A surge of dengue cases in North and South America is on track to make 2024 the worst year yet for the mosquito-borne virus as Puerto Rico scrambles to contain an outbreak.

The big picture: The combination of climate change and El Niño are fueling conditions allowing the disease-carrying Aedes aegypti mosquito to spread earlier than usual and in new areas, according to the World Health Organization.

• Puerto Rico this week declared an epidemic as dengue fever cases and hospitalizations sharply increased, while the WHO's Pan American Health Organization says the threat to the U.S. mainland remains relatively low.

By the numbers: There have been over 3.5 million dengue cases in the Americas so far this year, three times more than at the same time in 2023 — a record year that saw more than 4.5 million reported cases and about 2,400 deaths.

• The vast majority of cases have been reported in Brazil (81%), followed by Paraguay, Argentina, Peru and Colombia.

Zoom in: Dengue typically poses the biggest threat in areas with hot and humid climates and where poor sanitation and standing water can create breeding grounds for the Aedes aegypti mosquito, which also carries the chikungunya and Zika viruses.

• Rising temperatures and the increased frequency of extreme weather events can accelerate spread and transmission, said PAHO director Jarbas Barbosa.
• In areas where "there used to be no transmission" — including Argentina, Paraguay and Uruguay — there's already a large number of cases this year, Barbosa told reporters Thursday.
• In the United States, dengue is more common in the territories, but there have been cases of local transmission reported in some southern states, including Florida, Texas and California.

About 1 in 4 people infected with the virus will develop symptoms, which can range from mild to severe, according to the Centers for Disease Control and Prevention.

• Fever is the most common symptom, along with headache, vomiting and debilitating joint pain.

A relatively new dengue vaccine from Takeda Pharmaceuticals is available in limited quantities and ultimately won't make a big impact on efforts to tamp down the current outbreak, PAHO officials said.

• The vaccine is in highest use in Brazil and Argentina, which Barbosa said has good systems for tracking its performance and adverse events.
• "Because this is a new vaccine, we have to be extra vigilant to identify how it will behave in the real world," Barbosa said.
• Takeda last summer withdrew its application for U.S. approval of the vaccine, at the time citing conversations with the Food and Drug Administration about data collection.