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.