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The Centers for Disease Control and Prevention (CDC) issued a health advisory on Tuesday warning the public of a heightened risk of dengue virus (DENV) infections in the U.S. this year as cases continue to rise around the globe.

Countries in the Americas reported the highest number of cases ever, exceeding the record for a full year in less than six months. Since the start of 2024, there have been a total of 9.7 million dengue cases, more than twice the number recorded in all of 2023 (4.6 million).

The virus is spread by mosquitoes that thrive in warm weather and have been expanding their geographic reach as climate change brings warmer weather to new areas. The high number of cases so early in the year has caused concern among health officials about the reach of the virus.

In the United States, transmission is most common in six U.S. territories and freely associated states, which are classified as areas with frequent or contiguous dengue transmission. Local transmission in continental states has been more limited than in the territories, but it has still gotten worse in recent years as the climate has warmed.

This year, the high rate of transmission has led Puerto Rico to declare a public health emergency, with 1,498 cases recorded since the beginning of January, including 745 cases identified among U.S. travelers. The World Health Organization also declared an emergency in December.

Other states have recorded higher-than-expected numbers since January, including Florida (197 cases), New York (134 cases), Massachusetts (50 cases), California (40 cases), Colorado (14 cases), Arizona (9 cases), and the District of Columbia (8 cases), among others.

In a health advisory on Tuesday, the CDC warned healthcare providers to be on alert for the infection, especially in those who recently traveled to areas with higher rates of transmission. They should order appropriate diagnostic tests as needed, ensure timely reporting of new cases to public health authorities and promote mosquito bite prevention measures, the advisory said.

Most cases don’t lead to severe symptoms, but about one in four infections is symptomatic. Those cases can have mild or severe symptoms, including fever, headache, nausea, vomiting, rash, muscle and joint pain, and minor bleeding.

Severe dengue occurs in roughly 1 in 20 symptomatic cases. It usually requires hospitalization and can develop within a few hours. Severe cases can result in hemorrhage, shock, organ failure and death.

The CDC categorizes Global Dengue as a “level 1” risk, out of four levels, meaning travelers should “practice usual precautions.” The CDC encourages travelers to risk areas to use EPA-registered insect repellent, wear long sleeves, sleep in air-conditioned rooms or rooms with window screens.