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Federal health authorities warned that the risk of dengue in the United States has increased this year. This is a worrying sign as the number of cases of the mosquito-borne disease has reached record highs worldwide.
In the first half of this year, countries in North and South America reported twice as many cases 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 which originated in 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 “frequent or continuousDengue 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 for a year or two, including France, Italy and Chadin Central Africa.
There have even been a few hundred cases of local transmission in the United States. Health officials in Florida urged the public to take precautions — such as wearing bug 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 common?
Dengue, a viral fever, is transmitted by Aedes mosquitoes. It can cause excruciating joint pain. It is also known by a grim nickname: breakbone fever.
The Aedes aegypti mosquito, which causes 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.
The species adapted to urban areas, feeding on humans and breeding in small patches of airborne water, such as old tires, discarded bottle caps and air-conditioning drip trays.
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 can thrive.
“Aedes mosquitoes thrive in warm and moist environments, so climate change and rising temperatures as well as extreme weather events are certainly helping to expand their habitat,” says Dr. Gabriela Paz-Bailey, chief of the dengue division at the CDC’s National Center for Emerging. and zoonotic infectious diseases.
How dangerous is dengue?
Only one in four dengue cases is symptomatic. Some infections may cause only a mild flu-like illness. But others can cause terrible symptoms, including headaches, vomiting, high fever and aching joints. Full recovery can take weeks.
About 5 percent of people who become ill will progress to so-called severe dengue, which causes plasma, the protein-rich liquid 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 percent in people whose symptoms are treated. If left untreated, however, the mortality rate is 15 percent.
Severe dengue can go untreated because patients live far from medical care or cannot afford it. It can happen because hospitals are overwhelmed with cases during an outbreak, or because dengue is not diagnosed in time as it occurs in a new area.
Who is at risk?
Globally, 40 percent of people already 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 homes that do not keep mosquitoes away from them. In studies on communities along the US southern borderIn areas where the Aedes aegypti mosquito is well established, researchers found that there were as many or even more mosquitoes on the Texas side, but far fewer cases of dengue than on the Mexican side.
That’s because more people on the American side of the border had screens on their windows and air conditioning, which meant they were less bothered by mosquitoes. Moreover, they lived further away from each other and were less social.
By visiting friends and family fewer times, residents were less likely to carry the virus to new areas where a mosquito could pick it up from them and pass it on.
Dengue is unlikely to become a serious problem in the United States, “as long as people continue to live the way they are living now,” said Thomas W. Scott, a dengue epidemiologist and professor emeritus at the University of California, Davis.
Outside of Puerto Rico, most cases of dengue in the United States result from travel to countries where the virus is endemic. But scientists say dengue will continue to spread to places it hasn’t happened before.
In addition to climate change, increasing urbanization worldwide is also playing a role, said Alex Perkins, an associate professor of biological sciences at the University of Notre Dame and an expert in the mathematical modeling of dengue transmission.
If people recently come from rural areas, they are unlikely to be prioritized for immunity. The virus can then spread quickly among 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 had only a handful of dengue cases each year. In 2014, there were 42,000 cases in Guangdong province.
“All of a sudden it grew by several orders of magnitude in one year, without any real warning,” he said.
“In endemic areas, we continue to have record years year after year, and that’s what’s driving all of these imported cases into the United States and elsewhere,” he added.
“And when it comes to the more marginal transmission settings, like the southern United States, southern Europe, China — things aren’t getting better there either. So it’s not really getting better anywhere: it’s all bad.”
Is there a treatment for dengue?
There is no treatment for dengue infection. Patients’ symptoms are treated with medications, such as those needed to control pain. But pharmaceutical companies have antivirals in clinical trials.
Is there a vaccine?
Finding a vaccine against dengue was a long and complicated process.
Dengvaxia, a vaccine developed by French firm Sanofi, was widely rolled out in countries such as the Philippines and Brazil in 2015. But two years later, the company said it caused vaccinated people who contracted the virus to have more severe cases.
The CDC recommends Dengvaxia for use in endemic areas only in patients with laboratory-confirmed prior dengue infection.
The World Health Organization recently recommended a new vaccinecalled 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 United States anytime soon.
What else can we do?
Some countries have taken aggressive action against dengue and are managing to control the disease. Singapore, for example, uses a combination of methods, including inspecting homes and construction sites for breeding areas, with heavy fines for violations of the rules.
“It’s a successful approach, but they have a very large budget to support those activities,” Dr. Paz-Bailey said. “But not every country has that.”
Brazil and Colombia have had success using a bacterium called Wolbachia. When Aedes aegypti mosquitoes are infected with the bacterium, they can no longer transmit the dengue virus.
Researchers in South America are mass-producing mosquitoes infected with Wolbachia and releasing them to mate with wild insects in an attempt to get the bacteria through the mosquito population.
