Entomologist Kabirul Bashar blames bright light, one of modern life's accessories, for this behavioural change in mosquitoes
Published : 12 Jul 2023, 03:11 AM
It was believed that the Aedes mosquito, which carries dengue virus, only bites during the early morning and before dusk when the lighting conditions are neither fully dark nor completely bright. However, a recent study has revealed that this mosquito can bite throughout the day and night.
The study has shown that Aedes mosquitoes can breed effectively in various types of water, regardless of their cleanliness. This contradicts the previous understanding that stagnant clean water was the sole suitable breeding environment for these mosquitoes.
According to the World Health Organization, the Aedes mosquito has a greater tendency to bite during the early morning and late afternoon, just before sunset.
Additionally, fully grown Aedes mosquitoes typically find shelter indoors in dark spaces like closets, under beds, or behind doors and windows, as these areas provide protection from rain, wind, and potential intruders.
As dengue spreads across the country like wildfire during monsoon, government agencies, entomologists and other experts are urging the Dhaka residents to be aware and not allow water to get stored at any place in rooftops, balconies, porches, or in plant pots, trays, and bottles.
This is because those stagnated water could be an ideal breeding ground for mosquitoes, they said.
But the study conducted from 2021 to 2023 on adaptation of Aedes aegypti mosquito larvae in sewage, sea, brackish and drain water found that Aedes aegypti mosquitoes can breed in different types of water - including rainwater, saltwater, seawater, drain water, and sewage water.
This poses a new challenge for dengue control efforts as these mosquitoes have adapted to thrive in various aquatic environments.
The study found that the Aedes mosquito can survive equally well at 95.83 percent in rainwater, brackish water, seawater, and drain water, with a slightly lower survival of 92 percent in sewer water.
During the study, the research team conducted experiments on a total of 134,904 mosquitoes in various locations, including Jahangirnagar University, Chattogram, Cox's Bazar, and Savar. The purpose was to determine the timing of mosquito biting. The mosquitoes were captured specifically between 4 pm and 9 pm for the experiments.
The findings revealed that Aedes mosquitoes exhibit activity both at 9pm and 4pm, indicating that they are equally active during these time periods.
Entomologist Dr Kabirul Bashar, a zoology professor at Jahangirnagar University and the study's lead researcher, said that they captured the mosquitoes in a cup immediately after they came to bite. The mosquitoes were made unconscious using chloroform and taken to the laboratory for testing and species confirmation.
"The density of Aedes mosquitoes is slightly lower at night. However, Aedes mosquitoes still bite during nighttime. Previously, these mosquitoes used to bite only during the day, specifically in the morning and afternoon. Moreover, now they are also found in dirty water."
WHAT CAUSED THE CHANGE?
Dr Bashar blamed an increased use of bright artificial light in Dhaka, which he called light pollution, for the behavioural change in Aedes mosquitoes.
“People did not use so much light in the past, but bright light is a part of modern life now. They have more light than they need at home and marketplaces.”
Dr Shahriar Hossain, general secretary of the Environment and Social Development Organisation, did not rule out changes in mosquito behaviour because of excessive use of light.
“Light attracts insects. This is why their movement falls in cloudy weather. There is a myth that they can sense [rain coming], but actually they stop moving because of low visibility. They start moving when the darkness goes away.”
“But they don’t go towards the yellow light, mistaking it for fire. They are attracted by fluorescent, bright light.”
Dr Niatuzzaman, the director of Mugda Medical College Hospital in Dhaka, agrees with Dr Bashar and Dr Shahriar. “There weren’t so many dengue patients when we used sodium bulbs.”
“It is said that Aedes mosquitoes don’t bite at night, but large LED TVs at home and bright streetlights will confuse the mosquitoes.”
Entomologist Dr Manzur Ahmed Chowdhury, a former president of the Zoological Society of Bangladesh, said Aedes mosquitoes never bite in the dark.
“There must be day-like light for Aedes mosquitoes to attack,” he said, adding that the findings of the study should be peer-reviewed first for more confirmation.
WHAT’S THE REALITY?
Dr Mnzur said the idea that Aedes aegypti mosquitoes breed only in clean water is wrong. “What’ll the larvae eat in clean water?”
“Aedes mosquitoes generally lay eggs in the inner side of a wet pot with bacteria, fungus, zooplankton and phytoplankton so that the larvae can survive on them.”
The recent drives by the city corporations back his claim. Aedes larvae were found in dirty water during the drives on Jamuna Oil Company Ltd and Bangladesh Textile Mills Corporation offices.
Md Golam Mostafa, deputy chief health officer at Dhaka North City Corporation, said they found Aedes larvae in dirty water in the basements of some buildings in Mirpur on Tuesday. “We haven’t seen this before.”
‘NEED TO THINK DIFFERENTLY’
Dr Bashar said the authorities need to think differently to control Aedes mosquitoes.
“The city corporations can divide the wards into blocks and assign two workers every block. The workers will visit every house and kill mosquito breeding places with the help of the residents.”
He said such campaigns were effective in family planning and controlling malaria in Bangladesh.
“We must work in that way to control Aedes mosquitoes. Our city corporations say that they cannot visit every house. Dengue cannot be controlled until they can do it.”
[Writing in English by Osham-ul-Sufian Talukder and Arshi Fatiha Quazi]