by considering a range of pathogens that Aedes mosquito can transmit including Chikungunya, yellow fever, Zika virus, West Nile, Japanese Encephalitis, Eastern Equine Encephalitis, Ross River, Rift Valley fever, and the LaCrosse virus. The study showed that the control measures will be more effective if they can be introduced before the start of the rainy season.

There are no other magic bullets for controlling Aedes-borne diseases.

There are big concerns raised over the development of insecticide resistance across the world and no new insecticidesare on the pipeline. The development of a successful dengue vaccine is hampered by antibody-dependent enhancement (ADE) due to previous infections with different serotypes of the dengue virus. The prospect of Wolbachia-related intervention is bright but still far from being applied on a national scale considering the expenses and associated technicalities. In this situation, an integrated and holistic vector management plan while engaging the local communities is key for controlling Aedes-borne diseases, especially in resource-limited countries. Regular destruction of mosquito developmental sites and increasing surveillance for detecting active cases are key in controlling dengue virus infection. Continuous active

surveillance of DENV infections will enable early detection of cases and outbreaks. Public health authorities will be able to identify areas where the disease is spreading, take immediate action to control mosquito populations,isolate infected patients, and implement public awareness campaigns to educate people about preventive measures.

Fig 1: Top: Number of dengue cases and deaths over the period 2000-2022, Bangladesh.Bottom: Number of monthly dengue cases and deaths recorded in Bangladesh (2000-2022).

Fig 2: Monthly average growth factor for dengue cases in Bangladesh. The growth factor remains above 1 for the month of April-July, indicating the period when intervention could achieve higher success in reducing the burden of Aedes mosquitoes and dengue virus infection in Bangladesh.

Authors:

• Najmul Haider, Lecturer of Epidemiology, School of Life Sciences, Keele University, United Kingdom (n.haider@keele.ac.uk )
• Md. Ibrahim Khalil, Senior Scientific Officer, Field Diseases Investigation Laboratory, Department of Livestock Services, Ministry of Fisheries and Livestock, Barishal(dribrahim.dls@gmail.com)
• Mohammad Nayeem Hassan, Biostatistician, Shahjalal University of Science and Technology, Sylhet, Bangladesh(nayeem5847@gmail.com )

References:

1            Mohammad Nayeem Hasan, Ibrahim Khalil, Najmul Haider. Two Decades of Endemic Dengue in Bangladesh (2000-2022): Trends, Seasonality, and impact of Temperature and Rainfall Patterns on transmission dynamics. Medrxiv : https://doi.org/101101/2023071623292380 2023; 1: 1–25.