Alarming Trends in Dengue Incidence and Mortality in Bangladesh

Abstract

The past 50 years has seen a 30-fold increase in the incidence of dengue, an arboviral disease caused by the 4 dengue viruses (DENV 1–4). Since 2000, Bangladesh has been experiencing annual dengue outbreaks, but the epidemics have intensified in recent years [1, 2]. Two of the deadliest outbreaks on record occurred in 2019 and 2022, and this year's outbreak is on track to be the largest to date. A total of 203 406 dengue cases and 989 deaths have been recorded in Bangladesh between 1 January and 30 September 2023 [3], far outpacing previous years’ incidences for the same months (Figure 1A). The number of recorded cases this year in just the past 3 months—July to September—was 1.9 times the total cases reported in all of 2019, the last major outbreak.

Figure 1.

A, Weekly new cases of dengue in Bangladesh between August 2019 and 30 September 2023. B, Deaths attributable to dengue by month and year. C, Monthly normalized incidence (normalized to be between 0 and 1 for each year). D, District-level total cases per 100 000 population for the 3 largest outbreaks since 2000.

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Accurate scenario models and real-time risk maps that can inform vector control efforts and resource allocation during an outbreak can dramatically improve our ability to control and mitigate dengue epidemics. However, building these models requires a careful understanding of the major drivers of dengue transmission and risk of severe disease. Here, we report on the current dengue surge in Bangladesh, by analyzing daily reports from the Directorate General of Health Services [3], and we highlight 3 distinct features of this outbreak that warrants further research to develop accurate risk models for future outbreaks.

OUTBREAK TIMING

Dengue is endemic in Bangladesh and cases have been reported year-round. The incidence typically peaks between September and November, after the end of the monsoon season. This year's outbreak is unusual in terms of the magnitude of the outbreak as well as the seasonality. New cases began to increase in June, peaking in the first week of August (Figure 1C). This early sharp surge in cases was likely the result of multiple interacting factors, such as the timing of outbreaks in neighboring countries, timing of holiday travel [4], unusual episodes of rainfall earlier in the year [5], higher temperatures [5], and possible changes in transmission intensity due to antibody-dependent enhancement [6]. Understanding the impact of these drivers of dengue transmission will be crucial for forecasting outbreaks in the future, particularly in the context of predicted climatic changes, such as increases in mean temperatures across the tropics and the intensification of the monsoon season.

GEOGRAPHIC DISTRIBUTION

Historically, dengue cases in Bangladesh have been clustered around the 3 largest cities—Dhaka, Chittagong, and Khulna [7, 8]. A nationally representative serostudy conducted between 2014 and 2016, found that only 3% of individuals in the north had a history of prior infection with dengue, compared with >80% in Dhaka and Chittagong [7]. However, this landscape is changing rapidly, as more recent outbreaks have seen cases reported from across the country. This year, all districts in Bangladesh have reported dengue cases. Notably, there has been a sharp increase in cases in the south central and northern parts of the country (Figure 1D). The geographic expansion of dengue in Bangladesh may be an artifact of improved diagnostic capabilities, but it may also be due to increased travel volumes and greater connectivity across the country, climate- and environmental-driven changes in the vectoral capacity and range, and changes in transmission intensity.

MORTALITY BURDEN

The number of dengue-related deaths (593 deaths) in just the first 8 months of 2023 already far exceeded previous records (281 deaths in 2022 and 164 in 2019). The proportion of reported cases that resulted in death has also been notably higher in 2023 (4.9 deaths per 100 000 confirmed cases) compared with the last major outbreaks in 2019 (1.6 deaths per 100 000) and 2022 (4.5 deaths per 100 000). The unusually high mortality burden may be due to changes in the primary circulating serotype over time and an increased incidence of secondary heterologous DENV infection, which is known to be a risk factor for more severe disease. The recorded mortality and morbidity rates attributable to dengue in Bangladesh are likely just the tip of the iceberg, as most cases of dengue will not be captured in the hospital-based surveillance system. An increased focus on serotyping and the development of nationally representative serosurveys, paired with a robust surveillance system, is urgently needed for understanding the landscape of dengue immunity in Bangladesh and the implications for future outbreaks.

OUTLOOK

Vector-borne diseases, like other ecological systems, exhibit complex, nonlinear dynamics, driven by both intrinsic and extrinsic factors, making prediction and forecasting challenging. The ecology of dengue and its rapid emergence in South Asia over the past few decades have been shaped by a combination of demographic shifts—urbanization and increasing human connectivity—and climatic changes—increases in mean temperatures across the tropics and the intensification of the monsoon season. Forecasting the risks of severe disease and death for dengue is further complicated by the existence of multiple serotypes. The observed changing temporal patterns, shifting geographic ranges, and variations in disease severity can provide researchers with hints about what the future of dengue outbreaks may look like. Many other countries across the region, particularly Nepal, Sri Lanka, India, Thailand, and the Philippines, have also been experiencing unusually severe dengue outbreaks this year, possibly due to the impact of the El Niño climate pattern [9]. The alarming trends in dengue incidence observed this year are an urgent wake-up call for public health policymakers and researchers to pay closer attention to dengue dynamics in South Asia, strengthen the surveillance system and diagnostic capabilities, and develop tools and methods for guiding strategic resource allocation and control efforts.

Notes

Acknowledgments. We are grateful to the Directorate General of Health Services, Bangladesh for collecting the dengue data and releasing daily updates. We are also grateful to the Aspire to Innovate (a2i) program within the government of Bangladesh, for providing us with the data.

Data availability. Data are available for download from a public, open access website.

References

Author notes