Federal health officials have warned that the threat of dengue in the United States has increased this year, a worrying sign as global cases of the mosquito-borne disease reach record numbers.
In the first half of this year, twice as many cases were reported in the U.S. as in all of 2023, the Centers for Disease Control and Prevention said in an alert to health care providers on Thursday.
The region has seen nearly 10 million cases of the virus so far in 2024, most of them originating from outbreaks in South American countries such as Brazil and Argentina.
While local transmission of the virus has been limited in the continental United States, Puerto Rico, which is classified as having a “persistent or continuing” dengue risk, declared a public health emergency in March and has reported nearly 1,500 cases.
Cases of dengue fever, a deadly viral disease carried by mosquitoes, are on the rise worldwide. The increase is taking place in places that have long struggled with the disease and also in areas where outbreaks were unheard of until the past year or two, such as France, Italy and Chad in central Africa.
There have also been a few hundred cases of local transmission in the United States. Florida health officials have urged people to take precautions — such as using insecticides and draining out standing water — after a locally transmitted case of dengue was reported this month.
What is dengue fever and why is it becoming more widespread?
Dengue is a viral fever spread by Aedes species mosquitoes. It can cause severe joint pain. It is also known by a terrifying nickname: breakbone fever.
The Aedes aegypti mosquito, which is causing many of the current outbreaks, is native to Africa, where it originally lived in forests and fed on animals. But decades ago the species spread to the rest of the world via trade routes.
It adapted to urban areas, feeding on people and breeding in tiny particles of water trapped in old tyres, discarded bottle caps, and trays used to collect drippings from air conditioners.
Now, as more and more people move to urban areas — many living in low-quality housing in developing countries — they are more vulnerable to the virus. And climate change is bringing mosquitoes to new places, where they are thriving.
“Aedes mosquitoes thrive in warm and humid environments, so certainly climate change and rising temperatures and extreme weather events are helping them expand their habitat,” said Dr. Gabriela Paz-Belli, chief of the dengue branch at the CDC’s National Center for Emerging and Zoonotic Infectious Diseases.
How dangerous is dengue?
Only one in four dengue cases show symptoms. Some infections may cause only a mild flu-like illness. But other infections can cause terrible symptoms, such as headache, vomiting, high fever and joint pain. Full recovery can take several weeks.
About 5 percent of people who get sick will progress to so-called severe dengue, which causes plasma, the protein-rich liquid component of blood, to leak out of the blood vessels. Some patients go into shock, which can lead to organ failure,
The mortality rate is up to 5 percent in people who seek treatment for severe dengue symptoms. However, if untreated, the mortality rate can be as high as 15 percent.
Severe dengue may go untreated because patients avoid medical care or cannot afford it. This may happen because hospitals are overwhelmed with cases during an outbreak, or because dengue is not diagnosed in time because it appears in a new area.
Who is at risk?
Already, 40 percent of people worldwide live in areas where they are at risk of dengue infection; the disease is most common in tropical countries such as Brazil.
People most susceptible to dengue live in homes that do not protect them from mosquitoes. In studies of communities along the U.S. southern border, where Aedes aegypti mosquitoes thrive, researchers found that on the Texas side mosquito numbers were the same or sometimes higher, but there were far fewer dengue cases than on the Mexican side.
This is because most people living on the American side of the border had screens on their windows and air conditioners, which limited their exposure to mosquitoes, and they lived farther from each other and were less social.
By socializing less with friends and relatives, residents were less likely to carry the virus to new areas, where mosquitoes could pick up the virus from them and spread it to others.
“As long as people continue to live the way they do now, it is unlikely that dengue will become a serious problem in the United States,” said Thomas W. Scott, a dengue epidemiologist and professor emeritus at the University of California, Davis.
Aside from Puerto Rico, most dengue cases in the United States result from travel to countries where the virus is endemic. But scientists say dengue will continue to spread in places where it has not previously been an outbreak.
In addition to climate change, increasing rates of urbanization around the world are also playing a role, said Alex Perkins, an associate professor of biological sciences at the University of Notre Dame and an expert in mathematical modeling of dengue transmission.
If people have recently come from rural areas, they are unlikely to have developed primary immunity, so the virus can spread rapidly through the population.
“I think it’s a reasonable general expectation that this will be a growing problem in the United States,” he said.
Dr. Perkins said the experience in southern China offers a cautionary tale. Historically, the region saw only a handful of dengue cases each year. Then in 2014, 42,000 cases were reported in Guangdong province.
“Suddenly, in one year, without any prior warning, it increased several fold,” he said.
“In endemic situations, we are reporting record numbers of cases year after year, and that is why we are getting all these imported cases in the United States and elsewhere,” he said.
“And when it comes to more marginal transmission areas like the southern United States, southern Europe, China — the situation is not getting better there either. So the situation is not really getting better anywhere: everything is bad.”
Is there any cure for dengue?
There is no cure for dengue infection. Patients’ symptoms are controlled with medications, such as those needed to control pain. But pharmaceutical companies have antiviral drugs in clinical trials.
Is there a vaccine available?
The effort to find a vaccine for dengue has been long and complicated.
Dengvaxia, a vaccine developed by French company Sanofi, was widely introduced in countries such as the Philippines and Brazil in 2015. But two years later, the company said it was causing more severe cases of the virus among vaccinated people.
The CDC recommends using Dengvaxia in endemic areas only for patients with a previous laboratory-confirmed dengue infection.
The World Health Organization recently recommended a new vaccine called QDENGA to be used for children ages 6 to 16 years living in areas with high dengue infections, regardless of prior infection status.
The vaccine is already available in Indonesia, Brazil, Thailand and 16 European countries, including Britain and Italy. But it won’t be available in the United States anytime soon.
what else can we do?
Some countries have acted aggressively against dengue and have managed to control it. For example, Singapore uses a variety of methods, including inspecting homes and construction sites for breeding grounds, as well as imposing heavy fines for violations.
“It’s a successful approach, but they have really big budgets to support these activities,” Dr. Paz-Bailey said. “But not every country has that.”
Brazil and Colombia have found success using a bacteria called Wolbachia. When Aedes aegypti mosquitoes are infected with this bacteria, they cannot spread the dengue virus.
Researchers in South America are mass-producing mosquitoes infected with Wolbachia and releasing them to breed with wild insects, in an attempt to spread the bacteria through mosquito populations.
