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In Thailand, efforts to control Aedes mosquitoes and dengue fever have produced uneven results. Over the past few decades, the toll of the disease has diminished in some areas. In others, mosquito-borne infections remain high. To improve the situation, control measures are being reconsidered.

Mosquito display at the Division of Vector-borne Diseases gallery.

Statistics indicate that the number of dengue cases each year ranges from ten thousand to upwards of one hundred thousand, raising the question of whether existing control measures, which require a large pool of human resources, are truly effective.  Another question arises: what other tools could be used to control dengue?

Struggling with mosquitoes (1): The past and present of dengue control in Thailand

Like other countries around the globe, Thailand is researching new methods to contain vector-borne diseases and reduce the number of disease-carrying mosquitoes.  In this special report we survey new approaches to controlling Aedes larvae, methods that work for mosquitoes, as well as people.

Dengue-free mosquito

News reports indicate that over the past 5-10 years, dengue control measures have begun to focus on using technology and data analytics to make mosquitoes deal with their fellow mosquitoes.

Examples include the sterilisation of male mosquitoes to interfere with breeding and the biological modification of mosquitoes so that they either reproduce sterile offspring or offspring that no longer carry dengue. Both methods are being developed in a bid to prevent vector borne diseases.

In Thailand, a 2019 experiment led by Mahidol University, introduced bacteria from the Wolbachia family into male Aedes mosquitoes, which were then sterilised through exposure to weak radiation. Released, they bred with Aedes females without producing offspring. In the process, they also passed Wolbachia bacteria on to mosquitoes in the wild. 

A 2016 test using the same method in Pleang Yao district, Chachoengsao province produced a dramatic reduction in the number of wild Aedes mosquitoes, 97%, for 6 months.

In 2016-17 sterilisation was also used to control Asian tiger mosquitoes in a town on two river islands in Guangzhou, China. The number of adult female mosquitoes decreased for two years running - 83% in 2016 and 94% in 2017.

Sungsit Sungvornyothin, a lecturer from the Department of Medical Entomology, Faculty of Tropical Medicine, Mahidol University, explains why dengue control measures are increasingly focused on adult mosquitoes, not larvae.

“…New methods look beyond public cooperation. They only need acceptance. In Singapore, they invite people into their labs, invite them to look at public educational displays …they set out a mosquito cage, put their arm in and demonstrate that they don’t get bitten. Male mosquitoes don’t bite. And these males have been treated with a strain of [Wolbachia] bacteria. It lives in their cells. Mosquitoes carrying the same strain can breed with each other but the bacteria remains in their reproductive organs, in their ovaries and testis. And if they breed with a mosquito carrying a different strain, the strains clash, and the larvae have problems and die, “Sungsit explained.

In 1924, Wolbachia bacteria were found to be living in various insects. Some types of mosquitoes also carry the bacteria but not Aedes.  In 1987, it was learned that Wolbachia affect vector-borne diseases like dengue by disrupting viral transmission. Capitalising on this discovery in 2009, the World Mosquito Program succeeded in producing Aedes mosquitoes which pass Wolbachia along to their offspring, reducing the number carrying dengue.

In addition to reducing mosquito populations through the release of sterilised males, the Program now hopes that Wolbachia-carrying Aedes will naturally reproduce, a possible long-term solution to the spread of vector-borne diseases. As of September 2021, treated mosquitoes have been released in 11 countries in the Asia Pacific, Australia and Latin America.

Program data for the release areas shows a significant reduction in dengue infection rates. A recent release in a control zone at Yogyakarta, one of Indonesia’s major cities, reduced the number of dengue infections of all 4 serotypes by as much as 77%.  The number of patients needing hospital care also fell by 86%.

Still no light at the end of the tunnel

Sungsit thinks that limits remain to the use of Wolbachia. In real-world settings, Wolbachia-carriers mingle with dominant populations of local mosquitoes and, given their short lifespans, will need to be repeatedly released to have an impact.

He added that for countries like Thailand, places that are not island nations, new sets of mosquitoes can easily migrate into the area, and noted as well that there was also the possibility of mosquitoes overcoming control procedures. Probably because of such concerns, the Thai Ministry of Public Health remains cautious about relying on adult mosquitoes to control dengue.

Caution is warranted. A BBC report from Brazil noted that mosquito populations which had earlier been checked with biologically-modified mosquitoes rebounded in 2019, after an 18 month decline - an outcome contrary to what was supposed to happen.

“Different approaches need to be taken for island nations and places like Thailand which sit beside neighbouring countries. For archipelago countries, the sterile insect technique is likely to work - the population will decrease and no new mosquitoes can enter the country. But in a country surrounded by water like the United States, it still took ten years to control populations of biting flies using irradiation and release. They were trying to control a veterinary problem but solved it for people as well. That’s why, for about 10 years, I think, they tried releasing sterile males again and again and again, even after the fly population was decreasing. It worked because America has ocean on both sides.  It is a separate continent …”

“As for other countries that have succeeded … Vietnam, I guess. They succeeded but on an island. To do it in Thailand … we tried once already with the common fruit fly. We used radiation … to reduce the fruit fly population. We eradicated them in one garden and others flew over from the garden next door. Despite our diligence and ongoing efforts, there was no way to know if our work would ever succeed.”

“We’re choosing genetic modification, using genes from Wolbachia and the like. We are releasing modified creatures back into the environment. But we forget that it is in the nature of living beings to want to keep living. Living things don’t think about becoming extinct. They try to survive, especially insects. No matter how we try to kill them, they always find a way to survive. And when they do, we can’t go back and fix things,” Sungsit said.

With superior laboratory facilities, the Faculty of Science at Mahidol University still leads in researching the use of Wolbachia and sterilisation. According to Dr. Darin Arichokchai, Deputy Director of the Division of Vector-Borne Diseases, Ministry of Public Health, the Division’s main role is onsite testing.

For the immediate future, divisional policy is determined by the limited number of labs in the country.  The demand for modified mosquitoes cannot be met. As a result, new measures will still be used alongside traditional control measures - working with local organisations and communities to eliminate breedings areas and control mosquito larvae.

“To control disease by freeing sterilised and Wolbachia-infected mosquitoes, enough must be released to replace natural populations. Simply speaking, in the long term carrier mosquitoes in the wild need to replaced. To do this, a huge number need to be released.

Right now, we are limited by the fact that existing labs in Thailand cannot produce mosquitoes on such a large scale. Let’s say we need yellow fever mosquitoes for 100 households. We would need 100,000 mosquitoes, but we can only produce 20,000. We’re still not able to actualise our plans but it’s something we have to do, to expand our potential into the future.”

Convergence is the way forward

The Division of Vector-Borne Diseases is also using information technology to control of mosquito larvae. Raw data is gathered using an online platform and application developed for the Village Health Volunteers (VHV) and public health officers. The resulting database, dubbed the Outbreak Tracker, can be used to evaluate risks and follow outbreaks as they occur.

Other technological advances are also being made.

“There are some innovative new mosquito traps that not only trap and kill mosquitoes but also count them. Some of these are already being tested and there will be more in the future. In collaboration with the National Science and Technology Development Agency (NSTDA) we are looking to use AI to trap Aedes mosquitoes.

A photo of research conducted on mosquitoes by the Division of Vector-borne Diseases.

"Currently we produce risk assessments to see which areas have dengue outbreaks. Our people go onsite to survey mosquito larvae. It requires a lot of people but we’re thinking how AI could be used to help us survey, without sending people to catch mosquitoes.”

“We made heavy use of the Outbreak Tracker platform in 2018-2019 when we organised a volunteer program to eradicate mosquito breeding areas. That year, volunteer campaign programs were organised all across the country and use of the application increased. Controlling Aedes mosquito larvae was a part of the program. We initially anticipated that dengue would spread heavily, that we would have around 140,000 cases around the country.  As a result of the continuous campaigning, the number of cases for the year was only 80,000,” Dr. Darin said.

Interdisciplinary integration is currently another important step in controlling dengue, as can be seen from a Global Vector Control Response (GVCR) scheme that was approved by the World Health Assembly, WHO, in May 2017.

GVCR is a strategy to reduce the morbidity and mortality rates of vector-borne diseases by raising disease control awareness.  It calls for the cooperation of organisations outside of public health sector and coordination of approaches to make public health databases, conduct pandemic investigations, build community cooperation, and develop basic infrastructure to reduce carrier-insect reproduction.

In addition to the efforts of public health organisations, it calls for agricultural authorities to improve hygienic management of water system on farms; for communities to survey water sources at risk of becoming insect larvae breeding areas; for Public Works authorities to improve basic infrastructure to reduce breeding areas; and for the promotion of health knowledge by the media, as well as in schools, religious institutions and workplaces.

Using 2016 morbidity rates a baseline, GVCR aims to reduce vector-borne disease infections worldwide by 25% a year from 2020, 40% in 2025 and 60% in 2030.  The goal is to all but eradicate the disease within 2030.

In an online seminar hosted by the Thailand International Cooperation Agency (TICA) and the Technical Cooperation Directorate of Singapore in January 2021, GVCR plans for Thailand were discussed as a part of a vector-borne disease prevention and control curriculum focused on dengue fever.

Dr. Darin, a speaker at the event, told Prachatai that it was important to use existing tools together with public cooperation, instead of focusing on a single method.

“There is no single solution to controlling mosquitoes. Control requires that lots of measures be used together. If the question is, would eradicating grown mosquitoes still require fogging or not? In the next 5-10 years, probably yes. But the method of fogging may change. Before, people laboured carrying back-pack foggers that weighted over 10 kilos – in the future we may use drones, but fogging will still be needed.

Mosquito control rally outfit at the Division of Vector-borne Diseases.

“Getting rid of mosquitoes breeding areas still requires public participation. But in any case, we all need to look after ourselves, look after our surroundings and avoid getting bitten by mosquitoes. With regard to innovation, we can do risk assessment faster, and reduce the mosquito population in outbreak areas more rapidly. Lastly, we can continue to develop an effective information system that links data on people, the virus and mosquitoes so that we can analyse them together and have a clear picture of the risk of infection in each area.

“As for dengue vaccines, honestly, right now vaccines are not sufficiently effective. There is one (Dengvaxia vaccine) but data indicates that its effectiveness is not as good as it should be. In the near future, another new vaccine may come out and be registered in Thailand. But vaccines are an expense that comes out of the national budget. If we develop them, they come at a cost and it will take some time before they are cheaper. That’s why I say there is no one single solution, no one method that will solve everything. All the measures need to be utilised together.”

Sungsit at Mahidol University agrees that integrating disease control measures, a so-called “One Health” program, is important. In the case of Aedes mosquitoes, they not only carry the dengue virus, but also yellow fever, zika, and chikungunya. They can also cause lumpy skin disease on cows with viral infections in their mouths. As a result disease control measures needs to consider livestock as well as people.

A Bangkok sewer and mosquito fogging smoke.

“We cannot say for certain that in the future, Aedes mosquitoes from our region will also spread yellow fever like in Africa and South America. But there are lots of other viral diseases. If we go south, Aedes mosquitoes are carriers of elephantiasis and Wuchereria pahangi, a name derived from Pahang state (in Malaysia).”

“Aedes breeding areas are close to humans. We make these breeding areas for them.  It is almost as if we make them to hurt ourselves. And now, we are responsible for the bigger picture … we have pets and livestock, and we are the source of Aedes mosquitoes,” Sungsit said.

International guideline aside, integrated control measure face challenges on the streets of Thailand where public cooperation is often lacking and land-usage regulations remain weak.

“Clean water is on private land.  Water in public places is not that clean. It flows from abandoned buildings and plugged drainage pipes. When the rain falls, the water pools up.”

“Elsewhere in the world, people living in urban areas are told to take care of their property. Because if it is left unattended, if a fire happens, if wild animals move in, it will affect adjacent houses. If you don’t take proper care of the water on your property, if you don’t thoroughly drain it, the ground becomes saturated, creating problems for you and your neighbors.”

So said Thai City Planners Society president Assoc Prof Panit Pujinda, a man who once contracted dengue fever at a construction site. Discussing the problem of careless land management. Panit explained that the well-being of people in urban areas depends upon two key factors: owner accountability for land mismanagement and strict law enforcement. Thailand lacks both.

“If we assume that everyone is a good person, we might conclude that they are unaware of the negative impact their actions have on others. In this case, the solution might be to engage in public relations so that they know what they are doing to others. This might reduce the problem by half.”

“But there is another group that thinks ‘I don’t give a damn’. These people require more management.  They need to be advised.  Active measures need to be taken to the extent of using the law to force them to change.  Measures from light to heavy should be employed … but at present, not even the first steps have been taken.”

In terms of law, Asst Prof Panit suggests that current land tax regulations that are not used in line with city zoning which determines the category of land in question. Landowners in urban areas can still avoid paying fines on vacant land by planting gardens. Gardens in densely-populated areas expose more people to mosquitoes.

When a city expands without good management, those who reside in undeveloped areas with cheaper places to live face the most mosquito exposure.

“Other cities in the world use the mechanism of land tax. Core urban areas fill up first. When land in the city is fully utilised, the undeveloped areas that are a source of disease, crime, illicit gatherings and dangerous animals disappear. As adjacent undeveloped areas decrease, less people die from contagious disease.”

“But in our cities, undeveloped areas can be found in the heart of town … Something else should be there but in the middle of Si Lom [in downtown Bangkok], people are still planting mango and lime trees. The area is prepared with power and water for 30-40 story buildings. It even has electric rail transport. And then they say that planting lime and mango trees is all it takes to avoid paying for infrastructure. There is no incentive to solve the problem of underdeveloped urban land like this.  Instead, the city expands outwards.”

“Eventually, suburban areas are also infected with dengue because they are located beside farm areas. If it was all farm, the virus wouldn’t affect people so much but when housing estates are located close to farm areas, mosquitoes from the area reach people,” said Panit.

This special report series is supported by Internews' Earth Journalism Network.

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