Federal health officials have warned that the risk of mosquito-borne dengue fever infections has increased in the United States this year, a worrying sign as cases of the mosquito-borne disease reach record highs worldwide.
The US Centers for Disease Control and Prevention (CDC) reported on Thursday, in a warning to health workers, that the number of infections reported in the Americas in the first half of this year was twice as high as in all of 2023.
The region has reported around 10 million cases of the virus so far in 2024, most of which are due to outbreaks that began in South American countries such as Brazil and Argentina.
While local transmission of the virus in the continental U.S. is limited, Puerto Rico, which is classified as having a “frequent or persistent” risk of dengue infection, declared a public health emergency in March and has reported about 1,500 cases.
Cases of dengue fever, a deadly mosquito-borne viral disease, are skyrocketing around the world, not just in places that have long struggled with the disease, but also in places where dengue was unknown until the past year or two, such as France, Italy and Chad in Central Africa.
Hundreds of community-acquired cases have been reported in the United States, and health officials in Florida urged residents to take precautions such as spraying insect repellent and discarding standing water after local cases of dengue were reported this month.
What is dengue fever and why is it becoming more prevalent?
Dengue is a viral fever transmitted by the Aedes aegypti mosquito that can cause severe joint pain. It's also known by the ominous nickname “breakbone fever.”
The Aedes aegypti mosquito, responsible for many current outbreaks, is native to Africa, where it originally lived in forests and fed on animals, but decades ago the species spread around the world via trade routes.
The pest has adapted to urban areas, feeding on humans and breeding in small pockets of water such as old tires, discarded bottle caps and air conditioner drip trays.
Now, more people are migrating to cities, often to lower-quality housing in developing countries, making them more vulnerable to the virus, and climate change is introducing mosquitoes to new places where they can breed.
“Aedes mosquitoes thrive in warm, moist environments, so climate change, increasing temperatures, and extreme weather events are certainly contributing to the expansion of Aedes' range,” said Dr. Gabriela Paz Bailey, director of the Dengue Section in CDC's National Center for Emerging and Zoonotic Infectious Diseases.
How dangerous is dengue fever?
Only one in four cases of dengue fever causes symptoms. Some infections cause only mild, flu-like symptoms, while others can be horrific, with headaches, vomiting, high fevers and joint pain. It can take weeks for people to fully recover.
About 5 percent of cases progress to what is known as severe dengue, in which plasma, the protein-rich liquid part of blood, leaks out of the blood vessels, causing some patients to go into shock and suffer organ failure..
Severe dengue has a mortality rate of up to 5 percent in people whose symptoms are treated, but if left untreated, the mortality rate is 15 percent.
Severe dengue may go untreated because patients live far away from medical care or cannot afford it. This can happen during epidemics when hospitals are overwhelmed with patients, or when dengue appears in a new area and is not diagnosed in time.
Who is at risk?
Already, 40 percent of the world's population lives in areas at risk of dengue infection, with the disease most prevalent in tropical countries such as Brazil.
People most susceptible to dengue live in homes that don't keep mosquitoes out. In studying communities along the southern U.S. border where Aedes aegypti mosquitoes are common, researchers found that there were far fewer dengue cases on the Mexican side, despite there being as many or more mosquitoes on the Texas side.
That's because on the American side of the border, many people have screens on their doors and air conditioners, reducing exposure to mosquitoes, and because people live farther apart and are less social.
With residents visiting friends and relatives less, mosquitoes are less likely to carry the virus to new areas where it can be picked up and transmitted.
Dengue is unlikely to become a serious problem in the United States “as long as people continue to live the way they are 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 U.S. result from travel to countries where the virus is endemic, but scientists say dengue will continue to spread to areas that have never experienced it before.
In addition to climate change, rising 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.
People who have recently arrived from rural areas are unlikely to have developed priority immunity, so the virus can spread rapidly through a population.
“I think it's reasonable to expect generally that this will be a growing problem in the United States,” he said.
Dr Perkins said the experience in southern China was instructive: Historically, the region had only a few cases of dengue each year, but in 2014 there were 42,000 cases in Guangdong province.
“All of a sudden, out of nowhere, it went up by several orders of magnitude in a year,” he said.
“We are seeing record numbers every year in endemic areas, which is leading to an increase in imported cases in the United States and elsewhere,” he added.
“And in places where the spread is at its limits, like the southern U.S., southern Europe, China, things aren't getting better. So it's not getting better anywhere. It's all bad.”
Is there a cure for dengue fever?
There is no cure for dengue infection – patients' symptoms are managed with medication, including painkillers – but pharmaceutical companies have antiviral drugs in clinical trials.
Is there a vaccine?
The effort to find a dengue vaccine has been long and complex.
A vaccine developed by France's Sanofi, called Dengvaxia, was widely rolled out in countries including the Philippines and Brazil in 2015. But two years later, the company said it was causing severe symptoms in people who contracted the virus after getting the vaccine.
The CDC recommends the use of Dengvaxia only in endemic areas for patients with a laboratory-confirmed history of dengue infection.
The World Health Organization recently recommended a new vaccine, called QDENGA, for children ages 6 to 16 living in areas with high dengue infection rates, regardless of whether they have had previous dengue infection.
The vaccine has already been rolled out in 16 European countries, including Indonesia, Brazil, Thailand, Britain and Italy, but it won't be available in the United States any time soon.
What else can you do?
Some countries have taken aggressive measures to successfully control dengue, such as Singapore, which uses a combination of measures including inspections of homes and construction sites to identify breeding sites and heavy fines for violations.
“This is a successful approach, but there are very large budgets to support these activities,” Dr. Paz Bailey said. “But not all countries have that.”
Brazil and Colombia have successfully introduced a bacterium called Wolbachia, which infects Aedes aegypti mosquitoes and prevents them from transmitting the dengue virus.
Researchers in South America are trying to mass-produce Wolbachia-infected mosquitoes and have them mate with wild insects to spread the bacteria through mosquito populations.
