Dengue has now become an almost year-round disease in Bangladesh. How do you explain this change?
When an infectious or viral disease first emerges, we must stop it immediately. If we fail to contain it, it gradually grows at a geometric rate. This is exactly what has happened with dengue. At one time, dengue was confined mainly to Dhaka. When we failed to control it within the capital, it gradually spread.
Every day, large numbers of people travel from Dhaka to different districts by public transport. People travel on those vehicles, and mosquitoes travel with them. In this way, Aedes mosquitoes have spread to every district and upazila town across Bangladesh.
What is your forecast for dengue in the coming months?
At the Insect Rearing and Experimental Station (IRES) of Jahangirnagar University, we regularly conduct mosquito research across the country. We also carry out routine surveillance in Dhaka. By analysing several parameters—including Aedes mosquito density, the number of dengue patients, rainfall, temperature and humidity—we continuously develop forecasting or prediction models. Every forecasting model we have developed so far has proved accurate.
This year, our findings suggest that August and September could be particularly severe months for dengue unless we bring the situation under control immediately. I can specifically identify the districts that are likely to face a high risk of dengue.
Several districts in the Barishal Division, including Barishal, Barguna, Pirojpur and Patuakhali, are at high risk. Nearby districts such as Bagerhat and Khulna are also vulnerable. In addition, Chattogram, Cox's Bazar, Chandpur, Cumilla, Dhaka, Gazipur, Mymensingh, Narsingdi, Manikganj, Munshiganj and Narayanganj are expected to face significant dengue risks.
These districts are likely to record a much higher number of infections. Compared with previous years, Dhaka may not experience the same level of infections because the chance of infection there has declined considerably. Therefore, we now need much stronger dengue preparedness not only in Dhaka but also across districts outside the capital.
People generally believe that the risk of dengue is highest during the monsoon. Is that actually the case?
It is true that the risk of dengue is higher during the monsoon. However, it is wrong to assume that dengue does not occur in winter. Bangladesh now experiences dengue throughout the year—in winter, summer and the monsoon alike. During the monsoon, rainfall causes water to accumulate in various places.
Water collects in containers of different sizes where Aedes mosquitoes lay their eggs. The eggs develop into larvae, the larvae become pupae, and the pupae mature into adult mosquitoes. If an adult mosquito lives in an area where a dengue patient is present, the mosquito transmits the virus, allowing it to spread to other places and increase at a geometric rate.
That is why dengue cases rise during the monsoon. In Bangladesh, the highest risk occurs in August, September and October. We describe this as the post-monsoon period. Aedes mosquitoes breed during the monsoon, which is why dengue cases increase significantly afterwards."
Prothom Alo :
A few years ago, your research found that Aedes mosquitoes also bite at night and breed in dirty water. Those findings have now been confirmed.
Aedes mosquitoes are extremely adaptable. I describe them as highly cunning mosquitoes because they can adjust to almost any environmental change. We previously believed that Aedes mosquitoes laid eggs only in clean water. We now find that they also lay eggs in dirty water.
Just the day before yesterday (9 July), I joined a Japanese research team for fieldwork in Dhaka. We observed Aedes mosquitoes breeding in dirty water, and the Japanese scientist was astonished. He said, “I have never witnessed Aedes mosquitoes breeding in dirty water at any point in my life.”
There appear to be fewer mosquitoes in Dhaka now. Why, then, are dengue cases still occurring?
That is true. Most city residents would say that the overall mosquito population is now very low. Yet dengue cases continue to occur. Why? In Bangladesh, around 99 per cent of mosquitoes are Culex mosquitoes, and their numbers decline during the monsoon. Aedes mosquitoes account for less than one per cent of the mosquito population, but their numbers increase during the monsoon. That is why the overall mosquito density appears lower at this time.
Many city residents assume that because Aedes mosquitoes bite during the day, they only need to remain cautious during daylight hours. They also believe that because they do not see Aedes mosquitoes inside their homes, there is no need to use a mosquito net. However, even a single Aedes mosquito can pose a significant risk of dengue.
For that reason, people should make it a habit to sleep under a mosquito net whenever they sleep, whether during the day or at night, even if there are few or no visible mosquitoes indoors. As the next three to four months carry a high risk of dengue, it would be unwise to neglect this precaution.
Prothom Alo :
How effective do you consider the city corporations' mosquito control activities?
Bangladesh has spent substantial amounts of money on mosquito and dengue control over many years. Yet dengue has not declined; it returns every year. We need to identify why dengue remains uncontrolled. We continue to implement various measures, but the disease persists. During my 26 years of research, I have tried to identify the gaps. In my view, the city corporations' mosquito control programmes primarily target Culex mosquitoes.
Culex mosquitoes breed in polluted water found in drains, ditches and sewers. You will often see spray operators using fogging machines or spraying along drains, ditches and roadsides. However, these are not the places where Aedes mosquitoes typically breed.
Aedes mosquitoes breed in stagnant water found around homes, in basements, car parks, drums, buckets, household containers, flower pots and plant saucers. However, our current control measures do not target Aedes mosquitoes; they target Culex mosquitoes instead. That is where the mistake lies. We will only be able to control dengue if we direct our mosquito control efforts specifically towards Aedes mosquitoes.
Can the smoke produced by fogging machines effectively control mosquitoes?
The Japanese team spent the past few days working with us. They travelled around Dhaka and observed fogging machines releasing smoke. They were astonished to see smoke being dispersed in the presence of people. This practice is not approved under any circumstances.
Many countries around the world have prohibited fogging or fumigation through legislation. It is not an effective control method. Instead, it creates health risks for people, harms the environment and damages beneficial pollinating insects. Good-practice countries no longer use this approach.
Yet we continue to spend large sums of money on a method that is fundamentally ineffective. However, larvicides can be applied because they target the aquatic larvae that develop in water. Even then, we must carefully select products that are environmentally suitable and do not pose risks to human health.
Prothom Alo :
What advice would you give people on protecting their households from Aedes mosquitoes?
Every household must ensure that no water remains stagnant in any container on the premises. If water collects in any container in the yard, basement or vehicle parking area, it can become a breeding site for Aedes larvae.
Every resident should therefore make sure that there is no stagnant water anywhere around their home. If the water cannot be drained away, they should apply an insect growth regulator (IGR) or a larvicide, if necessary, and seek assistance from the city corporation. Whatever the situation, they must ensure that no stagnant water remains around their home.
Which three measures are the most urgent to implement right now?
At this moment, we need to prioritise three measures. First, we must strengthen community engagement to eliminate Aedes mosquito breeding sites. Second, we should apply insect growth regulators (IGRs) or biological larvicides to control Aedes larvae. Third, hospitals must prepare for a potential surge in dengue cases by providing physicians and nurses with specialised training so that they can effectively manage the increased patient load.