May 27, 2024

Narrator: This is the dengue virus. If you were infected right now, you might not even notice. But symptoms can range from a high fever to severe headaches  and vomiting. Or in the most extreme cases, death.

After a record-breakingly hot 2023, dengue has become alarmingly prevalent. Last year, across Latin America and the Caribbean, there were at least 3.5 million infections, with Jamaica witnessing ten times more than the year before. Bangladesh experienced its deadliest year of infections. And Peru’s health minister even resigned in the face of the country’s worst outbreak.

The thing is, 2023 doesn’t come as a surprise. Over the last 20 years, reports of dengue have increased tenfold. Roughly half of the world’s population is now at risk. So – what’s really to blame for the rise in dengue?

Okay, it’s complex. There’s lots of interconnected factors. Everything from vaccine access to maybe even the latest El Niño event. But even then, it can still be traced back to the main spreader of the disease: Aedes mosquitoes. Specifically, Aedes aegypti and Aedes albopictus.

These two pretty small species are common culprits when it comes to spreading dengue, alongside other viruses like chikungunya, Zika and yellow fever. With bodies and legs wrapped in stripes, they bite throughout the day, easily transmitting the virus from human to human. These mosquitoes used to live mostly in tropical and subtropical regions where they were found in forests, living in small crevices like tree holds. Today, they can be found on every continent except Antarctica, thriving within towns and cities as successfully as they do in rural environments. So, how have they spread so quickly?

Well – us humans are helping. A combination of growing international travel and urbanisation allows the disease-carrying mosquitoes to reach more places. And they’ve adapted exceptionally well to their new urban home. Aedes mosquitoes can lay their eggs in water tanks, buckets or bottle caps, even pet drinking bowls. Essentially anything that holds water. They’ve also become more resistant to insecticides, meaning they’re hard to repel and kill. And because Aedes aegypti and Aedes albopictus are active during the daytime, common interventions like bed nets aren’t effective.

Of course – there’s another factor at play. Climate change.

Dr Felipe Colón-González: Countries with suitable climatic conditions, particularly those in tropical and sub-tropical regions are at higher risk of new populations of Aedes mosquitoes and dengue.

Narrator: This is Dr Felipe Colón-González, Technology Lead, here at Wellcome.

Dr Felipe Colón-González: For example, South East Asia has the highest risk globally. Rising temperatures increase the climatic suitability and the geographic range of Aedes mosquitoes, and so, this expansion can lead to the introduction or establishment of mosquito populations and dengue fever in areas of the world where the disease and the mosquitoes have been absent or have been sporadic.

Narrator: Look at Europe, where changes in temperature and rainfall patterns are affecting the behaviour of the Aedes mosquitoes already as they venture further North and West across the continent. Today, there are nearly triple the number of regions where Aedes albopictus are fully established, compared to a decade ago.

Dr Felipe Colón-González: Changes in rainfall, either through excess rainfall or limited rainfall, can influence for example, water storage practices. If people start storing and collecting more water, that creates opportunities for mosquitoes to thrive.

Narrator: And more Aedes mosquitoes means more transmission potential for the dengue virus. In 2022 alone, the number of locally-transmitted dengue cases reported in Europe was as high as the previous ten years combined. Last year, in 2023, this number jumped even higher to at least 128 cases. But it’s not just that they can reach a wider area. They’re also staying around for longer, too. Sometimes by months. Bangladesh experienced exactly this. Here, the climate has become increasingly suitable for Aedes mosquitoes to thrive. Over time, the dengue season has become longer and more intense.

Dr Felipe Colón-González: As temperature surpasses 16 degrees celsius, the mosquitoes become more able to transmit the disease. But this relationship with temperature is not linear. So that means that, as temperatures increase, the rate of transmission increases. But if it increases too much, then it starts declining again. So there’s kind of an inverse U-relationship with temperature. And the same you can say about precipitation or rainfall.

Narrator: With these mosquito species able to spread the virus into brand new territory, dengue is now endemic in over 100 countries. And many millions and millions more people will be at risk of dengue by the end of the century.

Thankfully, researchers and teams are working hard to offer solutions. Like Dr Sophie Yacoub, based in Vietnam.

Dr Sophie Yacoub: Dengue control programmes have predominately focused on vector control. But I think this hasn’t been working. So we do need new methods of controlling dengue. And that includes: improving our surveillance systems, we need a comprehensive vaccine programme as well as treatment strategies. Currently we have no treatments for dengue, no antivirals and no host-directed therapies.

Narrator: Early warning systems play a vital role in tackling the rise of dengue, because with enough advance notice, we can limit disease outbreaks. However, whilst many early warning systems for infectious diseases have been proposed, the transition from theoretical to reality is rare. There are projects all over the world aiming to improve this, though.

Dr Sophie Yacoub: We’ve just started a project, it’s called DART, which starts for Dengue Advanced Readiness Tools and it’s a dengue forecasting tool. It’s not only an early warning system for public health institutions, it’s also gonna be one of the first that can warn the general public.

Narrator: These kinds of tools are crucial as we continue to live alongside dengue. Sure, 2023 was intense but this year could be even worse for many parts of the world. Like South America. During the first few weeks of 2024, cases of dengue in Brazil’s capital already exceeded the total for the whole of 2023. And a failure to slow global warming will only increase the risk of infection. We need to act faster.

Modelling suggests that cutting emissions, for instance, can significantly reduce transmission in future. When we combine these commitments with further research, vaccine development, innovative mosquito control, and better disease monitoring, we’ll eventually slow the spread, protecting billions of people from dengue.

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