More than expected numbers of dengue cases in extreme weather conditions have raised eyebrows, once again putting the sorry state of the mosquito control programme under the spotlight.
So far this year, the health directorate has reported 2,726 hospitalisations and 33 deaths caused by dengue, higher than the 2,023 hospitalisations and 13 deaths recorded over the same period in the past year, the worst year in terms of dengue outbreak since official record-keeping began in 2000.
The figures are rather surprising, as the weather condition marked by relentless excessive heat was supposed to put insects to summer sleep, reducing their reproduction and ability to infect.
Entomologists, epidemiologists, and zoologists feared that the government might have missed the golden opportunity to prevent the dengue outbreak from going out of hand.
They said that the Aedes mosquito population could have been effectively controlled with the minimum deployment of resources, leveraging the effects of excessive forecasting on the insect.
Bangladesh has remained in the grip of prolonged hot weather, which saw its daytime air temperatures remain above 36C from the last day of March until Friday, except for eight days passed in two phases in the second week of May.
The entire country of Bangladesh is currently in the grip of a mild to moderate heatwave, with vast swathes getting baked at around 38C or higher.
This is the third heatwave to sweep Bangladesh since March 31. The first heatwave lasted 37 days in a row, with daytime air temperatures frequently exceeding 40C and staying at 40C or more for three weeks without any break in some areas.
鈥業nsects go to sleep when it gets too cold or too hot,鈥� Abu Faiz Md Aslam, who teaches zoology at Jahangirnagar University, told 抖阴精品.
The winter sleep is widely known as hibernation, he explained, while the summer sleep is called aestivation.
Adult insects, including mosquitoes, could die if day temperatures reach 38C, which also destroys larvae and eggs by warming water bodies.
A mosquito鈥簊 optimum temperature range is 20C to 35C.
A study published in February in the Journal of Insect聽聽 Conservation by the Berlin-based Springer said that insects, which depend on the outside environment for heat, become inactive under excessive heat.
The research revealed that in the afternoon of July 19, 2022, when the temperature rose above 35C majority of butterflies in a certain neighbourhood became inactive.
It apparently became too hot for the butterflies to fly and carry out behaviours such as feeding, mating, and defending territories, the research said.
鈥楬igh temperatures reduce mosquitoes鈥� flight range and flying speed,鈥� said AM Zakir Hussain, a former director of the Institute of Epidemiology, Disease Control, and Research.
Despite the extreme weather conditions, the high presence of Aedes mosquitoes is evident in the officially reported dengue patient and death figures.
Bangladesh witnessed the deadliest dengue outbreak on record last year, with over 3.21 lakh hospitalisations and 1,705 deaths.
Over 1,000 hospitalisations were recorded in January, which was halved because of the rise in temperatures and the growing unfavourable weather in the following months.
Insects such as mosquitoes retreat to bushes during excessive temperatures, where they face a lack of the supply of warm blood required for their reproduction, explained Razzab Ali, who teaches entomology at Sher-e-Bangla Agricultural University.
Entomologists advise using chemicals on water bodies in such a way that they leave a layer over the surface, preventing mosquitoes from laying eggs there or the larvae in there from dying.
Epidemiologists also advised that government-run awareness campaigns in hospitals among the infected people so that the circulation of the dengue virus could be effectively controlled.
The level of awareness about dengue among people is manifested in the high number of deaths this year.
鈥楧elayed hospitalisation is mainly responsible for high deaths,鈥� Dhaka Medical College principal Md Titu Miah told 抖阴精品.
Most patients got admitted with dengue shock syndrome, implying that they stayed at home despite developing severe symptoms such as diarrhoea and bleeding, probably exposed to more mosquito bites, facilitating the spread of the disease, he said.
鈥榃e have to find out how Aedes mosquitoes breed in such unfavourable weather conditions,鈥� he added.
So far, high day temperatures have mostly been accompanied by low humidity, which hinders the spread of viruses.
Over the past week, the humidity in the air increased, exceeding even 80 per cent at times in a day. The optimum level of humidity for insects鈥� growth is 80 per cent.
A spike in the dengue outbreak, however, is unlikely to happen soon, experts said, for it takes at least two weeks for a spike in dengue cases to occur even after the environment turns favourable.
The prospect of heavy rain, which accompanies the onset of monsoons in June, is likely to delay the onset of favourable weather for mosquito breeding, experts said.
Dhaka South City Corporation鈥檚 chief health officer, Fazle Shamsul Kabir, however, claimed that their effective control measures have kept mosquitoes in check so far this year.
鈥極ur mosquito control programme is always on,鈥� he said, claiming that they were spraying 4,500 litres of insecticide every day through fumigation for mosquito control.
He said that they were also cleaning water bodies and possible breeding grounds.
Residents of DSCC, however, paint a completely different picture.
According to Mizanur Rahaman, a Kadamtali resident and activist, Zurain has artificial and natural water bodies filled with mosquito larvae.
Two weeks ago, Mizan said, authorities used insecticides following complaints, and that was the end of mosquito control, for there was no follow-up.
鈥榃ater bodies became filled with larvae once again, and even adult Aedes mosquitoes could be spotted around,鈥� said Mizan.
The 鈥笰uthorities鈥� mosquito control programme is nothing but a show-off. It does not mean to effectively control mosquitoes,鈥� he said.聽
Experts called fumigation a waste of public money, particularly because of the random use of insecticide doses and the manual way of doing it.
