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It’s already a record-breaking year for dengue infections in Central and South America, with almost 10 million cases diagnosed so far. 

Now, the Centers for Disease Control and Prevention is warning of an increased risk of the mosquito-borne virus in the U.S., as summer weather and vacation travel heat up.

 health care providers to be on the lookout and to test for cases, especially among people with fevers who have recently returned from places where dengue is surging.

“Currently, there is no evidence of an outbreak in the continental U.S.,” says 

, chief of the CDC’s Dengue Branch, based in San Juan, Puerto Rico. “But around the world, dengue cases have risen at an alarming rate. Particularly

in the summer months, we are expecting people to travel more to areas where dengue is common, and this could lead to more local transmission in the United States.”

 so far this year. And about 1,500 of those cases have been locally acquired, mostly in Puerto Rico, where dengue virus is considered endemic – that is, in constant, continuous circulation.

 over dengue in March, after cases rose quickly at an unseasonably early time. Locally acquired cases have also been reported in the U.S. Virgin Islands and Florida this year, Paz-Bailey says.

To be clear, the CDC does not expect to see large outbreaks across the U.S. this summer. Instead, the agency is anticipating more travel-related cases, and small chains of local transmission linked to those cases, says Paz-Bailey. These chains can arise in any state with an established population of

, the species most associated with their transmission in the Americas.

, local dengue cases have been seen in Arizona, California and Texas. “With increasing temperatures, we need to continue to be prepared and continue to strengthen the surveillance system so that we monitor the occurrence of dengue in new areas,” Paz-Bailey says.

A few intersecting threads related to weather, waning immunity and human behavior are contributing to the “explosive outbreak that has evolved in the last year,” says Dr. Albert Ko, a professor of public health at Yale University who has worked with dengue patients in Brazil for 30 years.

First, it’s been a warm, wet year in South America, providing ideal breeding conditions for mosquitoes. Populations of the potential dengue carriers are thriving. This year, the mosquitoes brought the disease to parts of southern Brazil and Argentina where it hasn’t historically been found – “a testament to climate change,” which is expanding the insects’ range, Ko says.

Second, dengue outbreaks tend to be cyclical. Big outbreaks happen every few years, and the last one 

. The cyclical pattern of dengue outbreaks is related to how population-level immunity rises and falls, Ko says.

There are four distinct strains of dengue, and a person who recovers from one type is protected against all of them for a couple of years. But that immunity wanes over time “and you then become 

,” Ko says. On a population level, immunity is high after a large outbreak and then declines in the years after, setting the scene for a new wave of dengue infections.

And third, the dengue virus is hitching a ride on human travelers, who are going to see family, friends and places they missed out on when travel shut down during the COVID-19 pandemic. 

“Human mobility, either short or longer distances, plays a significant role in moving the virus around,” says 

, an environmental scientist and global health researcher at Emory University, “Humans are the vector, the ones that are moving the virus even a longer distance than mosquitoes.” They’re getting bitten by mosquitoes with dengue and bringing it, often inadvertently, to wherever they’re going next.

Some 75% of people who get dengue infections show mild or no symptoms. “So that could translate into someone traveling to an area where there is active dengue transmission, acquiring dengue, returning to their home and then transmitting dengue to a mosquito,” all without knowing they’re carrying the dengue virus, Paz-Bailey says. That mosquito could go on to bite other people, potentially starting a chain of local transmission.

If most people are asymptomatic, how bad can dengue be? 

In a quarter of cases, people infected with dengue feel terrible. “About three to four days after being bit, the virus disseminates in the body, causing systemic illness,” says Ko, who has treated thousands of dengue patients. “Symptoms [include] fever, very severe body aches, joint pains and very, very severe headaches.”

A few patients will develop severe dengue, which can include a condition called 

. “It makes our blood vessels leaky, and people become dehydrated and go into shock … at which point they need urgent medical care, such as resuscitation with intravenous fluids, in order to save their lives,” says Ko at Yale. People with fevers and headaches from dengue should stick with treating themselves with Tylenol or acetaminophen, he says, and avoid aspirin, because aspirin thins the blood and can exacerbate the disease’s bleeding effects.

Dengue can be severe and fatal whether a person gets it for the first, second, third or fourth time. But there’s a particularly pronounced risk of serious illness the second time around, says Paz-Bailey with CDC. That’s because of a phenomena associated with dengue known as 

, where a first infection with dengue can prime a person’s immune system to help the virus infect cells more easily on a second infection.

Groups most at risk of severe illness include infants, pregnant women and the elderly.

Individuals can protect themselves from mosquito bites by wearing long-sleeved shirts and pants, and using 

They can also help reduce the buzzing of mosquitoes in and around their homes by “dumping standing water, using window screens and, if possible, using air conditioning, because that helps keep mosquitoes out,” she says.

fevers, severe headaches or other symptoms

 consistent with dengue should seek medical care, and health care providers should be ready to assess their symptoms and travel history and, if warranted, 

Dengue is a nationally notifiable disease – so any cases that are found should be reported to the local health authorities. That will help track where the virus is spreading, and could spur local education and mosquito control efforts, says Ko.

A vaccine against dengue has been discontinued

A dengue vaccine, Dengvaxia, is approved for use in the U.S. where the virus is endemic, such as Puerto Rico and the U.S. Virgin Islands. However, the three-dose vaccine, which requires several blood tests and repeat visits to the doctor’s office, has been hard to administer and slow on the uptake. Sanofi-Pasteur has 

 the vaccine, citing a lack of demand, and the last remaining doses expire in 2026, Paz-Bailey says.

The hope for the future, says Yale’s Ko, is twofold: better mosquito control measures that decrease dengue transmission and better vaccines that protect the unexposed population.

“The bad part of decreasing transmission is that people become susceptible because they haven’t been infected,” he says, “But if we have both a vaccine and [better] vector-control methods, we mitigate that risk.”

Ko sees progress on both fronts – citing developments with 

 that has been approved in some countries, though not in the U.S.

With better interventions that tackle mosquito-borne illnesses from different angles, Ko says, the country’s response to diseases like dengue could become “substantially effective” and many more people can be saved. 

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The Centers for Disease Control and Prevention warns of an increased risk of dengue infections in the U.S. this summer. The mosquito-borne virus is surging, and human travel is expanding its reach.

Travelers beware: It’s a big year for dengue

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