Major disease outbreaks in Brazil, Argentina, Puerto Rico, and elsewhere have dengue in the news. Those in the Mountain West are no strangers to mosquito-borne illnesses – in 2023, Colorado had the most West Nile virus cases of any state by far. So, is dengue next?

The short answer is “no.” But there’s more to it, as Dr. Daniel Pastula helps explain. He’s chief of Neuro-Infectious diseases and Global Neurology at the University of Colorado School of Medicine who sees patients at UCHealth University of Colorado Hospital on the Anschutz Medical Campus. Pastula studied a 2014 dengue outbreak in Fiji while a commissioned officer with the U.S. Public Health Service, and the CU School of Medicine Neuro-Infectious Disease Group he leads brings together clinicians, researchers, and epidemiologists to work with patients and study dengue, West Nile, Zika and other viruses that can impact the brain and nervous system.

What is dengue?

Dengue (break-bone-fever) is a viral disease transmitted mainly via certain Aedes species of mosquitoes, which also transmit yellow fever, chikungunya, and Zika viruses. These mosquitoes are native to tropical and subtropical climates, but their potential range extends into the southern United States, possibly as far as southeastern Colorado, according to the U.S. Centers for Disease Control and Prevention.

Aedes aegypti what Pastula describes as a “domesticated mosquito,” one that’s most at home around humans and prefers relatively clean water sources for breeding. Unlike West Nile virus transmission, which happens when mosquitoes bite birds and then infect humans, dengue transmission happens when mosquitoes bite infected and then uninfected humans, he says.

How widespread is it?

Globally speaking, it’s extremely widespread. About 4 billion people – roughly half the world’s population – live in areas where dengue mosquitoes (Aedes aegypti and, to a lesser degree, Aedes albopictus) buzz about. An estimated 400 million people get infected each year. The majority have no symptoms, making precise case counts difficult. About 100 million of those infected get sick, and about 40,000 die from severe dengue annually.

Dengue surges such as those happening at the moment happen, Pastula says, “when there are a lot of mosquitoes, a lot of humans, and overwhelmed public health and vector control systems.”

Have there been dengue cases in Colorado?

Yes, but they’ve been associated with travel to regions where dengue is present. CDC data show 133 reported cases in Colorado from 2010-2023 and five cases so far this year – all of them travel-associated. So far this year, there have been three cases of locally acquired dengue in the United States, all of them in Florida.

What are dengue symptoms?

According to the World Health Organization, if you’re symptomatic, symptoms set in 4-10 days after infection and last 2-7 days, with some patients taking weeks to get their energy back. Symptoms may include a high fever (104°F), severe headaches, serious muscle and joint pain (hence the dengue nickname “breakbone fever”), pain behind the eyes, nausea, vomiting, swollen glands, and rash. Serious abdominal pain can be sign of severe dengue, which can lead to internal bleeding. Speedy medical treatment is vital for severe cases.

Is there a cure for dengue?

There are no antivirals for dengue, so medical care is about supportive care. Those with mild disease take acetaminophen to control fever and pain (ibuprofen or aspirin can increase risk of bleeding). Research continues on multiple fronts, but there’s no curative treatment in sight for now. Part of the challenge is that there are four distinct dengue viruses, numbered one through four. Recovering from being infected by one does not protect against other types – quite the opposite, in fact.

Is there a vaccine for dengue fever I can take if I’m traveling?

There is a vaccine, but you probably aren’t eligible for it. The U.S. Food and Drug Administration approved Sanofi’s Dengvaxia in 2023, but with strict limitations. One is that it’s only approved in the United States for children and adolescents ages 6 to 16, Another has to do with dengue being four different viruses. People who have had one type of dengue virus may be at greater risk of severe disease if they’re later infected by one of the other dengue viruses. Because of that, the FDA approved the vaccine only for those who have tested positive for having had a previous dengue infection. Finally, you have to live where dengue is endemic to be eligible for the vaccine and it’s not endemic in Colorado or neighboring states.

Is climate change expanding the reach of dengue?

That’s hard to predict, but, Pastula says, “As things heat up and there’s more moisture, we’ll have more mosquitoes in certain places,” and the ranges of Aedes mosquitoes and other disease-bearing bugs could expand. This appears to have happened with another flavivirus – Japanese encephalitis virus – in Australia, Pastula says. Previously endemic only in that country’s far northern regions, Japanese encephalitis has spread to the more populous southeastern Australia.

“I do think that, with climate change, we’re going to see mosquitoes and the diseases they carry jump far beyond their historic ranges,” Pastula said.

What can be done to keep dengue outbreaks from happening?

As with any other fast-spreading disease, you must break the chain, Pastula says. That starts with good disease surveillance to detect where the virus is taking root. Doing something about it means public-awareness campaigns, inspections to spot standing water, larvicide treatments in that water, possible spraying of insecticides, and distributing and advocating for insect repellent.

Longer term, the introduction of Aedes aegypti mosquitoes hatched from eggs that carry a bacteria that hampers the mosquitoes’ ability to spread dengue looks promising, and Brazil is now releasing mosquitoes genetically engineered to kill female offspring. Similar mosquitoes were released in the Florida Keys to reduce the number of disease-causing mosquitoes in 2021.

What lessons can we take from dengue outbreaks elsewhere?

While dengue presents no imminent public-health threat in the Mountain West, mosquito-borne illness is already a serious concern here. In Colorado, West Nile virus remains a serious concern. It , hospitalized 375 and killed 48 people in state last year. Serious infections can leave people with permanent disability.

Breaking the chain of mosquito-borne disease chain starts at home. While Colorado summers can be dry, standing water is an invitation for mosquito reproduction. Dump it out if not needed. If your windows or doors don’t have screens, leave them closed. Use insect repellent when you head outside, particularly in the morning and evening when mosquitoes are busiest, and wear long pants and sleeves when possible.

“It turns out that the prevention for West Nile virus is largely the same as for dengue,” Pastula said.