There are now more ‘mosquito disease days’ in 95% of major US cities: See the map

Rising temperatures driven by climate change are creating more favorable conditions for mosquitoes to spread disease across much of the United States, according to a new analysis.

Using daily average temperature data collected between 1970 and 2025 from 242 weather stations across the contiguous United States, Climate Central analyzed long-term trends in "mosquito disease days" – days when temperatures fall within the range most favorable for West Nile virus transmission by the common Culex mosquito species.

There are more ‘mosquito disease days’ in most US cities

By the numbers:

The report, published Wednesday, found that nearly all major U.S. cities are experiencing more days each year with temperatures suitable for the transmission of West Nile virus, the nation's most common mosquito-borne disease. 

Researchers say longer warm seasons are allowing mosquitoes to emerge earlier in the spring, remain active later into the fall, and potentially spread disease over a longer period.

The results showed that 227 of the 239 cities analyzed, or 95%, experienced an increase in annual mosquito disease days since the early 1970s. On average, those cities now see 18 additional mosquito disease days each year.

California cities see largest increases

Dig deeper:

California cities recorded some of the largest increases. San Francisco topped the list with 52 more mosquito disease days annually, followed by Reno, Nevada, with 50 more days, Santa Maria, California, with 49, Albany, Georgia, with 41, and Salinas, California, with 40. Each city experienced increases more than double the national average.

Regionally, the Southeast, Northeast, and Northwest recorded the largest average gains, each adding approximately 20 mosquito disease days per year over the past five decades.

Temperatures plays critical role

Why you should care:

Scientists say temperature plays a critical role in mosquito biology. Within certain limits, warmer conditions help mosquitoes develop faster, survive longer, reproduce more efficiently, and allow the viruses they carry to multiply more quickly. These factors can increase opportunities for disease transmission.

However, researchers emphasize that temperature is only one part of a complex picture. Mosquito populations and disease spread are also shaped by rainfall patterns, humidity, ecological changes, urban development, land use and international travel.

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Heavy rainfall can create more breeding sites for mosquitoes, while drought conditions may increase West Nile virus transmission by concentrating birds – natural hosts of the virus – and mosquitoes around limited water sources. According to scientists, climate change is expected to increase both heavy rainfall and drought in different parts of the country, further influencing mosquito activity.

But scientists also say 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 actually lead to local decreases in disease transmission.

Big picture view:

More than 200 mosquito species are found in the United States, although only about a dozen commonly spread diseases to humans. The primary disease-carrying mosquitoes include the Culex species, which transmit West Nile virus; Aedes mosquitoes, which spread dengue, Zika, and chikungunya viruses; and Anopheles mosquitoes, which can carry malaria, though most U.S. malaria cases are linked to international travel.

West Nile virus remains the country's most common mosquito-borne illness, with approximately 2,000 reported cases each year. Infections typically peak during August and September.

Globally, mosquito-borne diseases pose a far greater public health threat. Malaria causes more than 600,000 deaths annually, primarily among children under five in Africa and Asia, according to the World Health Organization.

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