This is a big year for dengue

This is a record-breaking year for dengue infections in Central and South America, with nearly 10 million cases reported so far.

Now, the Centers for Disease Control and Prevention is warning that the threat of mosquito-borne viruses has increased in the U.S. as summer weather and holiday travel increase.

This week, the C.D.C. asked health care providers to remain vigilant and investigate cases, especially in people with fevers who have recently returned from areas where dengue outbreaks are occurring.

“At this time, there is no evidence of an outbreak in the continental U.S.,” says Gabriela Paz-Belli, chief of the CDC's dengue branch in San Juan, Puerto Rico. “But dengue cases have increased at an alarming rate worldwide. In particular, dengue cases have increased very rapidly in the United States. “In the summer months, we are expecting people to travel more to areas where dengue is common, and this may lead to more local transmission in the United States.”

So far this year, about 2,200 cases have been reported in the U.S. About 1,500 of those cases are locally transmitted, most of them in Puerto Rico, where the dengue virus is considered endemic — that is, it continues to spread.

Puerto Rico declared a public health emergency in March after an unseasonably high number of dengue cases. Paz-Bailey says locally transmitted cases have also been reported this year in the US Virgin Islands and Florida.

To be clear, the C.D.C. doesn't expect to see a massive outbreak across the U.S. this summer. Instead, the agency anticipates more travel-related cases and smaller chains of local transmission linked to those cases, Paz-Bailey says. These chains could arise in an established population in any state The Aedes aegypti mosquito is the species most associated with transmission in the United States.

In recent years, local dengue cases have been seen in Arizona, California and Texas. “With rising temperatures, we must be prepared and continue to strengthen surveillance systems so that we can monitor the occurrence of dengue in new areas,” says Paz-Bailey.

Why is the dengue virus increasing now?

Dr. Albert Ko, a professor of public health at Yale University who has worked with dengue patients in Brazil for 30 years, says some intertwining threads related to weather, waning immunity and human behavior are contributing to “the explosive outbreak that developed last year.”

First, it's been a warm, wet year in South America, providing ideal breeding conditions for mosquitoes. The population of potential dengue carriers is growing. This year, mosquitoes carried the disease to parts of southern Brazil and Argentina where it wasn't historically found — “a testament to climate change,” which is expanding the insects' range, says Ko.

Second, dengue outbreaks are cyclical. Major outbreaks occur every few years, and the last outbreak was in 2019. Ko says the cyclical pattern of dengue outbreaks is related to how population-level immunity waxes and wanes.

There are four different types of dengue, and a person who recovers from one type is protected from all of them for two years. But over time this immunity wanes “and then you become susceptible to the other three,” says Ko. At the population level, immunity is high after large outbreaks and then wanes in subsequent years, setting the stage for a new wave of dengue infections.

And third, the dengue virus is arriving on human travellers who are visiting family, friends and places they were unable to visit because of travel restrictions during the COVID-19 pandemic.

“Human mobility, whether it's short distances or long distances, plays a key role in moving the virus around. Humans are the vectors that spread the virus, carrying it even farther than mosquitoes,” says Gonzalo Vazquez-Prokopec, an environmental scientist and global health researcher at Emory University. They're being bitten by dengue mosquitoes and often unknowingly carrying it with them.

About 75% of people with a dengue infection develop mild or no symptoms at all. “This could mean that someone travels to an area where there is an active dengue infection, becomes infected with dengue, returns home and then spreads dengue to a mosquito,” says Paz-Bailey, all without knowing that he or she is carrying the dengue virus. That mosquito can then bite other people, potentially starting a chain of local infections.

How dangerous can dengue be if most people don't show symptoms?

In about a quarter of cases, people infected with dengue feel very bad. “About three to four days after the bite, the virus spreads through the body, causing systemic illness,” says Ko, who has treated thousands of dengue patients. [include] Fever, severe body ache, joint pain and a very, very severe headache.”

Some patients can develop severe dengue, which can include a condition called capillary leak syndrome. “This causes our blood vessels to leak, and people become dehydrated and go into shock … in which case they need immediate medical care to save their lives, such as resuscitation with intravenous fluids,” says Ko at Yale. He adds that people with fever and headache from dengue should treat themselves with Tylenol or acetaminophen and avoid aspirin, because aspirin thins the blood and can exacerbate the bleeding effects of the disease.

Dengue can be serious and life-threatening whether a person gets it for the first, second, third or fourth time. But the C.D.C.'s Paz-Bailey says the risk of getting severe illness the second time around is particularly high. That's because of a phenomenon associated with dengue known as antibody-dependent enhancement, where a first infection with dengue can prime a person's immune system to help the virus infect cells more easily upon a second infection.

Groups at highest risk of severe illness include infants, pregnant women, and the elderly.

What precautions can people take?

The CDC's Paz-Bailey says people can protect themselves from mosquito bites by wearing long-sleeved shirts and pants and using EPA-registered insect repellents.

They can help reduce mosquito buzzing in and around their homes by “draining out standing water, putting screens on windows and, if possible, using air conditioning, as this helps keep mosquitoes out,” she says.

People with fever, severe headache, or other dengue-related symptoms should seek medical care, and health care providers should be prepared to assess their symptoms and travel history, and test their blood if necessary.

Dengue is a nationally notifiable disease – so any cases found should be reported to local health authorities. This will help identify where the virus is spreading, and can boost local education and mosquito control efforts, Ko says.

Vaccine against dengue has been discontinued

The dengue vaccine, Dengvaxia, has been approved for use in the U.S. where the virus is endemic, such as Puerto Rico and the U.S. Virgin Islands. However, the three-dose vaccine, which requires multiple blood tests and frequent visits to the doctor's office, has been difficult to administer and has delayed adoption. Sanofi-Pasteur has stopped making the vaccine, citing a lack of demand, and the last remaining doses will expire in 2026, Paz-Belli says.

Yale's Ko says the hope for the future is twofold: better mosquito control measures that will reduce dengue infections and better vaccines that will protect unaffected populations.

“The downside of having fewer infections is that people become more susceptible because they haven’t been infected,” he says, “but if we have a vaccine and [better] By using vector-control methods, we reduce that risk.”

The U.S. sees progress on both fronts — citing the development of bacteria that inhibit mosquito reproduction, and another dengue vaccine that has been approved in some countries, though not the U.S.

Ko says with better interventions to tackle mosquito-borne diseases from different angles, the country's response to diseases like dengue could be “significantly more effective” and more lives could be saved.

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