Mosquitoes are spreading dengue fever across the Americas at historic levels, and U.S. travelers are bringing this potentially life-threatening virus back with them. The sharp rise in cases has prompted health officials in the United States to issue warnings about the increased risk.
In South America, dengue cases are currently declining due to cooler winter weather following an unprecedented outbreak. However, experts anticipate a rise in infections across Central America, Mexico, and the Caribbean as the Northern Hemisphere enters summer and hurricane season.
While the risk of local transmission in the contiguous U.S. remains relatively low, there are concerns about how well the country can control dengue in the future. Climate change is exacerbating conditions with extreme droughts followed by intense rainfall, which could help sustain the dengue-carrying Aedes aegypti mosquito, a species that remains present in the U.S.
Aedes aegypti mosquitoes thrive in urban areas and are adept at spreading dengue, raising concerns about the potential for sustained local transmission in the U.S.
“It’s expected that we will see increases,” said Dr. Albert Ko, an infectious disease epidemiologist at the Yale School of Public Health. “Climate change will drive the expansion of dengue and other mosquito-borne viruses. The real question is how we respond to it.”
In the first half of 2024, the number of dengue cases reported in the U.S. and its territories has already surpassed previous years. These cases are largely linked to travel, with individuals contracting the virus abroad or in Puerto Rico, where a public health emergency was declared in March due to local transmission.
In late June, the Centers for Disease Control and Prevention (CDC) issued a health advisory about the heightened risk of dengue in the U.S. Extreme weather events, such as Hurricane Beryl, further contribute to the problem by creating breeding grounds for mosquitoes in even small amounts of standing water.
Dengue Symptoms and Risks
Most people infected with dengue do not show symptoms. For those who do, symptoms include high fever, body aches, nausea, and rash, with most recovering within a couple of weeks. Severe cases of dengue, which occur in about 5% of infections, can be life-threatening and require hospitalization. Symptoms of severe dengue include vomiting, restlessness, rapid breathing, and bleeding. Infants, the elderly, and pregnant women are at higher risk for severe disease.
Severe dengue is more likely with repeat infections, as there are four distinct types of dengue virus, with immunity to each lasting only a few years. The same Aedes aegypti mosquitoes that spread dengue are also responsible for outbreaks of other deadly diseases like yellow fever and Zika.
Infected individuals can introduce the virus to local mosquito populations, which can then spread the virus for the rest of their lives. This cycle helps maintain and amplify dengue transmission.
Rapid and Widespread Transmission
Aedes aegypti mosquitoes can lay eggs in very small amounts of standing water, such as in bottle caps, pipes, or broken pots, making them highly adaptable and difficult to control.
Globally, dengue cases have surged dramatically. In 2000, there were 500,000 reported cases; by 2019, this number had risen to 5.2 million. The year 2023 saw over 6.5 million cases and 7,300 related deaths. Dengue is now appearing in regions that previously had little to no cases.
In the U.S., nearly 2,700 dengue cases have been reported this year, with the majority occurring in Puerto Rico. Travel-related cases have been seen in states like Florida, Texas, Arizona, and California, where Aedes aegypti mosquitoes are prevalent. Northern states such as New York and Illinois have also reported cases, although Aedes aegypti is not endemic there.
Dr. Gabriela Paz Bailey of the CDC highlights that increased travel since the COVID-19 pandemic has contributed to the rise in cases. In Puerto Rico, dengue rates are high partly because immunity from the 2015-2016 Zika epidemic has waned. Additionally, the dengue virus is shifting from types 1 to types 2 and 3.
Future Risks in the U.S.
Many southern U.S. regions provide favorable environments for Aedes aegypti mosquitoes. Climate change is expanding these hot and humid habitats, which could facilitate the reintroduction of dengue. Urbanization and poverty also contribute to the risk, as seen in areas like Houston, where discarded tires and other debris provide ideal breeding grounds for mosquitoes.
The U.S. currently lacks a widely available vaccine for dengue. Preventive measures focus on avoiding mosquito bites, which can occur at any time of day. For personal protection, Dr. Hotez recommends wearing long clothing and using insect repellent.
In summary, the surge in dengue cases underscores the need for heightened vigilance and preparedness as climate change and global travel continue to influence the spread of this dangerous virus.