On Monday, government leaders in Puerto Rico declared a dengue epidemic after a spike in cases of the mosquito-borne disease hit the island.

From the start of the year through March 10, there were 549 cases, including 341 hospitalizations and 29 severe cases, according to the most recent data provide by the Puerto Rico Department of Health. Cases are concentrated in cities including San Juan, Bayamon, Guaynabo and Carolina.

Between 2010 and 2020, more than 30,000 dengue cases were reported from four U.S. territories, with Puerto Rico reporting the most. In 2012, Puerto Rico reported 199 deaths, which was the last time the commonwealth declared a dengue epidemic.

Dengue fever poses a significant public health threat in many parts of the world, particularly in regions with high mosquito populations and limited access to health care.

The Centers for Disease Control and Prevention said it is working with the Puerto Rico Department of Health and the Puerto Rico Vector Control Unit on surveillance of the virus and on training health providers. The CDC is also involved with cleanup campaigns, using insecticides where appropriate, and in education efforts.

The Puerto Rico Department of Health said it is working to enhance surveillance, laboratory, vector control and outreach interventions.

“We work on a community-based strategy approach. … Those interventions can [include]: educational materials, field visits to identify environmental health risks, coordination with municipalities to eliminate potential mosquitoes breeding sites and applications of larvicides or adulticides,” Melissa Marzán, chief epidemiologist officer of the Puerto Rico Department of Health, said in an email.

What is dengue fever, and how is it spread?

Dengue can cause high fever, severe headache, joint and muscle pain, rash and, in severe cases, death.

The virus that causes dengue infect 100 million to 400 million people worldwide each year, according to the World Health Organization.

It is transmitted to humans most commonly by the Aedes aegypti mosquito. Dengue is caused by four related viral serotypes, which are variations or strains of a virus. Because the variations can affect the immune response to the virus, infection with one serotype does not provide immunity against others. So people can suffer from dengue repeatedly, with the risk of severe complications increasing with each subsequent infection.

According to the CDC, about 1 in 4 people who get infected with dengue develop symptoms, and 1 in 20 people who get sick from the virus suffer severe illness.

Infants, older adults and those who are pregnant or immunocompromised have a greater risk of developing severe symptoms.

Where is it most commonly found?

Dengue is most common in tropical and subtropical climates — prevalent in countries with warm climates where the Aedes aegypti mosquito thrives. Regions in Southeast Asia, the Western Pacific, the Americas, Africa and the Eastern Mediterranean are particularly affected by dengue fever outbreaks. Countries such as Brazil, India, the Philippines, Indonesia and Thailand have reported high numbers of dengue cases in recent years.

Albert Ko, an infectious-disease physician and epidemiologist at the Yale School of Public Health, said the ongoing spike in Puerto Rico is unusual because transmission of the virus in drier months is typically low.

“The fact that it’s happening now is really kind of a heads up, a warning sign for us. … We’re having introductions of the virus by people traveling, and we’re seeing these outbreaks in Puerto Rico that are out of the season, which are really unusual,” Ko said.

Earlier this month, The Washington Post reported a surge in cases in Brazil, causing many states in the South American country to declare a state of emergency.

Public health experts say these surges are likely to become more common because of climate change and because diseases once believed to be contained to specific regions are spreading because of increased travel, facilitating easier transmission of disease worldwide.

“A good example is what we experienced during Zika, also transmitted by the same mosquito when infections in travelers spread the virus quickly across the Americas,” Ko said.

How is dengue treated?

There are no antiviral medications to treat dengue. Instead, treatment involves supportive care such as ibuprofen, acetaminophen and other nonsteroidal anti-inflammatory drugs, or NSAIDs.

“It can have a waxing and waning course, but you’re usually talking about a couple of weeks overall where it’s worse in the first week or so, and we start to see some recovery over time,” said Jack C. O’Horo, an infectious-disease physician at the Mayo Clinic.

Is there a vaccine?

Two vaccines are available for dengue. One is Dengvaxia, developed by Sanofi. The other is Dengue Tetravalent Vaccine, developed by Takeda Pharmaceuticals. Both shots provide protection against all four dengue virus serotypes.

Dengvaxia is the only Food and Drug Administration approved vaccine for children 9 to 16 years old. But one hurdle is that children must have proof of a prior infection — as shown by a blood test — before getting the shot.

Ko said that for people who have never been exposed to dengue and get the vaccine, it has been shown that they have a higher risk of developing severe dengue if they become infected. He said the reason is because there is “incomplete protection against the four serotypes (some more than others) so the problem is when breakthrough infections occur, there is an increased risk of severe outcomes such as hospitalizations in those who weren’t exposed previously to dengue.”

The other vaccine, Qdenga, manufactured by a Japanese pharmaceutical company, Takeda, is approved in the European Union, United Kingdom, Brazil, Argentina, Indonesia and Thailand. In July 2023, the company withdrew its application for FDA approval citing “aspects of data collection, which cannot be addressed” in the designated review timeline.