Dengue fever is surging in Latin America

For the second time in five years, Brazil’s army is building field hospitals in the capital, Brasília. The tents are accommodating a surge of patients from swamped emergency departments, as millions of Brazilians succumb to dengue fever that is spreading across the country. As with covid-19, the last disease to prompt the construction of field hospitals, many dengue infections are asymptomatic. The one-in-four people who do fall ill can suffer for several weeks with a painful condition known as break-bone fever. Unlike covid-19, the virus causing this wave of illness is carried by mosquitoes. As the climate warms, their range is expanding and the number of people they infect is increasing (see charts).

Brasília is a hotspot, but dengue is spiking across Brazil and Latin America. In 2023 the region hosted 80% of the world’s confirmed dengue cases, according to the UN’s World Health Organisation. Brazil’s health ministry reckons that 3.8m people caught dengue in the country between the start of the year and April 23rd, about 1.7% of the population. Dengue is already spreading in Mexico and Central America too, places which do not usually see many cases until much later in the year.

The disease is even showing up in countries which have never previously been seriously affected, like Uruguay and Chile. Overall Latin America has seen three times as many cases this year as it did through the same period of 2023, itself a record-breaking year. The long-run trend is bad, too. Every year between 2000 and 2005 saw an average of 535,000 cases, according to the Pan American Health Organisation, another arm of the UN. In 2023 there were 4.5m. There have already been 5.9m cases in 2024.

People catch dengue when bitten by a female mosquito of the Aedes genus which is carrying a flavivirus pathogen. Around 5% of victims need hospital treatment. Some develop a form of the disease called dengue hemorrhagic fever, which can be fatal. At least 40,000 people die from it every year worldwide, a number that has doubled in the past two decades. Risk increases with each subsequent bout of dengue. The virus has four strains. While you cannot catch the same one twice, you can be reinfected with one of the other three.

Climate change explains much of this year’s jump in dengue cases and the long-term increase in the disease over the past two decades. There are ever fewer places where temperatures drop below 15°C in winter, the level at which mosquitoes tend to die out, so there are more virus-carrying insects in circulation, ready to surge, once temperatures rise in the spring. Recent changes in a Pacific-ocean weather system called El Niño have raised temperatures even more across the region, meaning that more of Latin America is experiencing the kind of tropical weather that suits Aedes.

Emitting gases which raise the planet’s temperature is not the only way that human behaviour is contributing to the rise in cases. Urbanisation trends are boosting the disease too, because densely populated areas offer a given mosquito more victims during its lifetime. Latin America’s shantytowns are breeding grounds for mosquitoes because of the abundance of standing water, where the insects lay their eggs. Most houses have flat roofs, where water can make pools. Residents who lack basic plumbing often store water in open tanks. Patchy refuse-collection services leave piles of uncollected rubbish which also serve as a mosquito mecca.

Panic is making a bad situation worse. Videos of mosquito swarms in Buenos Aires went viral in February, prompting consumers to stockpile insect repellent. Supermarkets and chemists in Argentina have been sold out for weeks. Shelves are bare in Brazil, too, and it is unclear when new supplies will arrive. Retailers in Uruguay are also reporting panic buying. Venezuela’s president, Nicolás Maduro, has charitably offered to help restock Argentina, not missing a chance to lambast the newly elected president, Javier Milei, whom he accuses of spectacularly mishandling the epidemic.

Dengue kills a small proportion of those it infects; the case fatality rate sits near 0.03%. But the virus is stretching already struggling public-health services and hampering economic output. The Federation of Industries, a manufacturing trade body based in Minas Gerais state in Brazil, reckons the combination of extra health spending and lost productivity through workers off sick thanks to Aedes will cost Brazil’s economy 0.2% of GDP this year. When growth is already fairly feeble, that is quite a knock.

The suffering now

Governments have few short-term fixes available. Their efforts focus mainly on preventing mosquitoes from spreading. In badly affected areas, public-health workers are sent out with masks, white boiler suits and fumigation nozzles to spray insecticide where mosquitoes may lurk.

Public education also plays a role. Posters plastered across cities urge residents to empty containers that hold standing water, such as plant pots and buckets. In some Peruvian municipalities the authorities have prohibited putting saucers of water in graveyards. Mourners are tactfully advised to consider laying artificial flowers rather than real ones.

Part of the reason that dengue remains such a problem is that no useful vaccine has existed until now. A shot called Dengvaxia, manufactured by Sanofi Pasteur, a French pharmaceutical company, was first licensed in 2015. But it works only if recipients have had the virus before. For those who have not, it makes them more vulnerable to contracting dengue severely. Given that most infections are asymptomatic, and it is therefore hard to tell whether a recipient has had dengue before, it is too risky to roll out Dengvaxia en masse.

Second-generation vaccines look more promising. QDenga, produced by a Japanese firm, Takeda Pharmaceuticals, does not appear to have the same problems with secondary reactions as Dengvaxia does. QDenga seems to be effective across all four strains, though it is tricky to tell this for sure, as it is rare for all four to be circulating at the same time during trials. Brazil has snapped up available supplies, but the vaccine is new and is not currently being manufactured in large enough quantities to allow governments to buy in bulk.

QDenga is not the only new vaccine. The National Institutes of Health in the United States, Instituto Butantan (a public research institute in São Paulo) and Merck have developed a vaccine that is in the final phases of trials. But even assuming that it gets regulatory approval, it will not be available until 2025 at the earliest. Neither this vaccine nor QDenga will help curb the epidemic raging across Latin America.

Others pin their hopes on scientific trials of a different kind. Experiments to inject wolbachia (a type of bacteria found in some other insects) into Aedes eggs, before releasing these mosquitoes into the population, have found that it can reduce dengue significantly. Mosquitoes which hatch from those eggs struggle to pass the virus on. Studies indicate that transmission is reduced by 75%. The hope is that wolbachia-infected mosquitoes will eventually replace the population of regular, dengue-spreading ones, because females pass the bacteria on to their offspring, regardless of whether they mate with a wolbachia-carrying male or not. Pilot studies in Colombia and Brazil have been successful and programmes are starting to be rolled out more widely. Other countries are watching developments closely and will probably follow suit.

If that sounds too good to be true, it may well be. Tests have been run in cities but efficacy is probably weaker in rural areas, where dengue tends to be less acute. Trials have been successful across relatively small areas, but producing enough wolbachia-infected mosquitoes to have an impact across large swathes of Latin America will be a Herculean task. The required laboratory space vastly outstrips current capacity. Singapore, which has endemic dengue, has used wolbachia-release programmes to reduce case numbers; the authorities breed and release 5m mosquitoes every week. This is costly.

Don’t be sloppy

Singapore offers other salutary lessons to Latin America. Professor Neil Ferguson of Imperial College in London calls the country’s dengue-prevention efforts the world’s “gold standard”. Crucially, its approach is preventive rather than reactive. Surveillance and vector-control programmes run constantly, even when dengue is at a low ebb. Public-health officers traipse around houses and businesses, checking for receptacles that might collect water and imposing hefty fines on wrongdoers. Testing and reporting suspected cases happens quickly. Mosquito-collection devices known as Gravitraps are everywhere, gathering data on their distribution and density. The authorities feed all this information into forecasting models that provide an early warning of outbreaks. Copious amounts of insecticide-spraying then follow.

Latin America’s challenge is that it is too poor to easily adopt the proactive approach. In response to a rapid growth of cases, governments give the impression that they are taking dengue seriously. Doctors are getting better training in diagnosing and treating the illness. Public awareness has begun to improve. Eventually a combination of vaccination and wolbachia programmes may make a dent. But before that happens, there is little evidence that the authorities have the tools to ease the current outbreak, or to nip future ones in the bud. The army should not be too quick to dismantle those field hospitals. They will be needed for some time to come. ■

Correction (29th April 2024): Merck, a pharmaceutical company based in New Jersey, is developing a dengue vaccine, not Germany’s Merck Group as we wrote. Sorry.

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