Rising temperatures across Asia and the Americas have contributed to multiple severe outbreaks of dengue fever globally over the past six months, making 2019 the worst year on record for the disease.
In 1970 only nine countries faced severe dengue outbreaks. But the disease, which is spread by mosquitoes that can only survive in warm temperatures, is now seen in more than 100 countries. There are thought to be 390 million infections each year.
Thais Dos Santos, an expert adviser on the surveillance and control of arboviral diseases for the Pan American Health Organization, said dengue was reaching new areas as temperatures rise.
“The hotter the climate, the better the mosquito is at breeding, but it is now coming up higher and higher to cities like Bogotá and Mexico City that are well above sea level, cities traditionally believed to be safe from the Aedes mosquito.”
Across Asia the scale of outbreaks has been taking communities by surprise. In Pakistan one city hospital in Rawalpindi admitted more than 2,000 dengue patients in a single weekend in October. Across the country a record-breaking 47,000 people have been infected this year, almost double the previous high of 27,000 in 2011.
Other Asian countries, including Bangladesh, Thailand, Cambodia and Laos, also had outbreaks, with a particularly severe situation in the Philippines, where 1,000 people died of the disease, including hundreds of children.
Mosquitoes have also brought dengue to places where it has never been seen before, including the hills of Nepal, never warm enough in previous years for the disease to take hold. In Nepal there have been nearly 14,000 cases of dengue since mid-July. The previous record was 2,000 two years ago.
The dengue season in Kathmandu is usually short, mild and over by November. But Dr Dev Basu Pandey said he was still treating dengue patients on his ward at the Sukrara Tropical and Infectious Diseases hospital in the Nepalese capital. “Usually there would not be dengue still at this time of year,” he said. “But this year it is still warm, even once the sun has set. We have never seen an outbreak like this; people are afraid.”
Average maximum temperatures in Nepal increased between 1971 and 2014 with the overall average annual maximum temperature of the whole country rising by 0.056 degrees Celsius. The study by the Department of Hydrology and Meteorology also noted that temperatures increased most in some of the highest altitude parts of the country.
The spread of the disease is heavily related to human activity. The insects thrive in human habitation, laying their eggs in used tyres, flowerpots, tree holes and any water-filled container.
As climate change alters monsoon and rain patterns in many countries, this creates the damp and warm conditions that, along with rapid urbanisation, help the Aedes mosquitoes flourish.
The Americas also broke records for dengue. More than 2.7 million cases were reported across the region by mid-November, the largest number since records began, and a 13% increase on the total yearly number in 2015, the previous record year. Over 2 million of the cases came from Brazil.Dos Santos said there were clear changes in dengue patterns. “Dengue is episodic so we would expect to see a larger than normal epidemic every two to five years but this year was unusual. We are seeing a shift in seasonality, seeing outbreaks start earlier.
“Another unusual thing this year was a heavy circulation of the second type, dengue 2, which is well known for causing severe forms of the disease. We saw a lot of severe cases and death this year.”There is also another way that hotter temperatures help dengue spread. “After a mosquito bites an infected person and drinks up the virus in the blood, the virus needs to spend time in the mosquito’s gut before it can be passed on through a bite. As the outside temperature increases ... the time for the virus to survive and be able to be passed on is reduced.”
Dos Santos said he wants people to see how rising temperatures and climate change are beginning to harm human health. “We are detecting a shift to a younger population, most of the cases in Central America were in under-15s. Sadly, most severe cases and deaths were disproportionately affecting children.
“We see children dying but how this is related to the climate is not always made explicit. [We need to do] a better job of connecting these constellation of ways in which the change in climate is affecting us.”
Rachel Lowe, of the London School of Hygiene & Tropical Medicine, said the current situation with dengue spreading to new areas was caused by a multitude of factors, including poverty and slum-like living conditions.
“This year’s outbreaks are unprecedented, part of 30 years of a global spread. We have seen dramatic increases in global temperatures and also unplanned urbanisation. There are a combination of factors that lead to dengue, including living conditions and the way people are forced to live without refuse collection.”While many vaccine trials are under way, she and her colleagues are looking at preparing for outbreaks using early warning prediction systems.
“We have a UK space agency-funded project to develop early warning in Vietnam and we are working with the Met Office and UNDP to develop seasonal forecast systems in Brazil and Equador. We are trying to improve preparedness so we can estimate six months in advance where you can go and target limited vector control and make sure health facilities know there is an epidemic under way.
Researchers from LSHTM and Oxford University produced figures earlier this year to show that if warming continues globally the two main disease-spreading mosquitoes – Aedes aegypti and Aedes albopictus – will significantly expand their range.
The models predict that by 2050, 49% of the world’s population will live in places where these species are established if greenhouse gas emissions continue at current rates, and if they are not curbed, even greater areas will be at risk.
“If no action is taken to reduce the current rate at which the climate is warming, pockets of habitat will open up across many urban areas” said Moritz Kraemer, an infectious disease scientist at Boston Children’s Hospital and the University of Oxford, who co-authored the research.