Many people in the continental United States don’t hear much about dengue. But that is likely to change in the face of a record-breaking surge of cases – partly fueled by a warmer and wetter year – across the world.
With only half of 2024 behind us, public health officials at the U.S. Centers for Disease Control and Prevention are alerting health professionals and the public to arm themselves with information about the painful – and sometimes debilitating and deadly – infection spread through mosquito bites.
According to the World Health Organization, or WHO, by mid-June 2024 there were close to 9.4 million cases of dengue reported across the Americas. That is more than double the number of cases reported in all of 2023. The flood of cases globally has translated to an uptick of dengue cases diagnosed in the U.S. – mostly in Puerto Rico, where officials declared a public health emergency in March 2024, but also among U.S. travelers returning from places where dengue-infected mosquitoes are common.
Complicating matters is the fact that dengue infections can present with a range of symptoms – from fevers, severe pain, and bleeding to no symptoms at all. Often, it is the second bout with dengue that can lead to more severe disease and potentially life-threatening complications, so understanding prevention, getting an accurate diagnosis, and monitoring for symptom progression is critical.
This tracks with the experience of Brunilda Nazario, an Atlanta-based physician who frequently travels to Puerto Rico to visit family. Nazario’s first infection was at the age of 11. She remembers a high fever that resolved after a few days spent in bed.
“My second round was worse. My bones, especially my spine, hurt so bad,” Nazario recalls. Her first symptoms began a few days after returning to Atlanta. Along with the fevers and severe body aches, she had days of nausea and noticed bruises on different parts of her body. She ended up in the emergency room with dehydration.
With the rapid explosion of cases, dengue will rise higher on the list of possible culprits when managing fever-inducing illnesses, especially in recent travelers.
Dengue care in the U.S.
Dengue is a relatively new disease, first found in humans in the 1950s.
Even though it emerged relatively recently, dengue is the most common infection transmitted from mosquito bites to humans – and close to 4 billion people, about half the world’s population, live in places where dengue poses a threat.
The risk of dengue is higher in South America, Central America, Mexico, and the Caribbean. Dengue rates are also high in South Asia, Southeast Asia, and some African countries.
That spells trouble not only for people living in areas where dengue is prevalent but also for travelers who might not show signs of infection until after they return home.
“I think of dengue all the time,” says Carlos Salama, an infectious disease doctor at Elmhurst Hospital in New York City.
According to Salama, U.S.-based physicians, particularly those serving patients who travel, should always consider dengue if they treat anyone who has recently traveled to parts of the world where dengue is present and is showing symptoms of a fever.
“And was it at the top of our list 10 years ago? No, it wasn’t. We have seen enough dengue now so that we’re like, ‘This is real,’” Salama said.
Salama added that a rise in U.S. dengue cases could also cause ballooning costs from unnecessary medical care. He shared the story of a young, otherwise healthy woman who recently returned with dengue after a Caribbean vacation.
“She clearly has dengue, and she has a multimillion-dollar work-up done,” Salama said. He suggested that clinicians need to learn more about dengue – because there will be more cases – and how to properly assess individual risk for progression to more serious illness. Salama added that clinicians can focus on managing adequate hydration and observing for worsening symptoms without unnecessary and often high-cost tests and consultations.
“The challenge that we have in the United States is recognizing these tropical diseases for what they are,” Salama said.
Anatomy of a dengue infection
Dengue infections can come from four different types of viral strains and can lead to a wide spectrum of symptoms. Three out of four infections are subclinical, meaning that people either have no symptoms or the symptoms are mild enough that they would not require visiting a doctor.
Of the 25%of dengue cases that cause symptoms, most people will experience what doctors refer to as self-limited illness – their symptoms will end within three to 10 days.
People who have symptoms typically experience fever, severe headaches, pain behind the eyeballs, nausea, vomiting, and muscle, joint and bone pain. The joint pain can be so severe that dengue is sometimes referred to as “break-bone fever.”
In some cases, people might notice mild bleeding and a rash.
About 5-10% of dengue cases progress to a more severe form of disease, which was once called dengue hemorrhagic fever or dengue shock syndrome because people may experience severe bleeding from the infection or go into shock.
“The patient can get really sick really fast,” says Gredia Huerta-Montanez, a pediatrician and environmental public health leader in Puerto Rico.
The most common risk factors for severe dengue are age (infants are particularly at risk), pregnancy, and chronic medical conditions like diabetes, obesity, and heart disease.
But one unique complicating factor also plays a role in severe dengue. Huerta-Montanez noted that the immune response to the first infection can sometimes make a second infection – with a different strain of dengue – more dangerous and possibly increase the risk for severe complications. This risk is known as antibody-dependent enhancement, an immune system response to the second strain that enhances the severity of the subsequent infection rather than tempering it.
There are no tests or blood markers that help identify which patients might tip into severe illness, but Huerta-Montanez explains that there is a “critical window,” a period lasting around 48 hours after the fever breaks, that can provide warning signs of deterioration to severe illness.
New abdominal pain, persistent vomiting, fluid buildup in the abdomen, a swollen liver, and worsening lethargy combined with certain findings on routine labs point to an emergency that requires intensive care and monitoring.
“We have this vast experience with dengue in Puerto Rico for decades,” Huerta-Montanez said, which she said is knowledge that can be shared with clinicians in the rest of the U.S.
Dengue by the numbers
3-14 days: The incubation period, or the time from infection before symptoms, is usually between four and seven days, with a range of three to 14 days.
3-10 days: Symptom duration can range from three to 10 days. Symptoms usually come on abruptly.
2 weeks: If the travel was more than 14 days before illness with fever, dengue is unlikely.
48 hours: Critical stage of dengue after the fever goes down. These 48 hours require vigilance for adequate hydration and close observation for the development of warning signs and symptoms.
4 types: There are four distinct strains of dengue. This means that one person can have dengue infections up to four times in their lifetime.
2nd infection: Antibody-dependent enhancement is linked with severe dengue and happens most commonly with the second dengue infection. It’s not as common with the third or fourth infection.
The spread of a ‘human-adapted’ mosquito
Dengue most commonly spreads through the bite of infected Aedes aegypti mosquitoes. These mosquitoes feed almost exclusively on humans and do not follow the general eating pattern of other mosquitoes that feed at dawn and dusk. They hunt for blood meals throughout the day while people are active and outdoors.
“It’s a human-adapted mosquito,” said Albert Ko, professor of epidemiology and infectious disease at the Yale School of Public Health.
Aedes aegypti is also often referred to as the “urban mosquito,” because it is well adapted to urban environments. The mosquitoes can lay eggs in standing water found in refuse and trash in urban environments – even a small bottle cap filled with water can be home to hundreds of eggs.
The current home range of Aedes aegypti is limited predominantly to tropical and subtropical environments, with some spread to the southern United States and parts of Europe. Transmission of dengue is possible in parts of the continental U.S., though most cases are from travelers who have returned from areas where dengue is present. But that may not always remain the case.
Climate change, which is driving warmer, wetter weather, may help account for the longer duration of transmission season in many parts of the world and the spread of Aedes aegypti to higher latitudes and altitudes.
“Prior success for dengue predicts future success,” Ko said.
In other words, Miami, the Florida Keys, border towns in Texas, and other places that have seen local dengue transmission before may be the first places to see more spread in the future.
How to protect yourself from dengue
For most people, the key to limiting the risk of dengue infection involves following CDC guidance:
- Prevent mosquito bites.
- Use Environmental Protection Agency-approved repellents during travel to and after returning from areas with dengue transmission.
- Wear loosefitting, long-sleeved pants and shirts.
- Control mosquitoes at home indoors and outdoors.
- Use air conditioning and window screens when possible to lower the risk of mosquito bites indoors.
- Dump and drain containers that hold water to reduce egg-laying sites in your home and neighborhood.
- Seek medical care if you have a fever or have dengue symptoms and live in or traveled to an area with dengue outbreaks.
Though these steps can lower our risk as individuals, Huerta-Montanez says there is much more to be done at a systems level. Climate change, deforestation, pollution, and urbanization are all linked to the surge in dengue, but the burden is particularly felt by those who cannot implement some of these basic strategies.
She explained that residents of Puerto Rico live with a poorly maintained energy grid and face frequent power outages, forcing most people to open windows to balance heat risk with dengue risk.
“All these things influence how you can prevent and how you can control dengue. Dengue is a neglected tropical disease, but I think that it’s a disease of environmental injustice,” she said.
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