Dengue cases rise sharply outside Dhaka

The number of dengue patients outside Dhaka has risen sharply this year, with many cases believed to be locally transmitted, a development health experts say is a serious concern.

While the Aedes aegypti mosquito has long been identified as the principal dengue carrier, experts suspect another species, Aedes albopictus, is contributing to the spread in rural and semi-urban areas as an epidemic mosquito vector.

Principal vector means the main mosquito species responsible for transmitting the dengue virus in a region, while species that normally play a minor role but can contribute to transmission, especially during severe outbreaks or under certain ecological conditions, are referred to as epidemic vectors.

When the surge becomes severe, epidemic mosquito vectors can become active alongside principal ones. While Aedes aegypti remains the principal dengue carrier, Aedes albopictus is emerging as a significant epidemic vector, particularly in rural areas.

A joint survey by the CDC, Directorate General of Health Services (DGHS), and IEDCR, conducted between December 2024 and March 2025 in seven districts, found a significant presence of Aedes albopictus outside Dhaka.

Aedes albopictus -- commonly known as the Asian tiger mosquito -- is a small, dark insect with a white dorsal stripe and banded legs. It can transmit the dengue virus and breeds in both man-made and natural containers, such as water tanks, discarded tyres, tree holes, and bamboo stumps. Unlike Aedes aegypti, albopictus thrives in vegetated and forested areas as well as in urban settings.

According to DGHS, total dengue cases this year have reached 25,710, with 104 deaths so far. Barishal Division has recorded the highest number of hospitalisations (8,997), followed by Chattogram (3,700) and Khulna (1,365).

The survey covered areas in Khulna, Chattogram, Barishal, Faridpur, and Barguna.

According to the report, among Aedes mosquitoes, albopictus accounted for 55.04% in Khulna, 76.4% in Chattogram city corporation areas, 8.2% in Barishal city corporation areas, 93.9% in Jashore municipality areas, 83.8% in Faridpur municipality areas, and 4.5% in Barguna municipality areas.

In Dhaka, pre-monsoon surveys over the last six years found Aedes albopictus rates of 3.25% in 2025, 1.35% in 2024, 3.98% in 2023, 0.2% in 2022, 0.01% in 2021, and 1.81% in 2020.

EPIDEMIC VECTOR CONCERNS

An official from DGHS said albopictus prevalence is higher in rural areas. "We need to identify these hotspots and apply insecticide to kill both adult mosquitoes and larvae," he said.

Prof Kabirul Bashar, an entomologist at Jahangirnagar University, warned that if albopictus has become a dengue vector in rural areas, containment will be more challenging, as different control strategies are needed.

He urged local administrations, led by deputy commissioners, to take proactive measures and provide targeted training.

A government entomologist, speaking anonymously, said high-incidence areas require at least a month-long intensive campaign to eliminate adult mosquitoes and breeding sites, with strong community participation.

He also recommended using Bti (Bacillus thuringiensis israelensis), a naturally occurring bacterium that kills mosquito larvae, blackflies, and fungus gnats.

SHIFTING MOSQUITO DYNAMICS

Albopictus, once considered a "village mosquito," is now being found in cities. In Barishal and Khulna, where Aedes aegypti once dominated, albopictus is now more common.

Its ability to breed in a wide range of natural and man-made containers -- from tree holes to discarded plastics -- makes control harder, said the government entomologist.

Towhid Uddin Ahmed, principal scientific officer (retd) at the Department of Entomology, IEDCR, said rural cases may also be linked to infected patients travelling from Dhaka. In Dinajpur last year, 49 out of 50 dengue patients he surveyed were infected in Dhaka.

He added that while immunity from last year's dengue strain is reducing infections in Dhaka, many outside the capital remain vulnerable.

Towhid warned that controlling albopictus in rural areas with abundant trees and bushes will be difficult. "Given the current surge, authorities should prioritise adulticide spraying to kill adult mosquitoes," he said.