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 hit record numbers.
In the first half of this year, the Americas reported twice as many cases as in all of 2023, the Centers for Disease Control and Prevention said Thursday in a warning to health care providers.
The region has seen nearly 10 million cases of the virus so far in 2024, most of which stemmed from outbreaks in South American countries such as Brazil and Argentina.
While local transmission of the virus in the mainland United States has been limited, Puerto Rico, which is classified as a “frequent or ongoing” dengue risk, declared a public health emergency in March and reported nearly 1,500 cases.
Cases of dengue fever, a viral mosquito-borne disease that can be fatal, are on the rise 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 past two years, 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 — such as wearing bug spray and throwing away standing water — after reporting a locally acquired case of dengue fever this month.
What is dengue and why is it becoming more common?
Dengue fever, a viral fever, is transmitted by the Aedes mosquito species. It can cause excruciating joint pain. It is also known by a grim nickname: breakbone fever.
The Aedes aegypti mosquito, which has caused many of the current outbreaks, is native to Africa, where it originally lived in forests and fed on livestock. But decades ago the species spread to the rest of the world via trade routes.
Adapted to urban areas, it feeds on humans and breeds in small patches of trapped water in places like old tires, discarded bottle caps and trays used to catch air conditioner drips.
Now, as more people move into 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 and humid environments, so certainly climate change and rising temperatures, as well as extreme weather events are helping to expand their habitat range,” said Dr Gabriela Paz-Bailey, head of the of dengue fever at CDC’s National Center for Emerging Drugs. and zoonotic infectious diseases.
How dangerous is dengue fever?
Only one in four dengue cases is symptomatic. Some infections may only cause a mild flu-like illness. But others can lead to frightening symptoms, such as headache, vomiting, high fever and joint pain. Full recovery may take weeks.
About 5 percent of people who get sick will develop what’s called severe dengue fever, 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 fever has a mortality rate of up to 5 percent in people whose symptoms are treated. If left untreated, however, the fatality rate is 15 percent.
Severe dengue may go untreated because patients live far from or cannot afford medical care. It can happen because hospitals are overwhelmed by cases during an outbreak, or because dengue is not diagnosed in time as 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 more common in tropical countries, such as Brazil.
People who are most vulnerable to dengue live in housing that does not keep mosquitoes away. In studies of communities along the southern US border, areas where the Aedes aegypti mosquito is well established, researchers found that there were as many or sometimes more mosquitoes on the Texas side, but far fewer dengue cases than on the side of Mexico.
That’s because more people on the American side of the border had screened windows and air conditioners, which limited their exposure to mosquitoes, and lived further away and were less social.
By making fewer visits to friends and family, residents were less likely to carry the virus to new areas where a mosquito can pick it up and spread it.
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 dengue cases in the United States come 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, increasing 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 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 quickly 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 offered a cautionary tale. Historically, the region has had only a handful of dengue cases each year. Then, in 2014, there were 42,000 cases in Guangdong province.
“All of a sudden within a year, it went up by a few 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 in 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 — it’s not getting any better there either. So it doesn’t get better anywhere: everything is bad.”
Is there a cure for dengue fever?
There is no cure for dengue infection. Patients’ symptoms are treated with medications, such as those required for pain control. But drug companies have antiviral drugs in clinical trials.
Is there a vaccine?
The quest to find a dengue vaccine has been long and complicated.
Dengvaxia, a vaccine developed by French company Sanofi, was widely marketed in countries including 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 outbreaks.
CDC recommends Dengvaxia only for use in endemic areas for patients with laboratory-confirmed previous 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 United States anytime soon.
What else can we do?
Some countries have acted aggressively against dengue fever and are managing to control it. Singapore, for example, uses a combination of methods, including inspecting homes and construction sites for breeding areas, with heavy fines for rule violations.
“It’s a successful approach, but they have a really big budget to support these activities,” said Dr. Paz-Bailey. “But not all countries have it.”
Brazil and Colombia have had success growing a bacterium called Wolbachia. When Aedes aegypti mosquitoes become infected with the bacteria, they can no longer transmit 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 pass the bacteria through the mosquito population.
