Mosquitoes on the March: Climate, Cities and Travel Drive a Resurgence of Vector‑Borne Diseases in the U.S.

Mosquito-borne and other vector-borne diseases once considered confined to tropical regions are increasingly reappearing across the United States, driven by warming temperatures, expanding urban environments and greater global travel, Vox climate reporting has found.

West Nile virus is now deeply established in large parts of the country. Dengue and chikungunya — long associated with tropical climates — have produced locally transmitted cases in U.S. territories and are increasingly reported on the mainland. Malaria, largely eliminated from the United States in the mid-20th century, has begun to re-emerge in isolated foci. Researchers and public health officials are now tracking a new potential threat: Oropouche virus, historically restricted to the Amazon basin, which is spreading through South American cities and has been detected in travelers returning to the United States.

Reporting by Vox climate correspondent Umair Irfan, based on months of field work and national reporting, shows how those forces are reshaping where and how people are exposed to pathogens carried by mosquitoes and biting midges. In Texas, Irfan reported from one of the country’s most advanced mosquito surveillance systems, located in a small city that state and federal officials have pointed to as a model for local response. The system uses continuous trapping, laboratory testing and mapping to detect viruses in insect populations before large human outbreaks occur.

The picture emerging from that and other reporting is twofold: the ecological risk is rising while public health capacity to respond has become more fragile. Entomologists and epidemiologists say warming temperatures lengthen mosquito seasons and allow species to survive farther north. Urban sprawl creates new breeding habitats through standing water, containerized trash and poorly planned drainage. International travel and trade introduce pathogens and vectors into new regions, sometimes followed by local transmission when conditions permit.

Public health officials warn that the United States faces a steady stream of challenges from vector-borne diseases. West Nile virus, introduced to the U.S. in 1999, has become endemic in many regions and causes seasonal outbreaks that can lead to severe neurological illness. Dengue and chikungunya have caused outbreaks in parts of the southern U.S. and U.S. territories. Sporadic locally transmitted malaria cases have appeared in recent years after decades without endemic transmission.

Oropouche virus, transmitted by midges and certain species of mosquitoes and historically associated with febrile outbreaks in parts of South America, has been documented spreading through cities in the Amazon and other areas. While human infections with Oropouche in the United States remain rare and largely identified among travelers, the pattern of urban spread in South America has raised concerns among researchers about the potential for further geographic expansion.

Irfan’s reporting underscores the practical work of local scientists and public health teams who have expanded surveillance to detect early signals of emerging threats. The Texas jurisdiction highlighted in the reporting sustained a years-long investment in trapping networks and laboratory capacity that allowed officials to detect and respond to virus activity before major human outbreaks developed. Those investments stand in contrast to many U.S. jurisdictions where mosquito surveillance and vector-control capacity have eroded amid budget cuts and workforce shortages.

“There are more than 200 mosquito species in the country, but just a handful are significant vectors for human disease,” Vox reporting noted, and it includes a practical guide to identifying the seven mosquito species of most concern in the U.S. The guide aims to help clinicians, public health workers and the public distinguish species that are most likely to transmit pathogens such as West Nile, dengue, chikungunya and Zika.

Experts caution that surveillance and early detection are necessary but not sufficient on their own. Many local health departments lack sustained funding for routine trapping, species identification and laboratory tests. The result is a patchwork of preparedness: some communities enjoy robust monitoring and rapid response, while others operate with minimal entomological capacity and limited ability to diagnose or contain outbreaks quickly.

The recent cluster of reporting also highlights the role of human behavior and infrastructure in shaping risk. Urban development that increases standing water or creates microclimates favorable to mosquitoes can amplify transmission risk. Human mobility — including travel for work, leisure and trade — contributes to pathogen introductions. Public messaging, access to preventive services and community engagement all affect the potential for local outbreaks to expand.

The resurgence of vector-borne threats has prompted calls from scientists for renewed investments in surveillance, laboratory capacity and workforce training, along with coordinated federal, state and local planning. Public health officials say targeted investments can improve detection and response without requiring uniform capacity across every county, but sustained funding is needed to maintain those improvements through fluctuating political and budget cycles.

Vox’s reporting series argues that confronting the growing threat requires combining field-level entomology, clinical surveillance, and community-based prevention strategies. It documents both the technical measures — such as species-specific trapping and genomic testing of pathogens — and the organizational challenges, including staffing, data sharing and the need for consistent funding.

The emergence and spread of pathogens once considered “tropical” underscore a broader shift in the epidemiology of vector-borne disease. Climate-driven changes in vector ranges, coupled with urbanization and global connectivity, have altered the geography of risk. For residents, that means the mosquitoes that bite in summer are not just nuisances; they can be carriers of illnesses with serious health consequences. For public health systems, it means sustaining and modernizing surveillance and response to prevent localized transmissions from becoming larger outbreaks.