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Dengue outbreaks are spreading across the globe thanks to a “perfect storm” of climate change, rapid urbanisation and intercontinental travel, experts have warned.
So far this year 1.6 million cases of the mosquito-borne fever have been reported in South America, with 80 per cent of these in Brazil.
The Nicaraguan government has also issued an alert after 55,000 cases were reported in the last seven months, and Honduras is facing its worst outbreak for 50 years.
Numerous countries in southeast Asia have also identified a surge in the number of cases. The Philippines declared a national epidemic this week after 146,000 infections and 622 deaths were recorded so far this year, while one hospital in Bangladesh logged 1,300 infections in just 24 hours in July.
In the 1970s the flu-like virus – colloquially known as break bone fever because of the severe pain it causes – was endemic in just nine countries.
But now severe dengue has taken hold in more than 100 countries, with roughly half of the world’s population now at risk, and is estimated to affect around 100 million people every year.
The disease is fatal only in extreme cases but the symptoms are extremely unpleasant, including a high fever, severe headaches and vomiting. It also exacts a heavy economic burden on countries as sufferers are unable to work, as well as overwhelming health services when there is a severe outbreak.
Becca Standish, a 25 year old from West Sussex, developed dengue when she was travelling in Thailand six years ago. She likened having the virus to being “in a pressure cooker”.
“I felt like someone was trying to crunch my head, like they were standing on the inside of my brain,” she told The Telegraph. “I was sweating profusely, I was so hot and in so much pain, it felt like my bones were breaking.”
An ‘explosion’ of dengue fever
Dengue is spread by the Aedes aegypti mosquito – which also transmits chikungunya, yellow fever and Zika – and is common in tropical climates. The insects typically bite people during the day – unlike the mosquitoes carrying malaria, which are active at night.
After yellow fever was largely wiped out in South America in the 1970s authorities stopped controlling the Aedes aegypti mosquito, allowing the dengue virus to quietly expand.
Rapid, unplanned urbanisation in parts of Latin America and south east Asia and warming temperatures have also created the “perfect” circumstances for dengue to flourish.
Between 2007 and 2017 deaths from the fever rose by two thirds, from 24,500 to 40,500.
“Warmer climates have created the optimum conditions for dengue to spread more quickly, as the virus replicates faster and mosquitoes breed more quickly,” said Dr Rachel Lowe, assistant professor at the London School of Tropical Medicine and Hygiene.
“But at the same time, we have seen massive rates of population growth and increased levels of unplanned urbanisation, which often leads to inadequate services and infrastructure.
“This perfect storm of increased environmental suitability and close contact between mosquitoes and humans – particularly in poorly serviced and densely populated urban areas – has allowed explosive outbreaks to occur,” she added.
In particular poor waste systems, piles of rubbish and stagnant or stored water can provide the perfect breeding ground for the Aedes aegypti mosquito.
But globalisation has also played a part in the steady rise of dengue.
“Increased interconnected travel and trade can move the mosquitoes and the virus across borders,” said Dr Anne Wilson, epidemiologist at the Liverpool School of Tropical Medicine. “When an infected traveller gets off a plane in a new destination, they can start another outbreak.”
Although the fever is most common in South America and south east Asia – tropical regions which have seen rapid population growth and urbanisation – experts are concerned that it will spread further.
“It’s fair to say that dengue is one of the few infectious tropical diseases that is still showing a rapid increase in numbers,” said Dr Tim Knott from the Wellcome Trust. “This is driven largely by the rapid spread of the Aedes mosquito into areas it wasn’t previously found, and the bulk of that is due to man-made factors.
“This mosquito is relatively immobile, it doesn’t travel much more than 100 metres in its entire lifespan. So if it was reliant on natural measures, it would take a long time to see the global spread we’ve witnessed over the last few decades.”
According to research published in Nature Microbiology earlier this year, two billion more people will live in dengue areas in 2080 compared with 2015 if the current rate of climate change continues. People living in the southern Mediterranean, coastal areas of China and Japan and the south eastern states of America will be at risk of catching the disease.
But dengue is also becoming an increasingly large threat in Africa. The disease is already endemic in 19 countries, but surveillance is patchy and the fever is often misdiagnosed.
“Dengue presents a big risk for sub-Saharan Africa, but few people have studied it there so it’s not well understood,” said Dr Wilson. “Previously, all fevers in Africa were considered to be malaria, but as malaria is decreasing we may start to uncover more dengue.
“Vector control programmes in Africa are often ill-equipped to deal with Aedes mosquitoes,” she added. “As the mosquito is active during the day and has different habitats to malaria mosquitoes, many of the mosquito control tools used for malaria won’t work.”
Containing dengue’s emergence
But prevention is vital as it is the main way to control disease – there are four strains of dengue, making it difficult to develop treatment options. There is currently no specific drug to cure dengue and Dengvaxia, the first vaccine developed, is effective only in people who have already had the disease.
“We don’t yet have a well distributed vaccine and often mosquito control programmes disorganised or inadequately funded,” said Dr Lowe. “Preventive vector control measures and improved infrastructure are key to contain dengue.”
Dr Wilson added: “As Margaret Chan, former director general of the World Health Organization, said – we dropped the ball on mosquito control in the 1970s, and we’re paying for that now.”
While stopping the mosquito from proliferating is key researchers at the World Mosquito Program are trialling a more sophisticated method of controlling the disease.
They are releasing mosquitoes with Wolbachia – a natural bacteria present in up to 60 per cent of insect species – into the mosquito population in parts of Brazil, hoping that they will breed with the wild population. Wolbachia infected mosquitoes do not transmit dengue and other diseases such as Zika and yellow fever.
The first mosquitoes were released in parts of Rio in 2014 and five years on they are still present in the population.
Dr Luciano Moreira, project lead for the World Mosquito Program in Brazil, said that the beauty of the Wolbachia project is that it’s sustainable.
“The way it works is that we release 150 mosquitoes – males and females – every 50 metres. In some areas 16 weeks is enough for these mosquitoes to take over. When we see 60 per cent we have an idea that number will just grow over time. And we don’t have to do any more releases,” he said.
Researchers hope to have protected around a million people in Brazil by the end of the year. But at the moment it’s too early to say whether the insects are preventing dengue and other diseases – in part because 2018 saw low levels of dengue in Brazil.
Next year, US researchers will begin a trial where they will deploy Wolbachia mosquitoes in clusters – 33 areas will have the Wolbachia mosquitoes, and 31 will not. The researchers will look at rates of dengue in school children.
“It’s really important for us to find out whether our work has an impact but it will take three to five years to show whether illness is reduced,” Dr Moreira said. “We need to follow this for three to five years to see if there is any impact.”
Dr Knott said: “[The programme] is using mosquitoes carrying Wolbachia to essentially replace the naturally occurring mosquito population and reduce the transmission of dengue. This is not a strategy that involves eradication or suppression of mosquito populations. So the food chain and ecology is unaffected.”
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