2019 is a record year for dengue fever in Latin America. The mosquito-borne disease has surged across the continent, from Mexico down to Chile and Argentina, with nearly 3 million cases reported. That's more than 20% higher than the previous record in 2015.
Over 2 million of the reported cases have been in Brazil. Countries in Central America, including Belize, Honduras and Nicaragua, are also among the hardest hit.
Dengue fever can feel like the flu — bad headaches, vomiting, pain in muscles and joints. In severe cases, it can cause blood vessels to leak and organs to fail. More than 1,300 people have died from dengue in Latin America this year. Hospitals across the region have struggled to keep up with the number of patients who need care.
The reasons from the surge range from rain (or lack of it) to the Zika virus.
Leah Katzelnick, an epidemiologist at the University of California, Berkeley who works on dengue in Nicaragua, says climate definitely plays a role.
The Aedes aegypti species of mosquito, which can infect people with the virus, will lay eggs in any pool of clean water. After this year's heavy rains in Latin America, "you're just fighting against everything," Katzelnick says. The mosquitoes will breed in a puddle on the lid of a trash barrel — or even discarded tires.
Raman Velayudhan, a mosquito specialist with the World Health Organization, agrees that climate conditions such as warmer temperatures, high humidity and abnormal rains have contributed to this year's surge. But so does drought: "If you have less rainfall, people hoard water," he says, which can create places where mosquitoes can breed.
A second probable reason for dengue's surge in Latin America comes from another virus: Zika.
After Zika swept through Latin America in 2016, rates of dengue dropped to fewer than 600,000 cases per year in the two years following. That's the lowest number in the decade.
Gabriela Paz-Bailey, senior epidemiologist at the Center for Disease Control and Prevention's Dengue Branch in Puerto Rico, explains that Zika and dengue are closely related viruses. "It is possible that the Zika outbreak in the Americas provided some short-term protection against dengue," she says.
Researchers think antibodies the immune system created to fight Zika also protect against dengue. But this immune system response fades after a few years — which may have primed 2019 for a big dengue surge.
The nature of Latin American cities could also be a factor. "Increased urbanization provides the right environment for these mosquitoes, in areas with large populations and poor sanitation," says Paz-Bailey. "And that just provides breeding sites for the mosquitoes next to people."
The 2019 totals are worrisome to Maurício Nogueira, a virologist in Sao Paulo. But he is trying to keep it in perspective. Every three to five years, the caseload spikes but then comes back down. That's because people who recover from dengue are immune to it for a couple of years — so there's not as much dengue circulating for mosquitoes to pick up. And that protects folks who haven't yet contracted the disease.
AUDIE CORNISH, HOST:
It's been a record year for dengue fever in Latin America. The mosquito-borne disease surged across the Americas with nearly 3 million cases reported. That's more than 20% higher than the previous record in 2015. NPR's Pien Huang explores why.
PIEN HUANG, BYLINE: It's been a weird year for rain and one of the hottest on record, and mosquitoes are loving it. Tropical countries like Brazil, Nicaragua and Mexico have all had bouts of heavy rain and flooding and huge spikes in dengue. Leah Katzelnick, who's an epidemiologist at UC, Berkeley, says that after it rains, mosquitoes will lay their eggs in any pool of water.
LEAH KATZELNICK: You're just fighting against everything. The lids on top of your barrels become containers for breeding mosquitoes. Any tiny little piece of trash. They've found that tires are a huge source of larvae.
HUANG: Those larvae grow into hungry mosquitoes of the Aedes aegypti species that infect people with dengue when they bite. In some cases, dengue fever can feel like the flu. In severe cases, it can cause blood vessels to leak and organs to fail. More than 1,300 people have died from dengue in Latin America this year. But mosquito weather is only one reason for the surge. Mauricio Nogueira is a virologist in Sao Paolo, Brazil.
MAURICIO NOGUEIRA: Every outbreak that we have has becoming bigger than the last one.
HUANG: He says it could be that the virus is mutating to become more infectious or because more people are packed into cities, which also makes it easier for mosquitoes to spread dengue. But dengue outbreaks come in cycles, too.
NOGUEIRA: Every three or four years, you have a surge of dengue cases.
HUANG: Nogueira says that people who recover from dengue are immune to it for a couple of years. So after big outbreaks, there's a time of herd immunity, where there's enough people who just had dengue to protect those that haven't. So weather, outbreak cycles, and another reason for this year's explosion is a different virus, Zika. Gabriela Paz-Bailey, epidemiologist at the Center for Disease Control's Dengue Branch in Puerto Rico, says rates of dengue have been super low since the Zika virus swept through in 2016.
GABRIELA PAZ-BAILEY: Zika and dengue are closely related viruses, so it is possible that the Zika outbreak in the Americas provided some short-term protection against dengue.
HUANG: Researchers think the antibodies the immune system's created to fight Zika protect against dengue. But this immune system response is a temporary effect that fades after a few years, which may have primed this year for a big dengue surge. Around the world, nearly 4 billion people are at risk of getting dengue, and climate change and increased air travel means that risk is growing. Researchers are working on better vaccines and ways to hack mosquitoes to control the spread.
Pien Huang, NPR News. Transcript provided by NPR, Copyright NPR.