How Climate Change Is Helping Tropical Diseases Gain A Foothold In The U.S.

Mosquito-borne diseases like dengue, malaria, and yellow fever have long been associated with tropical regions rather than the continental United States. However, a shift is occurring as these diseases are now becoming a concern in the U.S. In Florida, locally acquired dengue cases have been reported for over a decade. In 2023, malaria cases acquired in Texas and Florida made headlines. Recently, a new paper published in the CDC journal Emerging Infectious Diseases highlighted a significant example of sustained dengue transmission in Los Angeles County over a seven-week period.

During the fall of 2024, fourteen individuals in LA County contracted dengue, with eight cases concentrated within a mile in the San Gabriel Valley and the remaining six scattered across five neighborhoods over 20 miles apart. This outbreak underscored the precarious environmental conditions in the southern U.S. that are conducive to vector-borne disease transmission. While the presence of mosquitoes and warm climate create opportunities for transmission, the lack of sustained local chains has previously prevented the establishment of diseases like dengue. The LA outbreak demonstrates the consequences when conditions tip in favor of sustained transmission, even temporarily.

Dengue transmission relies on a relay system, where the virus is transmitted from infected individuals to mosquitoes and then to new hosts. The fourteen confirmed cases likely represent only a fraction of the total infections, as many dengue cases are asymptomatic or mild. With a seven-week chain of transmission in LA County, the relay likely completed multiple cycles, leading to additional undetected infections.

The increase in travel-associated dengue cases in LA County in 2024, coupled with a surge of cases across the Americas, created a perfect storm for local transmission. Infected travelers returning to areas where the Aedes aegypti mosquito vector is present can spark outbreaks, highlighting how global dynamics can impact local communities. Climate change further exacerbates the situation by expanding the range and season length of Aedes mosquitoes, intensifying transmission in tropical regions.

Managing diseases like dengue becomes a threshold problem once transmission reaches a certain level. Below this threshold, introductions die out naturally, but above it, each infection leads to more cases, increasing the burden on public health systems. Florida’s experience with dengue outbreaks demonstrates the importance of sustained investment in surveillance, diagnostics, and vector control to manage local transmission.

The 2024 dengue outbreak in LA County ended as mosquito activity decreased in late October. The public health response, including extensive surveys and outreach efforts, likely contributed to breaking the transmission chain. However, the impact of climate change on disease transmission patterns suggests that proactive measures to prevent disease introductions are more cost-effective than managing outbreaks once they occur.