Federal health officials 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 hit record numbers.
In the first half of this year, countries in the Americas reported twice as many cases as were reported in all of 2023, the Centers for Disease Control and Prevention reported Thursday in an alert to health care providers.
The region has seen nearly 10 million cases of the virus so far in 2024, most of which originated in outbreaks in South American countries like Brazil and Argentina.
While the local transmission of the virus in the mainland United States has been limited, Puerto Rico, which is classified as having “frequent or continuous” dengue risk, declared a public health emergency in March and has reported nearly 1,500 cases.
Cases of dengue fever, a mosquito-borne viral illness that can be fatal, are surging 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 year or two, including France, Italy and Chad, in Central Africa.
There have even been a few hundred cases of local transmission in the United States. Florida health officials urged the public to take precautions — like wearing bug spray and dumping out standing water — after reporting a locally acquired case of dengue this month.
What is dengue fever, and why is it becoming more widespread?
Dengue, a viral fever, is transmitted by Aedes species of mosquitoes. It can cause excruciating joint pain. It is also known by a grim nickname: breakbone fever.
The Aedes aegypti mosquito, which has been driving 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 small bits of trapped water in places such as old tires, discarded bottle caps, and trays used to catch air-conditioner drips.
Now, as more people move to urban areas — many to 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 is flourishing.
“Aedes mosquitoes thrive in warm and humid environments, so definitely climate change and rising temperatures and also extreme weather events are helping extend their habitat range,” said Dr. Gabriela Paz-Bailey, chief of the dengue branch at the C.D.C.’s National Center for Emerging and Zoonotic Infectious Diseases.
How dangerous is dengue?
Only one in four dengue cases are symptomatic. Some infections may produce only a mild flulike illness. But others can result in dreadful symptoms, including headache, vomiting, high fever and aching joint pain. Full recovery can take weeks.
About 5 percent of people who get sick will progress to so-called severe dengue, which causes plasma, the protein-rich fluid component of blood, to leak out of blood vessels. Some patients may go into shock, causing organ failure.
Severe dengue has a mortality rate of up to 5 percent in people whose symptoms are treated. Left untreated, however, the mortality rate is 15 percent.
Severe dengue may go untreated because patients live far from medical care or can’t afford it. It can happen because hospitals are overwhelmed with cases during an outbreak, or because dengue isn’t diagnosed in time as it appears in a new area.
Who is at risk?
Already 40 percent of people globally live in areas where they are at risk of dengue infection; the disease is most common in tropical countries, such as Brazil.
The people most vulnerable to dengue live in housing that doesn’t keep mosquitoes away from them. In studies on communities along the U.S. southern border, in areas where the Aedes aegypti mosquito is well established, researchers found that there were as many or sometimes even more of the mosquitoes on the Texas side, but far fewer dengue cases than on the Mexican side.
That is because more people on the U.S. side of the border had screened windows and air-conditioners, which limited their exposure to mosquitoes, and lived farther apart and were less social.
By making fewer visits to friends and relatives, residents were less likely to take the virus into new areas where a mosquito might pick it up from them and pass it on.
It is unlikely that dengue will become a serious problem in the United States, “as long as people keeping living like they’re living now,” said Thomas W. Scott, a dengue epidemiologist and professor emeritus at 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 that haven’t experienced it before.
In addition to climate change, rising rates of urbanization around the world are playing a role, said Alex Perkins, who is an associate professor of biological sciences at the University of Notre Dame and an expert in the mathematical modeling of dengue transmission.
If people have recently come from rural areas, they are unlikely to have priority immunity, so the virus can move swiftly 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 cautionary tale. Historically, the region saw only a handful of dengue cases each year. Then in 2014, there were 42,000 cases in Guangdong Province.
“All of a sudden in one year, it grew by a couple of orders of magnitude without any real forewarning,” he said.
“In endemic settings, we’re continuing to have record years, year in and year out, and that’s what’s driving all these imported cases in the United States and elsewhere,” he added.
“And when it comes to the more marginal transmission settings, such as the southern United States, Southern Europe, China — it’s not getting better there either. So it’s really getting better nowhere: it’s all bad.”
Is there treatment 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 firm Sanofi, was rolled out widely in countries such as the Philippines and Brazil in 2015. But two years later, the company said it was causing vaccinated people who caught the virus to have more severe cases.
The C.D.C. recommends Dengvaxia only for use in endemic areas for patients with a laboratory-confirmed previous dengue infection.
The World Health Organization recently recommended a new vaccine, called QDENGA, which can be used regardless of prior infection status, for children aged 6 to 16 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 United States 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 the inspection of homes and construction sites for breeding areas, with high fines for rules violations.
“It’s a successful approach, but they have a really large budget to support those activities,” Dr. Paz-Bailey said. “But not every country has that.”
Brazil and Colombia have had success deploying a bacteria called Wolbachia. When Aedes aegypti mosquitoes are infected with the bacteria, they can no longer pass on the dengue virus.
Researchers in South American are mass-producing mosquitoes infected with Wolbachia and releasing them to breed with wild insects in an effort to get the bacteria through the mosquito population.
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