Dengue reaches historic levels in the Americas and closer to home

Mosquitoes are infecting people across the Americas with dengue at historic levels and U.S. travelers are bringing the potentially life-threatening virus home with them. The alarming rise in infections has spurred American health officials to warn about the risk.

In South America, cases of the tropical disease are now decreasing during cooler winter months after record numbers of people were sickened by dengue. But experts warn more people will get infected across Central America, Mexico and the Caribbean as summer and hurricane season take their holds in the Northern Hemisphere.

While risk of local transmission in the contiguous U.S. remains low, officials are concerned about the extent to which the country will be able to harbor dengue in the years ahead. Climate change is creating scorching droughts followed by intense rainfall ? which could sustain dengue’s transmission vector, the pesky Aedes egypti mosquito, never fully eradicated from the United States.

A. egypti loves to feast on people’s blood and dense urban areas offer opportunities for it to spread, posing a risk for sustained local dengue transmission in this country.

“It’s to be expected that we’re going to have increases,” said Dr. Albert Ko, an infectious disease epidemiologist at the Yale School of Public Health, who specializes in dengue fever. Climate change, he added, “will drive the expansion of dengue and other viruses transmitted. I think the question is, what do we do about it?”

The number of people in the U.S. and its territories sickened by dengue in just the first half of 2024 has surpassed any year in the last decade. The cases are occurring via travel, with people bitten by the pesky Aedes egypti mosquito in foreign countries, as well as in Puerto Rico, where local transmission led officials to declare a public health emergency in March.

In late June, the Centers for Disease Control and Prevention issued a health advisory warning about increased risk of dengue in the U.S.

Recent extreme weather events, such as Hurricane Beryl, pose additional risks with just tiny amounts of standing water making suitable bases for mosquitoes to breed and spread dengue.

Dengue symptoms, cause

Most people who get dengue never show symptoms. But those who do can get high fever, body aches, nausea and rash. Most recover in a couple weeks, according to the World Health Organization.

Some infections are so severe they can require hospitalization or even, in rare instances, kill. Severe dengue occurs in about 1 in 20 infections and can include vomiting, restlessness, rapid breathing and bloody gums, nose or stool. Infants, elderly people and pregnant women are at increased risk for severe disease, the CDC said.

Severe dengue is more likely with repeat infections.

There are four different types of dengue virus, with immunity to each lasting only a few years.

The same A. egypti species that caries dengue is notorious for triggering historic epidemics of deadly diseases such as yellow fever and Zika.

While mosquitoes carrying dengue can infect people, infected people – including those who show no symptoms – can also introduce the virus to local mosquito populations. And once infectious, a mosquito can transmit the virus for the rest of its short life, WHO said. This helps sustain dengue’s spread.

Spread fast and far

A. egypti mosquitoes can lay eggs in standing water as small as a bottle cap, pipes, broken pots or anywhere that collects water.

“They’re really good at being everywhere,” said Dr. Gabriela Paz Bailey, the CDC's dengue branch chief.

Dengue’s increase is apparent globally, after international campaigns to eradicate dengue and the A. egypti in decades past failed. In 2000, there were half a million dengue cases. By 2019, there were 5.2 million. The year 2023 reached a historic high of over 6.5 million cases, with 7,300 dengue-related deaths.

“It’s increasing rapidly, and it’s also appearing in areas that didn’t have dengue before,” said Paz Bailey, who is based in San Juan, Puerto Rico.

The U.S. has seen nearly 2,700 cases so far this year, CDC data shows. The majority of those infections have been in Puerto Rico, during what’s normally considered the island’s dry season.

Nearly 900 Americans have contracted dengue while traveling abroad, returning to states such as Florida, Texas, Arizona and California, where A. egypti mosquitoes thrive and therefore the potential for local transmission is high. Northern states have also seen cases, with over 140 in New York and 35 in Illinois. Neither state is suitable for A. egypti.

Paz Bailey said increased travel since the COVID-19 pandemic has helped drive up infections. In Puerto Rico, she said dengue rates are high in part because immunity provided by the 2015-2016 Zika epidemic has waned. Meanwhile, the type of dengue virus is shifting, from dengue 1 to dengue type 2 and 3, she said.

Risk ahead in U.S.

Much of the southern U.S. has environments that allow A. egypti mosquitos to thrive. Climate change is expanding the range of such hot and humid habitats, officials said.

“We have to recognize the unique vulnerability to this part of the country,” Dr. Peter Hotez, co-director of the Texas Children's Hospital Center for Vaccine Development and dean of Baylor College of Medicine's National School of Tropical Medicine, told USA TODAY.

Along with climate change, he points to urbanization in areas such as Houston, a sprawling metropolitan area that's the nation's fourth-largest city, and poverty that can help foster dengue’s reintroduction into the U.S.

Hotez said a discarded tire can be the “Ritz-Carlton” for A. egypti mosquitoes, which are endemic throughout the region. Additionally, Texas' international airports allow travel to and from areas with widespread local transmission.

In muggy Houston, the recent effects of Hurricane Beryl, coupled with severe heat for days, are ideal scenarios for dengue and other vector-borne diseases to thrive, state officials said. There have been 10 cases of dengue in Texas this year, state health officials said. So far, all these are believed to be travel-related, not locally transmitted.

The U.S. currently doesn’t have a widely available vaccine to prevent dengue. The best way to prevent illness is to protect against mosquito bites, which can occur day and night.

Hotez said before he sets out on his morning walks, he covers himself in sweatpants and a sweatshirt, and sprays insect repellant to reduce mosquito exposure.

This article originally appeared on USA TODAY: As dengue fever rises in US, understand symptoms, transmission