Federal health officials have warned that the risk of contracting dengue in the United States has increased this year, a worrying sign as global cases of the mosquito-borne disease have reached record numbers.
In the first half of this year, countries in the Americas reported twice as many cases as in all of 2023, the Centers for Disease Control and Prevention said in an advisory to health professionals on Thursday.
The region has recorded nearly 10 million virus cases so far in 2024, most of which originated from outbreaks in South American countries such as Brazil and Argentina.
While local transmission of the virus in the continental United States has been limited, Puerto Rico, classified as having “frequent or continuous” risk, declared a public health emergency in March and reported nearly 1,500 cases.
Cases of dengue fever, a mosquito-borne viral disease that can be fatal, are increasing around the world. The increase is occurring both in places that have long struggled with the disease and in areas where its spread was unheard of until the last two years, including France, Italy and Chad, in central Africa.
In the United States, there have even been a few hundred cases of local transmission. Florida health officials urged citizens to take precautions — such as wearing mosquito spray and dumping standing water — after reporting a locally acquired case of dengue this month.
What is dengue fever and why is it becoming more and more widespread?
Dengue, a viral fever, is transmitted by mosquitoes of the Aedes species. It can cause excruciating joint pain. It is also known by a sad nickname: bone-breaking fever.
The Aedes a Egypti mosquito, which is the cause of many of the current epidemics, 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 has adapted to urban areas, feeding on people and breeding in small bits of water trapped in places such as old tires, discarded bottle caps and trays used to catch drips from air conditioners.
Now, as more people move to urban areas – many into lower-quality housing in developing countries – they are more vulnerable to the virus. And climate change is bringing the mosquito to new places, where it thrives.
“Aedes mosquitoes thrive in warm, humid environments, so certainly climate change, rising temperatures and even extreme weather events are helping to extend their habitat,” said Dr. Gabriela Paz-Bailey, head of the dengue branch at the CDC's National Center for Emerging. and zoonotic infectious diseases.
How dangerous is dengue?
Only one in four cases of dengue is symptomatic. Some infections may produce only a mild flu-like illness. But others can cause terrible symptoms, including headaches, vomiting, high fever and joint pain. Full recovery can take weeks.
About 5% of people who get sick will progress to so-called severe dengue, which causes plasma, the protein-rich fluid component of blood, to leak from blood vessels. Some patients may go into shock, causing organ failure.
Severe dengue has a mortality rate of up to 5% in people whose symptoms are treated. If left untreated, however, the mortality rate is 15%.
Severe dengue may go untreated because patients live far from medical care or cannot afford it. It may happen because hospitals are overwhelmed with cases during an epidemic or because dengue is not diagnosed in time as it appears in a new area.
Who is at risk?
Already 40% of people globally live in areas at risk of dengue infection; the disease is more common in tropical countries, such as Brazil.
People most vulnerable to dengue live in homes that don’t keep mosquitoes away. In studies of communities along the southern U.S. border, in areas where the Aedes a Egypti mosquito is well established, researchers found that there were just as many mosquitoes, or sometimes more, on the Texas side, but far fewer cases of dengue on the Mexican side.
That's because most people on the U.S. side of the border had screened windows and air conditioning, which limited their exposure to mosquitoes, lived further away from others, and were less sociable.
By visiting fewer friends and relatives, residents were less likely to bring the virus to new areas where a mosquito could infect and transmit it.
Dengue is unlikely to become a serious problem in the United States “as long as people continue to live the way they live now,” said Thomas W. Scott, a dengue epidemiologist and professor emeritus at the University of California, Davis.
Outside of 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 to places where they have never experienced it before.
In addition to climate change, increasing rates of urbanization around the world also play 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 are recently from rural areas, they are unlikely to have priority immunity, so the virus can spread rapidly through the population.
“I think the general expectation that this is going to be a growing problem in the United States is reasonable,” he said.
Dr Perkins said the experience in southern China offers a warning. Historically, the region recorded only a handful of dengue cases each year. Then in 2014 there were 42,000 cases in Guangdong province.
“All of a sudden, in a year, it grew a couple of orders of magnitude without any real warning,” he said.
“In endemic settings, we continue to have record years, year after year, and that's what's driving all these imported cases into the United States and elsewhere,” he added.
“And when it comes to the more marginal transmission locations, like the southern US, southern Europe, China, things are not getting better there either. So the situation doesn't really improve anywhere: everything is bad.”
Is there a cure for dengue?
There is no treatment for dengue infection. Patients' symptoms are managed with medications, such as those needed to control pain. But drug companies have antivirals in clinical trials.
Is there a vaccine?
The effort to find a dengue vaccine has been long and complicated.
Dengvaxia, a vaccine developed by the French company Sanofi, was widely distributed in countries including the Philippines and Brazil in 2015. But two years later, the company said it was causing more severe cases in vaccinated people who had contracted the virus.
The CDC recommends the use of Dengvaxia only in endemic areas for patients with a prior laboratory-confirmed dengue infection.
The World Health Organization recently recommended a new vaccine, called QDENGA, which can be used regardless of previous infection status, for children aged 6 to 16 years living in areas with high dengue transmission.
The vaccine has already been introduced in Indonesia, Brazil, Thailand and 16 European countries, including Britain and Italy. But it won't be available in the US anytime soon.
What else can we do?
Some countries have acted aggressively against dengue and are managing to control it. Singapore, for example, uses a combination of methods, including inspecting homes and yards for breeding areas, with large fines for violations of the rules.
“It's an effective approach, but they have a really large budget to support these activities,” Dr. Paz-Bailey said. “But not all countries have it.”
Brazil and Colombia have had success using a bacterium called Wolbachia. When Aedes a Egypti mosquitoes become infected with the bacterium, they can no longer transmit the dengue virus.
South American researchers are mass-producing Wolbachia-infected mosquitoes and releasing them to breed with wild insects in an effort to spread the bacterium throughout the mosquito population.