More Mosquito Disease Days in 95% of Major U.S. Cities

KEY FACTS

• Mosquito bites are a nuisance and a health risk. As the climate warms, mosquitoes can emerge earlier in the year and spread diseases over a longer season.

• Climate Central analyzed the changing annual frequency of mosquito disease days in 239 major U.S. cities from 1970 to 2025.

• Mosquito disease days have temperatures suitable for mosquitoes to spread West Nile virus — the most common mosquito-carried disease in the U.S.

• Nearly all (95%) of the cities analyzed experienced an increase in mosquito disease days — 18 more days on average compared to the early 1970s. 

• Regionally, the Southeast, Northeast, and Northwest have experienced the largest increase in annual mosquito disease days.

VISUALS

LEARN MORE

FULL REPORT

Warming temperatures can help mosquitoes spread diseases 

Mosquitoes are bothersome pests. But they’re also the world’s deadliest animal because of the diseases they spread to people. 

Both mosquitoes and the pathogens they spread are highly sensitive to temperature. Mosquito-borne diseases spread most easily within specific temperature ranges, which largely determine where and when these diseases occur across the globe.

As the climate warms because of heat-trapping pollution, spring is arriving earlier and summer temperatures are lingering later into the fall. As a result, mosquitoes can emerge earlier, remain active longer, and spread disease over a longer season.

Up to a limit, warmer temperatures also help mosquitoes and the pathogens they carry to develop, survive, and reproduce most effectively, increasing rates of disease transmission.

A new Climate Central analysis assessed long-term trends in mosquito disease days across the U.S. In this analysis, a mosquito disease day has a daily average temperature within a suitable range for transmission of West Nile virus by various Culex mosquito species common across the U.S. (see Methodology for details). 

West Nile virus is the most common mosquito-borne disease in the U.S., with about 2,000 people diagnosed each year and cases typically peaking in August and September. 

More mosquito disease days in 95% of major U.S. cities

Climate Central tallied mosquito disease days each year from 1970-2025 in 239 U.S. cities. 

• The annual number of mosquito disease days has increased in nearly all (95% or 227) of the cities analyzed. 

• On average, these 227 U.S. cities now experience 18 more mosquito disease days annually than during the early 1970s. 

• The top five cities with the largest increases in annual mosquito disease days were: San Francisco, CA (52 more days); Reno, NV (50 more days); Santa Maria, CA (49); Albany, GA (41); and Salinas, CA (40). Each experienced more than double the national average increase. 

• Regionally, the Southeast, Northeast, and Northwest have experienced the largest average increases in annual mosquito disease days since 1970 (20 more days in each region). 

Averaging city data regionally over the last 30 years (1996-2025), the U.S. regions with the highest average number of mosquito disease days annually are:

Mosquitoes are sensitive to temperature and more

More mosquito disease days means more opportunities for mosquitoes to bite people and potentially transmit disease.

But warming temperatures are not the only climate-related factor influencing mosquito-borne diseases. Mosquito activity and disease transmission are also influenced by:

• Changing precipitation patterns. Heavy rainfall can make more suitable habitats for mosquitoes. And droughts can increase West Nile virus infection rates, possibly because birds (virus hosts) gather around scarce water sources that also attract mosquitoes (virus vectors). Climate change is likely to increase both precipitation extremes (heavy rainfall and drought) in different regions.

• Ecological change. Climate change can affect the behavior and populations of other animals such as birds that are important for certain parts of the life cycle for mosquito-borne diseases.

• Higher humidity. Warmer temperatures cause more moisture to evaporate into the air, promoting the humid conditions that support mosquitoes. 

These climate-related factors interact with other conditions — including changes in land cover, urbanization, and international travel — to influence mosquito range, activity, and disease transmission to people. 

The dynamics of mosquito-borne diseases are complex, and not all mosquitoes transmit disease to humans.

Mosquito-borne diseases in the U.S.

There are more than 200 species of mosquitoes in the U.S. — about 12 of which spread diseases dangerous to humans. The most common disease-spreading mosquitoes in the U.S. are: 

• Culex species, which carry West Nile virus.

• Aedes species, which carry chikungunya, dengue, and Zika viruses.

• Anopheles species, which spread malaria and lymphatic filariasis, are also common in the U.S., although most reported cases are linked to travel. 

While most vector-borne diseases in the U.S. are spread by ticks, West Nile virus is the most common mosquito-borne disease in the country. 

West Nile virus is spread by several Culex mosquito species, which have widespread suitable habitats. Reported cases of West Nile virus are distributed across the U.S. By comparison, reported cases of tick-borne diseases are concentrated in the Northeast and Upper Midwest.

Overall, rates of infection and the severity of resulting health effects from mosquito-borne diseases are much lower in the U.S. than in tropical and subtropical regions of the world. 

Climate change and mosquito-borne diseases around the globe

About half of the global population is at risk of malaria or dengue, both of which can be life-threatening in the absence of effective prevention and treatment. Each year, malaria causes more than 608,000 deaths worldwide, mostly among children under the age of 5. The global burden of these diseases falls disproportionately on people in Africa and Asia. 

The geographic range of these diseases is likely to expand with future warming. A 2021 study found that rising global temperatures are likely to lengthen transmission seasons and expand geographic ranges for both malaria and dengue — vastly increasing the number of people at risk in the decades ahead.

A 2019 study estimates that hundreds of millions more people globally are likely to be exposed to viruses carried by Aedes mosquitoes due to continued warming by 2080.  

An estimated 18% of dengue cases in Asia and the Americas during 1995-2014 are attributable to warming that has already occurred, with further increases projected by mid-century. 

Climate change is not increasing the risk of mosquito-borne disease everywhere. In some areas, warming may cause temperatures to exceed the optimal range for some mosquito-borne viruses, which could lead to local decreases in disease transmission.

LOCAL STORY ANGLES

Have cases of mosquito-borne disease been reported in your area?

Use the Centers for Disease Control and Prevention’s ArboNET map tool to find data on annual rates of various mosquito-borne diseases in the U.S., including West Nile virus, dengue, and Zika. Less common diseases that mosquitoes spread in the U.S. include Cache Valley virus, Eastern equine encephalitis, Jamestown Canyon virus, La Crosse encephalitis, and St. Louis encephalitis. 

Does your area face current or future dengue risk? 

Although dengue is not currently common in most of the U.S. (with the exception of Florida and Puerto Rico), the season length for dengue transmission by Aedes aegypti mosquitoes has increased since the 1950s. Even under more moderate future warming scenarios, significant dengue risk expansion is projected throughout Gulf Coast states by 2080. 

Is the range of disease-carrying mosquitoes expanding in your area?

The range of disease-carrying Aedes mosquito species has expanded in the decades since they were introduced to the U.S. Continued range expansion is projected from the South and Southeast into the Ohio Valley, Upper Midwest, and up the East Coast by 2050 — when tens of millions more people in the U.S. and Canada could be at risk from Aedes-transmitted viruses such as dengue and Zika due to continued warming. Locally-acquired dengue cases have been recently reported in California; a 2026 study estimates that about 4 million more California residents could face risks of dengue transmission due to climate warming and urban expansion. 

CONTACT EXPERTS

To request an interview with a Climate Central expert about this analysis, please contact Abbie Veitch, aveitch@climatecentral.org.

Erin Mordecai, Ph.D. 
Associate Professor of Biology
Stanford University
Relevant expertise: Effects of climate, species interactions, and global change on infectious disease dynamics in humans and natural ecosystems
Contact: emordeca@stanford.edu  

Sadie Ryan, Ph.D.
Professor of Medical Geography
University of Florida
Relevant expertise: Climate and health interactions; modeling vector-borne diseases; quantifying social-ecological risk of disease emergence and exposure in a changing world
Contact: sjryan@ufl.edu 

James Mutunga, Ph.D.
Assistant Professor
The Pennsylvania State University - Harrisburg
Relevant expertise: Climate change and vector-borne diseases; disease vector control strategies; infectious disease ecology
Contact: jjm7962@psu.edu

FIND EXPERTS

Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts. 

Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.  

Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices. 

Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area. 

METHODOLOGY

Daily average temperature data (1970-2025) for 242 U.S. weather stations within the contiguous U.S. were obtained from the Applied Climate Information System (ACIS), which is developed, maintained, and operated by NOAA’s Regional Climate Centers. 

In this analysis, mosquito disease days are defined as days with average temperatures within the thermal limits for transmission of West Nile virus (WNV) by three common Culex species, based on thermal ranges developed by Shocket et al. (2020); Table 2 therein. 

For each of the 242 stations analyzed, we used the thermal range for WNV transmission by the dominant Culex species in the geographic region, based on range maps from Gorris et al. (2021) and Hort et al. (2023).

For locations with multiple dominant Culex species, we selected the species with the largest range between the lower and upper thermal limit for WNV transmission, thus reflecting the total potential occurrence of WNV transmission by locally present species. 

The species assigned to each location analyzed is listed in the available data file. Generally, thermal ranges for WNV transmission by C. tarsalis were used for locations west of the Mississippi River. Ranges for C. pipiens were used for the Midwest and Northeast, and ranges for C. quinquefasciatus were used south of the Ohio River. 

Additionally, following Fay et al. (2025), temperature data were filtered to include only those days that occurred within 13 days of another day within the thermal range for WNV transmission. This is to remove days that are isolated from other qualifying days by a duration that spans the timeline for mosquito development. 

Long-term trends (1970-2025) in the annual number of mosquito disease days at each location analyzed are based on linear regression. 

For reported data summaries based on linear regression, only 239 of the 242 stations are included in this brief due to data completeness measures that were not met by three stations: Bend, OR; Hazard, KY; and Twin Falls, ID.

This analysis was limited to the contiguous U.S. because Alaska, Hawaii, and Puerto Rico have virtually no reported historical cases of West Nile virus. 

Thanks to Dr. Erin Mordecai and Dr. Sadie Ryan for their input in the development of this Climate Matters brief.