Dengue isn’t normally a disease that people in the U.S. are concerned about. But cases in the U.S. this year are already closing in on the total number of cases in all of 2023. More than 9.7 million cases have been reported in North and South America, which is twice as many cases as were reported throughout last year. The global picture is also alarming: The World Health Organization (WHO) reported 6.5 million cases in 2023, a record high.
Here’s what to know about the rise of dengue in the U.S.
What is dengue?
The infection can have very different effects, ranging from no symptoms to death. But its nickname is break-bone fever, for the severe chills that accompany a fever that can spike as high as 104°F. While not fatal in most cases, it can be if it progresses to severe dengue, in which people develop dangerous bleeding as their blood vessels become compromised and start leaking, and clotting factors begin dropping.
Why cases are increasing
Since dengue is carried by the Aedes mosquito, anything that contributes to increased mosquito populations will also fuel rises in dengue (along with other mosquito-borne diseases like West Nile Virus, Zika, and malaria.) In 2023, 43 states in the U.S. reported West Nile cases, and so far in 2024, 11 states have. Mosquito populations are booming around the world, thanks to climate change, which is providing more of the warmer climates that mosquitos favor, as well as heavy rains, which provide the pools of water where the insects lay their eggs. The dengue virus is taking full advantage of these fertile circumstances.
Dr. Gabriela Paz-Bailey, chief of the U.S. Centers for Disease Control and Prevention’s (CDC) dengue branch, says there’s another reason for the dengue surge this year. Dengue outbreaks tend to be cyclical, caused by any combination of four main virus types, known simply as dengue 1, 2, 3, and 4. “People who get infected with one type of dengue are protected for a couple of years, but the immunity wears off and they are susceptible to getting another type,” she says. “In 2019, the large epidemic in the Americas was caused mainly by dengue 1 and 2, and now dengue 3 is circulating, so people have less immunity.”
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How many cases of dengue have been reported in the U.S.?
According to the CDC, more than 2,500 cases have been reported so far this year. About 800 of them were contracted when people were traveling abroad, and the rest were locally transmitted. Most of the U.S. cases occur in Puerto Rico, where high concentrations of mosquitos led health authorities there to declare dengue a public health emergency in March.
Infectious disease experts say that most local cases in the continental U.S. likely occur when a mosquito that has bitten an infected recent traveler then passes the disease to someone else. In the continental U.S., locally acquired cases of dengue have been reported in Florida.
“The reservoir is already seeded, and mosquito populations [in the continental U.S.] already have [the virus],” says Dr. Robert Murphy, professor of infectious diseases and executive director of the Robert J. Havey, MD Institute for Global Health at Northwestern University. “There is very little in the way of stopping it, other than being more aware and ready to diagnose cases.”
What is the government doing?
The CDC issued a Health Alert Network advisory on June 25 urging doctors to be aware of the rise in cases and to test people who report symptoms of fever and chills and who have recently traveled, especially to parts of the world where dengue is endemic, like Puerto Rico, the U.S. Virgin Islands, Micronesia, the Marshall Islands, and Palau. Identifying people who are infected can also help limit spread of the virus if they restrict their time outdoors when they might be bitten by local mosquitos that would then become additional vectors for the disease.
Paz-Bailey says the blood test is available at public health labs and a few commercial labs. For now, “there is no expectation that there will be large outbreaks within the continental U.S.,” she says. “But there can be small chains of transmission, so we need to do everything we can so clinicians can identify dengue early, order the right tests, and help everyone protect themselves.”
Is there a treatment for dengue?
There is no antiviral treatment; care involves lowering fever and making sure people remain hydrated. Doctors should closely monitor patients who have other health conditions, such as heart disease or diabetes, as the infection can make those conditions worse. “Dengue is a systemic illness, and the virus can hit any part of the body,” says Dr. Sharon Nachman, professor of pediatrics at Stony Brook Children’s Hospital. “There is no part of you dengue doesn’t touch.”
While most people will recover from the infection after two to seven days, some will develop severe dengue and require hospitalization. In those cases, patients continue to vomit, feel extremely fatigued and lethargic, and develop dangerous bleeding that can lead to shock. Doctors should watch for these warning signs 24 to 48 hours after the fever subsides.
Is there a dengue vaccine?
There is a vaccine, Dengvaxia, approved in the U.S., but only for children 9 to 16 years old who have already had a dengue infection and lived in areas where the disease was endemic. The shot involves three doses given six months apart. The vaccine, made by Sanofi, was approved in 2019, but because of the small population it targets, demand for it has been low. A spokesperson said the company has decided to stop manufacturing the vaccine by 2026 “due to low global demand.”
Other countries around the world have approved vaccines that the WHO recommends and that can protect against severe disease, but those are not approved in the U.S. One, made by a Japanese manufacturer, is effective against dengue 1 and dengue 2 but not against dengue 3, and the company is currently studying its effectiveness against dengue 4, says Paz-Bailey.
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If I’ve had dengue before, do I have immunity?
Yes, to a certain extent. But because there are four types of dengue, people can be infected multiple times and become equally sick if a different version of the virus infects them each time. And the dengue virus has a unique feature that makes repeated infections potentially more severe. “Something interesting happens after you get infected the first time,” says Paz-Bailey. “After the first infection, the antibodies you develop actually help the virus infect cells during a second infection, which results in more virus produced, a higher viral load, and a higher risk of severe dengue. Because of how the immune system behaves, the antibodies you develop after a first infection will not help you during a second infection, but may actually make that infection worse.”
Is there anything I can do to protect myself?
The best advice is to avoid getting bitten by a mosquito in the first place, since so many of them now carry different diseases. Using insect repellent, and wearing long sleeves and long pants to cover any exposed skin, can protect them from feeding off your blood. And if possible, Paz-Bailey suggests using an air conditioner and closing windows to prevent mosquitos from flying indoors. Getting rid of their breeding grounds is also critical. “Inspect your home for any standing water and empty, scrub and turn over any receptacles that can hold water,” she says. “Even a bottle cap and can hold 100 mosquito eggs.”