Health officials in the Florida Keys have issued a dengue fever alert after two confirmed cases of the mosquito-borne disease were reported there.

In the alert, issued this week by the Monroe County Department of Health, officials said they were taking precautions to curb the spread of dengue fever. Those measures include stepping up door-to-door mosquito inspections; enhancing mosquito surveillance; spraying aerial mosquito treatments as necessary; and driving through neighborhoods and spraying mosquito treatments from trucks as needed.

But the threat of dengue fever is not limited to Florida: Just last week, the U.S. Centers for Disease Control and Prevention issued a nationwide advisory about the increased risk of dengue fever infections in the United States.

Puerto Rico has been the hardest hit: Of the 2,241 U.S. dengue cases reported so far this year, 1,498 occurred in that U.S. territory in the Caribbean. In March, a public health emergency was declared there after cases of the mosquito-born illness broke historical records. 

For comparison, 3,036 dengue cases were reported in all of 2023 in the U.S. and its territories.

Internationally, the incidence of dengue fever has been the highest on record this year, especially in Latin American countries, where more than 9.7 million dengue cases have been reported so far. That’s twice as many as in all of 2023 (4.6 million cases).

Why the spike? Many nations have reported increasingly hot temperatures, which create ideal breeding conditions for the mosquitoes that spread dengue.

"Dengue transmission peaks during the warmer and wetter months in many tropical and subtropical regions," the CDC noted in its advisory. "Dengue cases are likely to increase as global temperatures increase."

About 1 in 4 people who get infected with the dengue virus will get sick, according to the CDC. Among those who fall ill, the symptoms can be mild or severe. The most common symptoms are fever along with joint, muscle, bone or eye pain, headache, nausea, vomiting or rash.

About 1 in 20 cases will develop severe dengue, which can be life-threatening and require hospitalization since it can result in shock, internal bleeding and even death.

Those who have had dengue in the past are more likely to develop severe symptoms. People can get sick with dengue fever up to four times in their lives, once for each type of the virus that can cause the disease, according to the CDC.

Some prevention methods include staying in places with air conditioning when possible and installing window screens to keep mosquitoes out of the home. When outdoors, use insect repellent, wear long sleeves and pants and clean up any areas with stagnant water where mosquitoes could breed.