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As the climate warms, dengue is spreading to new areas and cases are surging

‘Dengue is the future. We’ve got to learn to deal with it.’

This picture shows an individual on the right of the image holding a pesticide sprayer gun. He is spraying towards the left where there is a bed. ULAN/Pool / Latin America News Agency
Health workers fumigate a home in the Peruvian capital, Lima, on 16 June 2023 to try to prevent the spread of mosquito-borne dengue fever. This year saw Peru suffer its worst dengue outbreak on record.

A number of countries have seen a dramatic rise in dengue fever in 2023, and experts attribute its spread to the warming climate and the greater movement of people around the world. 

With an El Niño weather phase predicted over the coming months — when Pacific ocean temperatures are warmer for an extended period and much of the planet is expected to be warmer than usual – they worry the situation could get out of control.

“We didn’t learn the lessons of the COVID pandemic,” said Christian Requena, regional head of the national medical association in Piura, the northern region of Peru that was hit hardest by the country’s worst dengue outbreak on record between March and July. “With the climate phenomena that are coming, it could be disastrous.” 

Dengue is now endemic in most countries in Latin America, but outbreaks have been spreading into new areas globally, and case numbers around the world have been rapidly increasing this year. Scientists say the warming climate is not only bringing more countries and regions in temperate, cooler zones into the range of dengue-carrying mosquitoes, but it is also increasing the pace of breeding and transmission.

In late September, both the Bangladeshi authorities and doctors in Sudan warned that hundreds had died and that health services were strained. Several other nations were on alert as well, including Argentina, Bolivia, Jamaica, Peru, Sri Lanka, and Thailand

Dengue, a mosquito-borne disease now found in 130 countries, is often symptom-free, but it can also cause high fevers, body aches, nausea, and rashes. Severe cases can be deadly. There is no specific treatment and no known cure. Dengue has been on the rise worldwide in recent decades, registering an eight-fold increase between 2000 and 2022, and more than half the world’s population is at risk, according to the World Health Organization. That figure is only expected to increase. 

Dengue case rates around the world May-July 2023

This map shows where most dengue cases can be found around the world.
European Centre for Disease Prevention and Control

Experts say an early response to the first cases, with the support of local communities to eliminate breeding sites around homes, can help prevent outbreaks. Meanwhile, the scientific community is working on new vaccines and developing new methods to fight the mosquito responsible for the infection. 

Dengue receives less attention and less funding than malaria, which historically has killed 10 times more people in an average year. But that situation could be changing.

As temperatures rise, the Anopheles mosquitoes that cause malaria – and are less tolerant of high temperatures – are likely to decline. The Aedes species responsible for dengue will spread, Michael Macdonald, an entomologist who has worked as a consultant to the WHO and other international groups, told The New Humanitarian.

“Malaria will soon be in the past, but dengue is the future,” Macdonald said. “We’ve got to learn to deal with it.”  

Dengue strains Peru’s health system

The outbreak in Peru, which has the highest case fatality rate in the Americas this year, mirrors what is happening in several other nations, and offers a glimpse of what could be to come if predictions for a strong El Niño hold true. 

In Peru, El Niño usually brings higher temperatures, rain on the desert coast, and drought in other regions. But the pattern’s warming effects will be felt across the continent, as far away as northeastern Brazil, where dengue cases also spiked this year. 

In 2017, the arrival of a warm current off the coast of Peru — dubbed a “coastal El Niño,” because it was a local phenomenon — brought warmer weather and heavy rain to the north coast, causing hundreds of millions of dollars in damages and a spike in dengue cases. 

Reconstruction was slow and interrupted by the pandemic, and most of the funds went to bridge and road repairs rather than flood control. When new flooding hit Peru this year, the country was unprepared and the health system had collapsed under the soaring number of dengue cases by the time the outbreak peaked in July, Requena said. 

In Piura, both primary care clinics and hospitals have inadequate infrastructure and equipment, he said, and there’s a shortage of doctors and other health personnel.  

Through mid-September, there were more than 250,000 cases and 419 deaths in Peru this year, compared to 77 deaths during the previous outbreak in 2017. Over 75,000 of the infections occurred in Piura, one of the country's poorest regions.

As many as 90% of cases are mild or asymptomatic and can be handled at local health centres, according to Requena. But because of the poor coordination in Peru’s healthcare system, too many people sought care at hospitals, overloading facilities that should have been concentrating on high-risk or severe cases. This led to shortages of basic medication like rehydration salts and paracetamol. By June, criticism of the health ministry's handling of the outbreak led the health minister to resign. 

The Piura regional government, Requena said, requested emergency funds, which eventually arrived. But there has been little planning for the looming El Niño.

“We need at least a three-year plan for addressing arboviruses,” Requena said, especially those transmitted by Aedes aegypti and Aedes albopictus mosquitoes, including not just dengue, but also zika, chikungunya, and yellow fever. It should, he added, consider co-infections, as doctors in Piura saw patients who tested positive for dengue as well as COVID-19, leptospirosis, or influenza: “We need an integral management plan, and we don’t have one.”

Prevention by the community

Dengue is associated with standing water where mosquitoes can breed, which increases with unseasonal rains. Studies show a correlation with rising temperatures, which facilitate mosquito breeding and shorten the time it takes for the transmission of the virus after feeding on the blood of an infected person. 

Identifying mosquito breeding sites and quick responses are crucial, said Valerie Paz-Soldán, the Peru-based director of the Tulane Health Office for Latin America, which conducts research focused on emerging infectious diseases, including dengue. 

“By the time you have an outbreak, there's too many breeding sites and it's out of control,” she told The New Humanitarian. “You're going to need to fumigate in a lot of different places at the same time, and the number of cases you’re seeing is the tip, tip, tip of the iceberg.”

Governments often do not allocate emergency funds until an outbreak is under way, and humanitarian aid arrives late, when hospitals are already overwhelmed. 

“We don’t have [Médecins Sans Frontières] for prevention,” Paz-Soldán said. 

Unlike the Anopheles mosquitoes that transmit malaria, Aedes mosquitoes generally stay within about a one-block area and lay their eggs in damp places. Low-income neighbourhoods are most affected, because residents without running water store water in buckets or barrels. Open drainage ditches are likely to be nearby as well. 

Mosquito-control programmes that treat potential breeding sites with larvicide generally focus on household water that residents keep in containers. But studies in Iquitos, Peru’s largest Amazonian city, have found that problem areas at this breeding stage are more likely to be outdoors – in discarded items such as old tyres, in flower pots, or in other places where rainwater collects and which often go untreated, according to Amy Morrison, a project scientist with the University of California, Davis who has been studying dengue in Peru since 1998. 

When dengue cases spike, government health workers often go house to house to fumigate indoors and outdoors, but experts say those later response efforts are often unproductive. 

Once they breed, Aedes mosquitoes generally live indoors, so spraying outdoors is ineffective, and doing it on indoor walls often misses the insects, which tend to settle on clothing, explained Macdonald, the entomologist, who is also an adviser to AEGIS, a research group at the University of Notre Dame in Indiana that promotes spatial repellents.

A modelling study in Iquitos indicated that spraying targeted places, where the mosquitoes do in fact settle, would be more effective. 

Spatial repellents — chemically impregnated squares of cloth the size of a sheet of printing paper that can be hung in homes — appear to be an effective alternative to spraying, added Macdonald.  

New vaccines and a bacteria-based approach

Other measures are under development.

A vaccine approved by the US Food and Drug Administration (FDA) and the EU is available, but only for people who have had one confirmed infection. 

Four dengue strains – or serotypes – exist, and infection provides immunity, but only to that serotype. A person who has had one form can be infected with a different strain, and may have a more serious condition because of a strong immune response. Late last year, the EU authorised a second vaccine, regardless of previous exposure history. Several other vaccines are in the testing stages. 

The scientific community is looking for other ways to stop dengue.

One method, developed by the World Mosquito Program (WMP), uses a common bacterium called Wolbachia. Treating Aedes eggs with Wolbachia interferes with the adult mosquito’s ability to transmit arboviruses through its saliva, and while the bacterium alone does not reduce the mosquito population, it decreases the health hazard. 

Studies have shown the method can decrease dengue cases, although some experts say it would be difficult to do so on a large scale and worry that the virus could mutate to dodge Wolbachia. 

Edited by Daniela Mohor and Tom Brady. 

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