As Dengue Fever Rises in the United States, Understand the Symptoms and Transmission

Mosquitoes are infecting populations in the Americas at an unprecedented rate, and American travelers are bringing home the potentially deadly virus. The alarming rise in infections has prompted U.S. health officials to warn of the risk.

In South America, dengue cases are down during the winter months, when cases are on the rise. But experts warn that more people will be infected in Central America, Mexico and the Caribbean as summer and hurricane season sets in in the northern hemisphere.

Although the risk of local transmission in the contiguous United States remains low, officials are concerned about the extent to which the country could harbor dengue fever in the years ahead. Climate change is creating scorching droughts followed by intense rains, which could sustain the vector of dengue transmission, the Aedes egypti mosquito, which has never been completely eradicated from the United States.

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

“We should expect an increase in cases,” said Dr. Albert Ko, an infectious disease epidemiologist at the Yale School of Public Health who specializes in dengue. Climate change, he added, “will promote the spread of dengue and other transmissible viruses. I think the question is what can we do about it.”

The number of people infected with dengue fever in the United States and its territories in the first half of 2024 alone exceeded that of any other year in the past decade. Cases are occurring during 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 warning about an increased risk of dengue fever in the United States.

Recent extreme weather events, such as Hurricane Beryl, pose additional risks because tiny amounts of standing water provide fertile breeding grounds for mosquitoes and the spread of dengue fever.

Dengue fever symptoms, cause

Most people with dengue fever have no symptoms. However, those who do may experience high fever, body aches, nausea and a rash. Most people recover within a few weeks, according to the World Health Organization.

Some infections are so severe that they may require hospitalization or, in rare cases, death. Severe dengue occurs in about 1 in 20 cases and can cause vomiting, agitation, rapid breathing, and bleeding from the gums, nose, or stool. Infants, older adults, and pregnant women are at increased risk of severe illness, according to the CDC.

Severe dengue is more likely with repeated infections.

There are four different types of dengue virus, against which immunity lasts only a few years.

The same species of A. egypti that causes dengue fever is known to have triggered historical outbreaks of deadly diseases such as yellow fever and the Zika virus.

Mosquitoes that carry dengue can infect humans, but infected people – including those who show no symptoms – can also introduce the virus into local mosquito populations. And once infectious, a mosquito can transmit the virus for the rest of its short life, according to the WHO. This helps keep dengue fever spreading.

Spreads quickly and far

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

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

The increase in dengue fever is visible globally, after the failure of international campaigns to eradicate dengue and A. egypti in recent decades. In 2000, there were half a million cases of dengue fever. In 2019, there were 5.2 million. The year 2023 saw a historic high of over 6.5 million cases, with 7,300 dengue-related deaths.

“The disease is growing rapidly and is also appearing in areas where it was not present before,” said Paz Bailey, based in San Juan, Puerto Rico.

The United States has recorded nearly 2,700 cases since the beginning of the year, according to CDC data. The majority of those infections have occurred in Puerto Rico, during what is normally considered the island’s dry season.

Nearly 900 Americans have contracted dengue fever while traveling abroad, returning to states such as Florida, Texas, Arizona, and California, where A. egypti mosquitoes thrive and the risk of local transmission is high. Cases have also been reported in northern states, with more than 140 cases in New York and 35 in Illinois. None of these states are conducive to A. egypti infection.

Bailey said increased travel since the COVID-19 pandemic has contributed to the rise in infections. In Puerto Rico, dengue rates are high in part because immunity from the 2015-2016 Zika outbreak has waned. At the same time, the type of dengue virus is changing, from dengue 1 to dengue types 2 and 3, she said.

Risks Ahead in the United States

Much of the southern United States has environments that are conducive to the proliferation of A. egypti mosquitoes. Climate change is increasing the diversity of these warm, humid habitats, officials said.

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

Besides climate change, he points to urbanization in areas like Houston, a sprawling metropolitan area that is the country’s fourth-largest city, and poverty that may be fueling the reintroduction of dengue fever into the United States.

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

In muggy Houston, the recent effects of Hurricane Beryl, combined with days of intense heat, are a perfect scenario for the spread of dengue fever and other vector-borne diseases, state officials said. There have been 10 cases of dengue fever in Texas this year, state health officials said. So far, all of those cases appear to be related to travel and not local transmission.

In the United States, there is not yet a widely available vaccine to prevent dengue fever. The best way to prevent the disease is to protect yourself from mosquito bites, which can occur during the day or night.

Hotez said that before heading out for his morning walks, he covers himself with sweatpants and a sweatshirt and sprays on insect repellent to reduce exposure to mosquitoes.

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