Dengue fever, once confined to the tropics, now threatens the US

Meg Norris was traveling in Argentina in April when the first signs of dengue fever hit her. The weather in Salta, just south of the Bolivian border, was warm, but Norris, a 33-year-old from Boulder, Colorado, zipped a fleece sweatshirt around her body to stop herself from shivering.

“I thought it was sun poisoning,” she said. 

She woke that night in a sweat and spent the hours alternately burning up then freezing. In the morning, her eyes were sore and her lymph nodes were swollen. For the following week, there was nothing to do but sleep, stay hydrated and wait for the body aches that give the illness the moniker “break-bone fever” to pass.

Latin America is experiencing its worst dengue fever outbreak on record. Case numbers in the first 4 ½ months of 2024 are already 238% higher than they were by this time last year, which itself ended with a record 4.1 million cases, according to the Pan American Health Organization. Cases are more than 400% higher than the five-year average.

An unusually wet and warm summer season brought by the El Niño weather pattern has created ideal conditions for the mosquitoes that spread dengue to hatch en masse and carry higher amounts of the virus.

Experts warn this could be a preview of what dengue fever will look like in the future. Climate change is creating unusually balmy conditions, which are already expanding the range of mosquito-borne diseases.

“That’s concerning for places where dengue hasn’t occurred before in recent history: North America and Europe,” said Dr. Albert Ko, a professor of epidemiology of microbial diseases at the Yale School of Public Health.

Dengue is a viral fever caused by four different viruses and spread through mosquito bites. It’s common in many tropical regions across the globe, but has begun to appear in more temperate climates. The mosquitoes that carry dengue fever, Aedes aegypti are now regularly found in the southern parts of the U.S., but recently, the insects have been found as far north as the Bay Area and Washington, D.C. One 2019 study predicted an additional 2 billion people will be at risk for dengue fever by 2080.

“We are definitely worried,” Ko said.

Why are dengue cases rising around the world?

Dengue outbreaks have historically occurred in the Americas every three to four years, said Dr. Gabriela Paz-Bailey, dengue branch chief in the division of vector-borne diseases at the Centers for Disease Control and Prevention. “But now we are seeing them every year,” she said.

Part of the reason for that is tied to climate change.

A warming climate expands the mosquitoes’ habitat and allows them to breed all year long, rather than only in the warmer months. The hotter temperatures also cause the viruses to replicate faster, meaning mosquitoes end up carrying many more viral copies, increasing the likelihood that a person will become infected if bitten.

“We are also seeing dengue cause outbreaks at times when they usually don’t occur,” Ko said.

South America’s dengue cases weren’t just unusually high this year, but they also came unusually early in the season. Similarly, Puerto Rico, a place where dengue outbreaks can occur in the summer and fall, declared a public health emergency in late March after the U.S. territory was overtaken by dengue fever cases and more than 400 people were hospitalized.

In recent years, the epidemic has spread to parts of southern Brazil and northern Argentina, where dengue hasn’t previously been a big problem, Ko said.

“That gives us a snapshot of what we may see here in North America in the coming decades,” Ko said.

How would dengue get a foothold in the U.S.?

The fact that Aedes aegypti mosquitoes are found in places outside their normal range doesn’t mean the mosquitoes are carrying dengue viruses, but those first insects are a warning of what may be to come, Ko added

Locally transmitted dengue fever infections — meaning the infected person didn’t get sick abroad — are still rare in the continental U.S., but have recently been seen for the first time in some states. Last October, California health officials reported the state’s first case of locally transmitted dengue in Pasadena. Local transmission has also occurred in Arizona, Florida and the southern coast of Texas. Last summer brought record-breaking heat waves to Europe, where cases of local dengue transmission were seen in France, Italy and Spain.

“I think this means dengue will become more common,” said Paz-Bailey, adding that the main concern is still the significant increase in cases where the virus is already endemic.

This summer, she does not expect to see significant dengue outbreaks on the U.S. mainland, but she said there is likely to be some people who travel to regions that have higher-than-usual cases and bring the virus back home.

“Travel-associated cases do result in small chains of outbreaks,” Paz-Bailey said.

Humans are reservoirs for dengue, so in order to have widespread transmission, enough people must be infected for the mosquitoes to reliably bite someone with the virus so that they can spread it to another person.

“That’s why we’re seeing an outbreak of dengue in Puerto Rico right now,” said Michael von Fricken, director of the One Health Center of Excellence at the University of Florida in Gainesville. “They’ve reached this tipping point where there are enough infected humans that they’re subsequently infecting other mosquitoes that are continuing to transmit disease.”

Florida has logged 176 dengue cases so far this year, the vast majority in people who were infected in other countries, most frequently Brazil or Cuba. The Florida Health Department has recorded only seven cases of locally transmitted dengue transmission in the state so far this year. In all of 2023, the department documented 173 locally transmitted cases, most of them in Miami-Dade County.

What are the symptoms of dengue fever?

Dengue fever is caused by four viruses, so a person can be infected four times in their lifetime. Only about 1 in 4 people are symptomatic the first time they’re infected, according to the CDC.

Ko said a person’s initial symptoms are usually a fever and headaches. Fatigue, nausea, vomiting, a rash that looks like measles, as well as the extremely painful body aches.

Most people recover in a week or two, but about 1 in 20 people develop severe dengue, which can be fatal. The more times a person is infected with dengue, the higher risk they are for complications.

“After you’ve had your first exposure, your risk of having dengue hemorrhagic fever or severe symptoms increases exponentially,” Von Fricken said. Dengue also becomes deadlier with each infection.

While the U.S. does have a dengue vaccine, it’s approved only for children ages 9 to 16 who live in places where dengue is endemic, including Puerto Rico, American Samoa or the U.S. Virgin Islands.

What’s more, children can get the vaccine only if they’ve previously had a dengue infection. That’s because if a person were to get vaccinated and then get their first dengue infection, they still run the risk of getting very sick, just as someone gets sicker from their second infection. Since most Americans have not had dengue, “that vaccine is not very useful” for most, Ko said.

There’s no specific drug to treat dengue. Instead, doctors just aim to treat the symptoms and keep the patient comfortable until the virus runs its course. That means resting and drinking a lot of fluids. Ko said people should try to take acetaminophen (Tylenol) for pain and fever if they can, since nonsteroidal anti-inflammatory drugs (NSAIDs), which include ibuprofen and aspirin, can make bleeding worse if someone develops hemorrhagic dengue, in which their blood vessels are damaged and become leaky.

Paz-Bailey said it’s important for people traveling to places with dengue to stay in places with air conditioning when possible, use insect-repellant and wear long sleeves and pants to avoid mosquito bites.

Bed nets can be helpful, but the mosquitoes that carry dengue typically bite during the day, so they may be less helpful than they are at preventing other mosquito-borne diseases like malaria, Ko said.

At home, people can make their yards less appealing to mosquitos by reducing the amount of standing water, especially after a bout of rain.

“It’s difficult to control the mosquito population, so we need to hit it with all we have and layer our strategies,” Paz-Bailey said. “No single strategy will be good enough.”

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