“The fatigue was so bad I couldn’t stand, and that’s terrible when you need to take care of a child, right? My head hurt, my eyes hurt – I couldn’t keep them open for long,” remembers Ana Luísa Braga.
The 38-year-old, a social worker and mother of a three-year-old, from Belo Horizonte, the capital of Minas Gerais state in Brazil, fell ill with dengue fever in March.
She is one of millions to feel the effects of the virus this year as the world grapples with its largest ever outbreak. Cases reached their highest number on record in 2023, with 6.5m reported globally – and 2024 looks likely to double that figure, with 12.4m cases so far.
The mosquitoes that spread the virus are benefiting from increased urbanisation, and changes in climate and temperature. At the same time high obesity rates and other chronic health conditions are making people more vulnerable, scientists say.
Dengue is sometimes known as “breakbone fever” – a description that resonated with Braga. “I consider myself to have a high pain tolerance, but the pain was so intense.”
She needed hospital treatment, after deteriorating to the point that she was vomiting and could no longer eat or drink. “Even after being hospitalised for five days, I only gradually started getting better. The fatigue, in particular, didn’t leave me for about 15 days,” she says.
The World Health Organization estimates that 4 billion people are at risk of dengue and related viruses, rising to 5 billion by 2050. The rapid spread over recent years is “an alarming trend”, says WHO director general Dr Tedros Adhanom Ghebreyesus.
On 3 October, he launched a global strategy “to fight rising dengue” and other diseases spread by the same mosquitoes, such as Zika and chikungunya. Actions in the plan, ranging from surveillance to mosquito control efforts, will require $55m (£42m) over the next year.
Deadly combination of factors
Most of this and last year’s dengue cases are in Latin America, with high numbers also reported in parts of Africa and south-east Asia. But infections are cropping up worldwide, including in mainland Europe.
The world faces a combination of factors including the climate crisis, increased migration and urbanisation, says Prof Sophie Yacoub, head of the dengue research group at Oxford University’s clinical research unit in Ho Chi Minh City, Vietnam.
The mosquito species that tends to carry dengue is Aedes aegypti, particularly in south-east Asia. But a second type can also spread the virus: Aedes albopictus – the tiger mosquito. It can survive in a greater range of temperatures, and has established itself in new areas. It is well adapted to urban settings and can breed in a tiny amount of standing water.
“That is what is causing local transmission in Europe, for example, and it is also pushing up into China and into the southern states of America,” says Yacoub.
Changes in climate not only help the mosquitoes to survive in new areas, but may cause extreme weather events such as flooding, creating new breeding grounds. Heatwaves can speed up reproduction cycles.
Dr Najmul Haider, a lecturer in epidemiology at Keele University, published a paper in September in the journal IJID Regions, an official journal of the International Society for Infectious Diseases, highlighting surging case numbers. He wanted, he says “to raise an alert that this is an alarming number, and the number is increasing”.
It is also, he says, likely to be an underestimate: “Remember, most of the dengue cases are asymptomatic or very mild, they don’t go to the healthcare system, they are not diagnosed, so they remain unnoticed.”
Haider warns that a second infection could be riskier than the first. This is because dengue fever has four distinct versions, known as serotypes. Each interacts differently with antibodies in human blood.
People develop immunity to the serotype they are first infected with. But during a subsequent infection, with a different serotype, the body may produce antibodies to the first serotype it encountered. Those bind to virus particles, but fail to neutralise them. Instead, they end up creating an extreme immune response, which can end in organ failure.
Part of the reason for the rise in cases in Brazil is that all four serotypes appear to be circulating simultaneously for the first time, officials say.
Haider said dengue’s historically lower death rate than malaria, a disease spread by a different type of mosquito, had made it less of a priority – particularly in resource-limited countries.
However, its fatality rate is starting to change, says Yacoub. “A lot of people dismissed it as another viral illness – but actually we are seeing some higher mortality now.
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“There are similar risk factors to Covid-19 – so people with obesity, or diabetes, or other comorbidities have a higher risk of getting severe dengue and higher mortality. And I think as dengue is affecting many countries that have also increasing obesity or diabetes, these two pandemics are kind of colliding.”
In Vietnam, she says, “we are now seeing a lot of obesity in our teenage kids, and they’re the ones who are coming in [for hospital treatment]”.
Clinical trials and vaccines
There is no treatment for dengue, with hospital care typically supporting the body while it fights the infection. A trial of a promising antiviral being developed by Johnson & Johnson was discontinued earlier this month.
Yacoub’s group is conducting trials to find out whether repurposed antivirals already in use for other infections may be effective against dengue, and others to see whether therapies that focus on altering patients’ immune responses could help. She is hoping for results within the next few years.
There are also efforts to set up a large global trial looking at multiple treatments simultaneously, akin to the Recovery trial that during Covid-19 established that cheap steroid dexamethasone could help patients. “It would just be a constantly recruiting platform trial across 10 different countries, with thousands of patients being enrolled,” Yacoub says. “That kind of size trial is what we need to really generate that robust evidence.”
Two vaccines against dengue have made it to market but one, Dengvaxia, is only useful in people who have been previously infected – otherwise it can increase the risk of severe infection. This limits its usefulness, particularly for mass immunisation campaigns.
The second, Qdenga, is seen as more promising and is being used in Brazil to bring cases under control in dengue hotspots. Daniel Soranz, Rio’s municipal health secretary, says: “In the 1940s, Brazil organised itself to vaccinate the entire population against yellow fever and eradicate it. So, it’s possible to do the same with dengue. I believe that in about five or six years, once we have the vaccine produced on a large scale, we will be able to eradicate dengue in Brazil.”
The traditional remedy route
For now, and in the absence of new medicines, many people in hotspots are using traditional remedies such as papaya tea, often on the advice of medical professionals and despite a lack of scientific evidence to support their use.
Solange Fletcher-Herbert’s 16-year-old son Sjelan caught dengue while on holiday in Tobago. She knew to avoid aspirin and ibuprofen because they can increase the risk of bleeding complications. “We started to give him Pepto-Bismol to settle his stomach, paracetamol and just coconut water to keep his electrolytes up,” she says.
After he had been ill for a few days, she sought help at a private hospital close to her home in Diego Martin, Trinidad. “He couldn’t eat, he was really, really ill,” she says. “They had to put him on drips.”
When he was discharged the family was told to focus on raising his platelets. One close friend, a doctor, suggested they try brewing a tea with young papaya leaves for Sjelan – a remedy also endorsed by the teenager’s granny.
“So we got it for him, boiled it, and within two days, he had to go back to test his platelets. Within two days of drinking it, his platelets were all the way back up and he was doing so much better,” says Fletcher-Herbert. “Just to see how he turned around, I swear by those papaya leaves. And I know a lot of people who had the same experience say the same thing.”
Dr Michael Head of the University of Southampton says there is some evidence that compounds in papaya could have an impact on blood clotting, but little is known about what dose would be needed to be effective.
“Given dengue is a high burden and occasionally serious infection, treatment options would be useful,” he says. “Further research into potential antivirals, whether based on papaya or anything else, are likely to be required for long-term global control of dengue fever.”