December is supposed to be a month with a biting cold in Bangladesh, bringing down the Aedes mosquito population substantially. But the country reported over 100 dengue patients on Thursday with 31.2 degrees Celsius of temperature in Dhaka.

What is actually behind the dengue prevalence in Dhaka at this time of the year? Experts think Bangladesh is bearing the brunt of global warming.

They attribute the unusually high dengue cases during the dry season, mainly in Dhaka, to the prolonged rainy season, sporadic rainfall, and high humidity and temperature, Aedes mosquitoes' reproductive and behavioural changes caused by climate change and lack of people’s awareness and poor controlling measures by the two city corporations of the capital.

Also read: Climate change making dengue an annual affair in Bangladesh: Experts

They also said the climate conditions of Bangladesh are becoming more favourable for the Aedes mosquitos to prevail in all seasons with high or low intensity mainly for the increase in temperature and breeding sources.

The analysts emphasize the need for modernizing the mosquito controlling measures and carrying on separate drives to contain the Aedes and Culex mosquitoes in all seasons as dengue has become endemic in Bangladesh.

Dengue scenario

According to the Directorate General of Health Services (DGHS), 98 people died of dengue, including 90 in Dhaka alone, this year as of Thursday.

As many as 108 new dengue patients were hospitalised in the last 24 hours until Thursday morning, health authorities said.

Since January, 27,451 patients have been admitted to different hospitals with dengue in the country.

Possible causes

DGHS spokesman Robed Amin said the dengue cases are still very high in Dhaka city due to the prolonged rainy season, erratic rainfall and high level of humidity.

“Dhaka sporadic experienced rainfall last week while the humidity is not falling in December. That’s why the density of the Aedes mosquito is still high.”

Usually, he said, dengue cases start declining from October with the fall in mercury. “As the daytime temperature is still high and it’s suitable for Aedes mosquitos to breed and survive.”

Robed said it is also established by different studies that the dengue problem is deepening in Bangladesh due to abnormal shifts in the country’s traditional seasons as an adverse impact of climate change.

Also read: Dengue cases keep rising: 142 more hospitalized in 24 hrs

“Our rainy season is starting early, while the country is experiencing erratic rainfall even in November. And the warmer condition prevails during the winter. This is also a fact that there’re many suitable breeding grounds of Aedes mosquitos in many houses of the capital for lack of people’s awareness,” he observed.

Prof Kabirul Bashar, an entomologist of Jahangirnagar University (JU), said the rain pattern in the country is changing like the seasons. “We saw rain in late November this year which is unusual. Usually, dengue can sustain up to 20 days of rain.”

Besides, he said, vehicles are washed on the ground floors or parking spaces of multistoried buildings in Dhaka. “During our survey we saw most parking spaces have a small tank or tank-like arrangement from where water used for cleaning vehicles. And ultimately it remains stagnant. This is a breeding ground for the Aedes mosquito round the year.”

The entomologist said they also found a small water reservoir in almost every house where Wasa’s meter is connected. “Due to leakages in the reservoir, water remains stagnant there. The dengue virus-carrying mosquitos can breed in that water.”

As most houses in Dhaka do not have an uninterrupted water supply, the JU Professor said, many inmates used to store water in buckets, drums and other pots all-time for emergency use where Aedes mosquitoes can live and breed in any season.

Besides, he said, the temperature of about 22-25 degrees Celsius is now suitable for the breeding of the Aedes mosquitos as these insects are changing their reproductive nature. “Today’s daytime temperature is more than 30 degrees. So, this temperature is still conducive to the breeding of the Aedes mosquitos.”

Towhid Uddin Ahmed, Medical Entomologist and former Chief Scientific Officer of the Institute of Epidemiology, Disease Control and Research (IEDCR), said different kinds of discarded pots that can contain water are scattered throughout the capital, helping the Aedes mosquitos to increase.

Besides, water remains stagnant in many under-construction buildings in the city and those are one of the main sources of Aedes mosquitos breeding.

More worrying is that the city authorities have no visible and effective steps to contain the Aedes mosquitoes.

What to do?

Describing Aedes as a domestic mosquito, Prof Kabirul said both the city authorities and the city dwellers have to play a joint role round the year in containing such blood-sucking insects.

He said it is the responsibility of the city dwellers to remain alert about the sources of Aedes mosquito breeding in their houses while the city authorities need to continue anti-mosquito drives throughout the year.

The expert said separate mechanisms for containing the Culex and Aedes mosquito are necessary. “The city corporations are carrying out the same mechanism for controlling the two types of mosquitos. We need separate mechanisms as the breeding sources of Culex and Aedes are different.”

He said city corporations usually remain busy with anti-mosquito fogging drives, to kill both the Culex and Aedes mosquitos. “But I think fogging is not much effective for destroying the Aedes mosquitos as these insects remain inside houses or buildings.

“We need to think of Aedes mosquitos’ breeding source management and household spray to contain the dengue virus. People’s awareness is necessary in this regard,” the JU professor said.

Towhid Uddin said the city authorities should have strong surveillance to identify the areas where Aedes density is high and take action accordingly.

He said quality research is needed to know the change in characteristics Aedes mosquito, the effectiveness of insecticide the city corporations use to destroy such mosquitos.

He said there are now many effective insecticides and modern equipment in different countries to kill Aedes mosquitos. “The city authorities are still depending on fogging and some old sprayers.  We need to modernise the mosquito controlling system.”

Also read: 'National laws need to be revised to tackle dengue menace'

Improvement likely from mid-December

Prof Kabirul said the density of the Aedes mosquito is likely to decrease from the middle of this month if there is no rain while the infection rate may fall significantly in January if the temperature comes down below 20 degrees Celsius during daytime.

Towhid said the dengue cases may decline sharply in January if any cold wave hits the country.

He, however, said dengue has become endemic in Bangladesh and it may not be eliminated completely in any season.