If you can’t beat your enemy, you know: join him. Of course, to find new ways to beat it. In Singapore they have a problem with dengue, an infection transmitted by mosquitoes that, among other symptoms, usually causes fevers, pains and skin rashes, so its authorities have decided… to breed mosquitoes of the species that transmit the disease. And they also do it in large numbers, by millions, to then release them through the streets and urbanizations of the state.
It sounds crazy, but it makes all the sense in the world.
What happens in Singapore? That he wants to face dengue, an infection transmitted by mosquitoes and that is located mainly in tropical areas, such as certain regions of Africa, Asia or Central and South America and Pacific islands. Although it is not usually serious, it does cause discomfort to those who suffer from it and can even manifest itself in a more virulent type that causes bleeding and fatigue.
According to data from the US CDC, it is estimated that each year about 400 million people are infected with the dengue virus and about 100 million become ill. As for deaths, there are some 21,000 cases attributable to the disease.
Is the infection important there? The National Environment Agency (ANE) of Singapore keeps a detailed control of the registered infections, in the short and long term. Its statistics help to verify how the evolution has been over the last years… and days. During the week of February 5 to 11, for example, the agency registered 219 reported cases. So far in 2023, up to the sixth week, the Singapore agency had accounted for about 1,500.
And what is Singapore doing? A few years ago, in 2019, Undark specifies, the NEA activated a program that basically consists of breeding millions of Aedes aegypti mosquitoes, also known as “dengue” or “yellow fever”, nicknames that give a fairly accurate idea of how uncomfortable that I could be living with them. They take care of them conscientiously, watching each phase, to then release them around buildings and streets. The NEA even publishes maps and a schedule of the areas where it releases them, sometimes with the help of trucks.
Undark specifies that every week the insects raised by the NEA generate 24 million eggs. The mosquitoes began to be released in 2016, although the program itself started in 2019. Over time it has grown from releasing about two million insects each week to reaching five million by 2022.
And that… for what? Well, as paradoxical as it may sound, to limit the spread of dengue. The key is in the type of mosquitoes that the NEA technicians release: they are not just any insects, but Aedes aegypti that meet two fundamental conditions. First, that they are male, so they do not bite or transmit diseases. What they are looking for is sugar that they obtain from plants, unlike females, who do need proteins from the blood.
Their second peculiarity is that they are carriers of Wolbachia, a bacterium found in more than 60% of insect species, including butterflies, bees or dragonflies. Also in certain mosquitoes, such as Aedes albopictus or Culex quinquefasciatus. Those who do not transmit it to each other naturally are the Aedes aegypti. That is one of the keys to the Singapore program.
What is special about the bacterium? That it can become a valuable ally to reduce the risk of dengue infection. As? Using the mosquitoes bred by the NEA as a modern “Trojan horse”. “When male mosquitoes that carry Wolbachia-Aedes mate with urban females that do not carry it, the resulting eggs do not hatch,” highlights the agency, which advances the final goal: that, over time, the “continuous releases” of mosquitoes with the bacteria end up leading to “a decline in urban populations.” In other words, fewer swarming specimens of the annoying Aedes aegypti.
“This not only reduces the risk of dengue, but also of other diseases, such as Zika and chukungunya,” insist the Singapore experts. To achieve this they need to introduce the bacteria into the mosquitoes by injecting it directly into the eggs. “Wolbachia is transmitted through the mother. Female carriers transmit the bacteria to their offspring, thus generating a stable line.”
Is it giving results? The data handled by NEA certainly invites optimism. Its experts carried out a study in various locations and found that the release of male specimens with the bacterium reduced Aedes aegypti populations by up to 98%. “Consequently, the central areas of the study sites with at least one year of releases saw up to 88% fewer dengue cases compared to areas without releases,” he ditch.
The system, of course, also has its weak points, such as a temporary increase in male mosquitoes after their release, the possibility of a female slipping in or endangering other species in the ecosystem. The NEA, however, speaks of a “negligible” or “low” danger and claims: “It is safe, without risk to human health and with negligible risk to the ecology.”
Is it the only strategy with the battery? Not really. Experts have verified that there is another way to combat dengue infections that is also based on Wolbachia. The one they use in Singapore is called “suppression”, an eloquent label that clearly identifies what its objective is: to eliminate mosquito populations. Hence, only males with the bacteria are released.
The other is called “replacement” and what it proposes is to release males and females with Wolbachia so that they introduce the bacteria among the insect populations —when both members of the couple are carriers they can reproduce successfully— in the city. Because? Well, because mosquitoes with Wolbachia see their ability to transmit diseases, including dengue, reduced.
Is it only used in Singapore? The answer is negative again. A little over a decade ago, mosquitoes with Wolbachia were released in Australia precisely to combat dengue and the possibilities of the “replacement” method have encouraged the World Mosquito Program to launch initiatives in a dozen nations, such as Mexico or Vietnam. An interesting one has also been registered in California.
One of the great contributions of Singapore is the improvement of the breeding system, a complex process that requires, for example, careful counting of the larvae or the separation of the specimens by sex. In the city-state, for example, they use an automated counting system that allows them to guarantee that each of the larvae trays contains exactly 26,000, essential for the agency’s experts to maintain breeding conditions.
Imágenes: DFID – UK Department for International Development (Flickr)
