​How Many People Will Get Dengue Fever This World Cup?

Right now, about 600,000 people are pouring into Brazil for the World Cup, joining the two million Brazilians planning to shuffle around on some sort of World Cup pilgrimage. What many of the interlopers don't know, and what many of the locals do, but can do little about, is that being outdoors in Brazil puts you at a particular risk of contracting dengue fever, a potentially lethal mosquito-borne disease.

Brazil is the world leader in dengue infections. Between 2000 and 2013 there have been seven million cases of dengue in the country, higher than anywhere else on the planet. Dengue fever is caused by the dengue virus, and that virus is transmitted by a the females of a particular species of mosquito called Aedes aegypti. Unlike malaria-carrying mosquitoes, the Aedes aegypti bite only during the day. This makes avoiding them trickier—you can sleep beneath your mosquito net and wait out the night-biters, but unless you want to miss all the games there's no hiding from mosquitoes that bite during the day.

There is no treatment for dengue, and, unlike malaria, there's no pre-trip treatment for it either. So to avoid getting dengue, all you can do is avoid getting bitten by a dengue-carrying mosquito. So should World Cup fans be worried?

What do we know?

You couldn't design a better incubator for dengue than Brazil. It's got big concentrations of people, lots of standing water, and a nice, warm climate. So when Brazil won both bids for both the 2014 World Cup and the 2016 Summer Olympics epidemiologists around the world started wondering what the influx of people to the South American country might mean for all kinds of infections, including dengue. In the past year, several publications have reviewed and predicted the risk of dengue for travelers, finding that while nobody knows just how many people might get dengue this World Cup, it's definitely still something soccer fans should watch out for.

Of course, not all mosquitoes in Brazil carry the dengue virus, so whether or not people become infected depends on how often they get bit and how many infected mosquitoes are flying around. In the lead-up to the World Cup, Brazil has sent men in hazmat suits throughout cities like São Paulo to spray streets and homes with pesticides and remove standing water pools where mosquitoes might breed. Nobody knows just how many mosquitoes in Brazil carry dengue, but scientists can look back and see how past travelers have fared in Brazil.

That's exactly what a team of researchers at the Harvard School of Public Health did, using a database called the GeoSentinel Surveillance Network. Doctors in this network report back the number and types of diseases that travelers acquire on their jaunts around the world. By looking at what diseases different people return home with, they can get a sense of where diseases are moving to and from. When they looked for travelers who had returned from Brazil and wound up at a doctor's office, they found almost 1,600. Among the hospitalized, dengue and malaria were the most common diagnoses, and 92 of those were dengue.

The study is limited, obviously—only doctors that are part of the network contribute to the data set, and this doesn't address or track the many visitors who return perfectly healthy. But it can give them a taste for how many people get really sick when traveling to and from Brazil, and what they're coming down with. "We tended to see people who were sicker than the rest, or had something that seemed really strange and wanted medical attention," says Mary Wilson, the lead author on the study.

Another study tried to model the risk of dengue during this particular World Cup using information about past infection rates, current population density, demographics and projected weather conditions like rainfall and temperature. Modeling just how high the dengue risk might be months in the future is tough without knowing the exact environmental conditions, and according to lead author Rachel Lowe, this is the first time anybody has used seasonal forecasts to try and predict public health issues before a major sporting event.

What they found was that for most cities, the risk was low. Only three cities in the north of Brazil slipped into the high-risk zone: Recife, Natal and Fortazela, meaning the chances of dengue are greater than 300 per 100,000 inhabitants. Manaus, a stadium in the midst of the jungle, fell into the "medium" risk category. Lowe and her team hope to turn this forecasting system into an early warning system, that might be able to alert cities when their risk jumps.

So, what's the number?

Nobody can say for sure, but one team tried to estimate how many World Cup tourists would be infected, and came up with an expected 33. That's very specific!

Obviously, there are limits to these kinds of forecasts, says Lowe. "If areas have lots of breeding sites like discarded tires or water storage units, that increases the risk," in a way that their model can't predict. Wilson agrees. "Part of it will depend on how much the cities do, and how much they can control the mosquito populations before the games," she says. So far, things look a little messy.

If there's a lot of rain in the days leading up to the World Cup, as there has been this week, the mosquito population might boom and the risk might go up. In fact, this year has already been a big year for dengue in places like São Paulo, where they've seen 6,000 cases already this year in the city alone, and 60,000 in the surrounding areas.

Which isn't to say you should panic if you're reading this from a São Paulo beach. Only about 20 percent of people who become infected will ever know it. Of that 20 percent, only a very small percentage of them will have the severe symptoms that give dengue the nickname "the bone breaker"—things like high fevers, and severe aches, and pains. And if your friend gets dengue, you don't have to worry about catching it from them—the virus is only spread from mosquito to human, and never between two humans.

Still, not getting dengue is better than getting it, and to prevent infection Wilson recommends wearing long sleeves and spray your clothes with mosquito deterrents like Permethrin.

But Wilson also says that dengue might not be the World Cup travelers primary concern. Winter isn't dengue season in Brazil, it's flu season, and flu certainly is something you can catch from your sick friend. As are hepatitis A, measles, and meningitis. Malaria risk is low for most of the cities, but anybody traveling to Manaus or into the jungle—as Brazil hopes people will after dropping billions of the World Cup and pointing to tourism as a way to make that money back—should take malaria risks seriously. And for those going to the beach in Brazil, put down a towel. "One infection we saw very frequently was something called cutaneous larval migrans, where hookworms penetrate the skin," says Wilson.

The virus is certainly a huge problem for Brazil, and will continue to infect about a million of their people each year, but it might not be much of an issue during the World Cup. Outbreak or not, however, everybody can agree to stay the hell away from Brazilian mosquitoes at all costs.


Rose Eveleth is a writer, producer, and designer based in Brooklyn, New York. She's dabbled in everything from research on krill to animations about beer to podcasts about fake tumbleweed farms. In her spare time she makes weird paper automata and daydreams about hanging out with a pack of foxes.

Screamer is Deadspin's soccer site. We're @ScreamerDS on Twitter. We'll be partnering with our friends at Howler Magazine throughout the World Cup. Follow them on Twitter, @whatahowler.

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