Dengue virus is the most common mosquito-spread virus globally, but generally relatively rare in the continental United States. However, the US Centers for Disease Control and Prevention warned this week about a higher-than-expected number of dengue cases among US travelers — 745 cases through June 24 — as well as a public health emergency in Puerto Rico, where the virus is endemic.

Global infections in 2024 have been the highest on record, with cases in the Americas already reaching 9.7 million — exceeding the highest number ever recorded in a single year.

What is dengue, and where is it common?

Dengue is a mosquito-borne virus that is spread mainly via the Aedes aegypti mosquito, which is also known to carry several other viruses such as yellow fever, chikungunya and Zika, according to the CDC.

Dengue is endemic in more than 100 countries in tropical and subtropical climates, mostly in urban and semi-urban areas, according to WHO.

It’s also endemic in the US territories of Puerto Rico, the US Virgin Islands and American Samoa.

More than 34,000 locally acquired cases were reported in the US between 2010 and 2023, according to CDC data. A handful of outbreaks of locally transmitted cases have been reported in states with warmer climates, including Florida, Texas, Hawaii and Arizona. California reported its first locally-transmitted case in Pasadena last year.

But almost all US cases are reported in Puerto Rico, says Dr. Gabriela Paz-Bailey, chief of the Dengue Branch in the Division of Vector-Borne Diseases at the CDC’s National Center for Emerging and Zoonotic Infectious Diseases, told CNN previously. And that’s not a surprise.

“It has the perfect climate for it. It’s a tropical island,” she said. “There are indeed efforts from the health department and other organizations like the Puerto Rico vector control unit to control dengue in the area.”

This includes public education and integrated mosquito management such as “removing places where the mosquitoes lay eggs, and that is done through community engagement, but also clean-up campaigns,” Paz-Bailey said.

Similar efforts are also recommended in the rest of the US and its territories, she noted.

Dengue symptoms

Only about 1 in 4 people infected with dengue will have symptoms.

The most common symptom is fever, and it can also cause nausea, vomiting, rash or aches and pains – typically behind the eyes or muscle, joint or bone pain.

There are four strains of the virus, or serotypes, according to the World Health Organization. Once someone catches one of the strains, it can’t infect them again. But the more times someone is infected with different strains, the more likely they are to become severely ill.

Severe dengue is less common, with about 1 in 20 people developing it. But the symptoms are more troubling. It can cause shock, internal bleeding and even death.

About 100 million people get sick and 40,000 die of severe dengue around the world each year, according to the CDC.

Testing for dengue

If you get sick and live in an area where dengue might be spreading or have traveled to a place where the virus is common, the CDC recommends consulting your doctor.

The only definite way to know if you have the virus is through blood testing, which varies from place to place, Paz-Bailey said.

The general recommendation is to do a PCR test, she says, if your symptoms started within a week or less.

“If the person presents beyond the seven-day window, a serological test to measure IgM antibodies is recommended,” Paz-Bailey said.

It can take a day to a week to get those results, she says.

Dengue vaccine and treatment

The US Food and Drug Administration approved a vaccine for dengue in 2019 and the CDC recommends it to children ages 9 to 16 who have laboratory-confirmed evidence of a dengue infection and who live in areas of the United States where dengue is endemic or regularly occurring. The vaccine requires three doses, given six months apart.

However, Sanofi-Pasteur said it will stop manufacturing the vaccine, Dengvaxia, due to a lack of demand in the global market.

There is no dengue vaccine available for adults in the US. The CDC said there are two other dengue vaccines approved or in development, but they are not currently available in the United States.

There’s also no specific medicine to treat dengue, the CDC says. Over-the-counter meds like acetaminophen can ease fever or pain, but experts say not to take aspirin or ibuprofen because they can increase the risk of hemorrhaging sometimes seen with dengue.

Protecting yourself from dengue

Preventing mosquito bites and controlling mosquitoes in and around the home are the key ways to prevent dengue. If you’re going outside, the CDC recommends US Environmental Protection Agency-registered insect repellents to prevent bites from mosquitoes that may be carrying dengue. Wearing long-sleeved shirts and long pants is another option, along with treating your clothing with 0.5% permethrin, an insecticide.

The Aedes aegypti mosquito, which spreads dengue and other viruses, does not like to travel far and prefers to breed in our backyards, said Dr. Isik Unlu, acting director and operations manager of the Miami-Dade County Mosquito Control Division.

“They prefer to be around people. That is the problem,” she said, adding that the species is often found in containers that collect rainwater, especially during the summer months.

Almost everything you see in a backyard can become a breeding ground, Unlu says, including birdbaths, plant saucers, tires, extension gutters and kiddie pools.

Unlu and Paz-Bailey recommend removing standing water around your home as often as possible to prevent mosquitoes from gathering and breeding.

Mosquito surveillance in the US

In Florida, Unlu and her team are often called out to homes where mosquito infestations are reported. If they find a large population of adult mosquitoes, the team heads out in the middle of the night to spray the area with insecticide. Her county has hundreds of mosquito traps set.

“When we collect the mosquitos from these traps, especially if it’s an area that we suspect dengue is circulating or we have confirmed cases, our research team assists us to process these mosquitos,” Unlu said. The mosquitos are evaluated for sex, counted, put in test tubes and sent to a lab outside Orlando to be tested for dengue and other viruses.

Dengue is not considered endemic to Florida, according to the CDC. Unlu says this is largely due to disease surveillance operations. But the virus is present in the southern part of the state because of people traveling to places where it is endemic, such as Cuba or Brazil.

“We get residents who go on vacation or visit their family where dengue is endemic and more common, and they get bitten there, and then they travel back here,” Unlu said. “And since we have the vector, Aedes aegypti, here, when they get bit, the mosquitos then get infected, and then they bite another human being who never traveled outside Miami, and they get sick.”

The CDC’s Paz-Bailey says climate change will also encourage the spread of dengue around the world.

“Higher temperatures will probably expand the range of places where a mosquito can survive. And then it can also facilitate transmission through other ways, like faster viral replication in the mosquito and increased survival of mosquitoes,” she said.

CNN’s Jen Christensen contributed to this story.