HUMANS are passing through a crucial time. Along with the wars, massive global migrations and rapid climate change, vector-borne and zoonotic diseases are on the rise at an unprecedented rate. Dengue is one of the severe life-threatening diseases that is no more a secret to anyone living in Bangladesh. As I spend more than three decades of my professional career on the laboratory aspects of vector-borne and zoonotic diseases, I have never seen an unprecedented increased trend of dengue virus-positive cases like this year compared with past few years globally, including Bangladesh. So, the Dhaka city should be on guard so that a situation as it was in the past year would not repeat.
Dengue virus is a century-old virus that belongs to the family called flaviviridae and can cause mild to severe disease. Among 100 people exposed to this virus, only about 25 may fall ill and many may have fatal consequences if the situation is not properly managed. On an average, 400 million people are infected with dengue every year. About 100 million people fall ill from infection and 40,000 die from severe dengue virus infection. There are over 70 members in this virus family and many of the familiar mosquito-borne diseases such as dengue, West Nile, zika, yellow fever, Japanese encephalitis, St Louis encephalitis and tick-borne encephalitis virus belong to this family. All these viruses are transmitted through mosquitoes. Dengue virus can be transmitted by both Aedes aegypti and Aedes albopictus that abundantly present in Bangladesh.
Only in four months in 2024, more than 7.6 million dengue cases have been reported to the World Health Organisationin, including 3.4 million confirmed cases, more than 16,000 severe cases and more than 3,000 death,听surpassing the annual high of 4.6 million cases in 2023. Ninety countries have known active dengue transmission which highlights the acceleration of the health problem.听
The World Health Organisation also reports that in South-east Asia, Indonesia is experiencing a surge in dengue incidence with 88 593 confirmed cases and 621 death as of April 30, 2024 鈥� about three times higher than the figure in the same period in 2023. In 2024, Bangladesh, Nepal and Thailand have reported a higher number of cases compared with the same period of 2023.听 What is further concerning is that dengue patients are detected twice as much outside Dhaka where dengue cases were concentrated for centuries. No one needs to be a rocket scientist to predict that dengue cases would be more severe and be noticed throughout the country. Are we prepared to face this health problem? The health ministry has ordered preparations in advance to avoid the sufferings of patients and allocated some money as well. No doubt, this is a good piece of news and right approach, but what action has so far been taken?
Bangladesh is a haven for mosquitoes. There are than 120 mosquito species recorded in the country. Each of Aedes, Anopheles and Culex spp has several species that have been seen in Dhaka city. Because of the geographical location of Bangladesh, warm temperature and significant rainfall, mosquitoes can breed throughout the year. So, the total elimination of mosquitoes is an absurd idea. However, , keeping them under control requires a serious action the way it was taken to combat Covid-19.
The government must have a defined and actionable long-term mosquito control plan like the National Adaptation Plan of Bangladesh (2023鈥�2050) done to combat climate change. This plan should be well coordinated, consistently monitored year after year and continue with the action laid out in the plan. Basically, the plan should follow three C鈥檚 鈥� coordination, consistence and continuity. The execution of the plan needs commitment from the government, policymakers, academics, scientists, administrators and, most important, residents.
The usual practice in Dhaka city for years is to kill adult mosquitoes during the peak season by spraying here and there using a century-old manual sprayer in an unregulated manner although spraying is highly effective for killing adult mosquitoes. But that should be done properly and also should be considered the last resort. Vector-borne diseases are serious problems for Bangladesh and the situation would turn bad in coming years. So, it needs to be dealt with as a priority. The countries that managed to keep mosquito population at a tolerable level have adhered to both long- and short-term strategies. Bangladesh should do the same.
The long-term strategy should span more than five years, with community-level education and awareness.
Community education is the key to mosquito control. People should be reminded of it throughout the year using television-radio, newspapers, hoardings, social media, etc. We also need to include basic sanitation education from primary schools to junior high school curriculum so that the required knowledge could be ingrained in young people who will be responsible citizen. Speaking engagements for different groups, starting from villages, are needed as well and religious institutions such as mosques, temples, churches, etc,in addition to elementary and junior high schools, should be involved. The government should also distribute leaflets, with details on how to rid households of containers that could allow mosquito breeding.
An ongoing year-to-year mosquito and disease-causing pathogen monitoring surveillance is needed. Surveillance is one of the key parameters that allow timely detection and or the determination of changes in the abundance of vector population and pathogens over time. The knowledge gained from the data generated from the surveillance can be used by the city authorities or any other local government institutions in order control mosquito populations on time and on right locations, using standard protocols. Surveillance will not only allows having information on mosquito control decision-making. This will also help to understand species diversity, abundance, the emergence of new mosquito species, the detection of super breeding sites and their role as vectors in different diseases. Establishing a sample collection protocol is important and entomologists and bio-statisticians should be consulted to have an unbiased sample collections and a broad coverage of the Dhaka city. Most of the medically important mosquito species breed within 500-metre radius (not considering wind factor). So, both larval mosquito (by water sampling) and adult mosquito should be collected. A snapshot surveillance (one time, one season) will not provide enough information. So, surveillance should continue for years even when mosquito population goes down significantly. Coordination among academics, entomologists, microbiologists, bio-statisticians and data analyst is essential to have a successful programme. Most successful mosquito control programmes that use the information follow four major critical tactics: (a) source reduction (to remove mosquito habitats), (b) the use of structural barriers for mosquitoes, (c) mosquito control at larval stages and (d) adult mosquito control.
Short-term strategies, which could span three to five years, should include source reduction, the use of structural barriers, larval mosquito control and adult mosquito control. The foremost task should be to reduce the sources of mosquito breeding. The purpose of source reduction is to reduce the number of larval habitat areas available for mosquitoes. Regular inspections by house owners or flat dwellers are one of the most important methods to eliminate mosquitoes around houses and drains in residential areas. Everyone should work to remove the mosquitoes by ridding the households of standing water, thereby, removing places for the mosquito to lay eggs. The city should also provide education for city dweller on radio and television or through leaflet distribution etc, to effectively control mosquitoes. All standing water and water-holding containers in yards should be emptied.
Water in PET bowls, flower vases and birdbaths should be changed at least on a weekly basis. Rain barrels, water tanks and cisterns or fish should be screened. Hot tubs and pools should be kept in a working condition with appropriate chlorination. Plastic swimming pools, when not in regular use, should be emptied. Water-holding plants such as bromeliads should be flushed out. Rooftops should not have stagnant water. Garbage and recycle bins should have small holes for water to get out. Old tyres with stagnant water or any containers with water should be removed. There should not be any stagnant water in and around houses. Everyone knows it, but it needs to be ensured that everyone practises it, too. Other measures include as installing windows/door screens and using bed nets while sleeping (day and night) as the Dhaka city has both day- and night-biting mosquito species.
Larval mosquito control is extremely important and is the most efficient way to prevent mosquito-borne diseases. This method targets immature mosquitoes living in water before they become biting adults. In many countries, a naturally-occurring soil bacteria, Bacillus thuringiensis israelensis, Saccharopolyspora spinosa, or Lysinibacillus sphaericus is equally effective and can be found in both liquid and solid forms. Liquid larvicide products are applied directly to water using backpack sprayers and truck- or aircraft-mounted sprayers. The solid one can be purchased as dunks, tablets, bits, pellets, granules or briquettes. These forms of larvicide are also applied to areas where mosquitoes lay eggs. The Dhaka city was not properly planned and this city has rapidly grown over time with unplanned structures and a result access to house often become challenging. Newspaper reports say that every year, the city authorities do some spraying using trucks. Unfortunately, there is no access to trucks in most risky places. If any small areas are not covered, mosquitoes will generate there and will soon spread to other areas. The city corporations must consider buying crop duster planes, which are not that expensive when compared with the magnitude of mosquito problem, and use them for the whole city within a day followed by weekly application until water sampling data show no signs of larval mosquitoes in stagnant open water sources.
This bacterium was extensively studied and found to have no bad effects on non-target organisms when it is applied properly. There are also other naturally-occurring products that can be used safely as well. Many larvicides have the highest ratings for safety and environmental compatibility in western countries. According to the published literature, larval control is by far the most effective method of mosquito control. Small areas of less than an acre can be treated with liquid, granular, packet or pellet formulations of the bacterium Bacillus thuringiensis israelensis or methoprene by using hand-held applicators. The government may allow these products to become available for public purchase so that they can use them in their vicinity as well. Small fish (Gambusia) can also be used for the control of mosquito larvae in small ponds, cisterns and storm drains.
If mosquito defeats all these measures and adult mosquitoes are still hungry for blood, killing adult mosquitoes should be the last resort. It is better to have ongoing adult mosquito surveillance programme using local experts for effective results. Adult mosquito surveillance can be programmed in many ways depending on the budget and human resources.
Similar to larvicidal application, crop dusters should be used for adulticide spray as well. Entomologists should be consulted for the selection of effective adulticides, dosing and scheduling, etc. Residents should be adequately notified in advance so that exposure to pesticides can be minimised. Mosquitoes have been with us for centuries and will remain with us, no matter whatever we do. But, effective mosquito control planning and readiness will however, help us to keep their number at a minimum level. Bangladesh and especially Greater Dhaka city, should have a year-to-year plan the way western countries do about snow removal reediness. An effective implementation of mosquito reduction plan using such steps can keep mosquitoes at bay.
There are many other methods like genetically engineered mosquitoes which are geared to destroy female offspring gradually. I would hold this technology for now to have more data to see its long-term effects on ecological impact of using genetically-modified mosquitoes. Vaccine should be another choice along with other methods. Dengue vaccine is already available and the ICDDR鈥橞 is evaluating one.
It has been proved that the traditional way of controlling mosquitoes is not working in Dhaka city and other parts of the country and mosquito problem is increasing and compounding every year. A well-coordinated, integrated approach guided by a long-term mosquito control plan is essential to have an effective mosquito control. The key elements are (a) community awareness and education; (b) extensive mosquito surveillance (both larvae and adult), (c) staged mosquito control; and (d) keeping eyes on new research development in these areas such as the use of genetically-engineered mosquitoes. Apparently, all the steps seems quite doable and easy to execute but this is a challenging task. We all need to keep in mind and adhere to 3 C鈥檚 to achieve the targeted goal and this is everyone鈥檚 job.
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Dr Muhammad Morshed is a clinical microbiologist and programme head, zoonotic diseases and emerging pathogens, BC Centre for Disease Control, Vancouver, and clinical professor, pathology and laboratory medicine department, University of British Columbia.
