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More Mosquito Days Increasing Zika Risk in U.S.

Research Report by Climate Central

Hot and humid summer weather across the U.S. brings with it the rise of the mosquito season, and this year the threat of the Zika virus makes that more than a minor nuisance. Mosquito species found in the Lower 48 states are known to transmit this disease, thriving in tropical and subtropical climates. As the climate warms and humidity increases across the nation, the risk of mosquito-borne diseases, like Zika, is becoming more prevalent for Americans.


Climate Central’s States at Risk project has analyzed how the length of the mosquito season has been changing across hundreds of metropolitan areas across the Lower 48 states. We found that in most of the country, rising temperatures and humidity since the 1980s have driven an increase in the number of days each year with ideal conditions for mosquitoes. Warming temperatures lead to more evaporation, which puts more water vapor in the atmosphere and increases humidity. The overall increase in mosquito days in the U.S. is likely increasing the risk of several mosquito-borne diseases, including the Zika virus

  • Cities like Baltimore and Durham, N.C., have seen their annual average mosquito season grow by nearly 40 days since the 1980s. 

  • Dozens of cities across the Midwest, Northeast, and along the Atlantic Coast have all seen their mosquito seasons grow by at least 20 days over the past 35 years. 

  • More than 20 major U.S. cities have ideal climate conditions for mosquitoes at least 200 days each year. 

  • In a few hot Southern cities, rising extreme heat since the 1980s has actually caused the mosquito season to begin to decrease (though there are still hundreds of days each year with ideal conditions for mosquitoes in these locations).

Zika Virus Overview

A current outbreak of the Zika virus in Brazil and other South and Central American countries has made the disease a public health concern over the past year. While many people that contract the virus have mild symptoms, including fever, rash, and joint and muscle pain, or no symptoms at all, the virus can cause serious complications. Pregnant women that are infected have an increased risk of delivering babies with birth defects and the virus has also been associated with Guillain-Barre syndrome.

According to the CDC, the Zika virus isn’t currently being spread by mosquitoes in the U.S., but species known to transmit the virus (Aedes aegypti and Aedes albopictus) are found in the U.S. and the CDC has confirmed more than 1,400 cases of Zika in travelers returning to the U.S. These infected travelers have the potential to spread the virus domestically through mosquitoes.

The range for mosquito species able to transmit Zika spreads across much of the country, but these mosquitoes are particularly abundant in the Southeast and along the Atlantic Coast.

Mosquito Season Growing

Climate Central analyzed how the number of days each year with ideal conditions for mosquitoes has been changing since 1980. We found that most major cities in the country (76 percent) have seen an overall increase in days conducive for mosquitoes in the past 36 years, and many regions have seen the mosquito season increase by half a month or more. 

Among the 200 largest metro areas in the U.S., 10 cities have seen their seasons grow by a month or more over this relatively short period of time. Overall, 125 cities are now seeing their average annual mosquito seasons at least five days longer than they were in the 1980s. 

The top 25 cities that have seen the biggest increase in mosquito days since the 1980s are:

We also discovered that in a few locations, temperatures have increased so much since the 1980s that the overall mosquito season has decreased slightly. For example, five Texas cities (all of which have seen large increases in extreme heat over that time) have seen their mosquito season decrease by 5-30 days. However, four of these cities are still seeing an average of at least 100 days each year with ideal conditions for mosquitoes, so the overall threat has not decreased considerably. 

See how the annual average number of mosquito days has changed in all these metro areas, since the 1980s.

Florida cities face the greatest overall threat from mosquitoes; most of its major cities face hundreds of days each year with climate conditions that are ideal for these insects. Many of these cities have seen an overall increase in mosquito days since the 1980s, but a few cities, like Fort Myers and Orlando have slightly shorter seasons now than they did 35 years ago. 

The top 25 U.S. cities with the longest average annual mosquito seasons are:

The analysis of mosquito season length is based on the ideal temperature and humidity conditions for Asian Tiger Mosquitoes (Aedes albopictus), which is one of the species known to transmit the Zika virus. These mosquitoes have high mortality rates at temperatures outside the range of 50-95°F or at relative humidity below 42 percent.

Previous research has also confirmed that several U.S. cities also have climate conditions that support another transmitting mosquito species, Aedes aegypti. However, because the Asian Tiger mosquito exhibits greater cold tolerance, it is expected to have a greater transmission risk in more cities.

Zika and Climate Change

In the coming decades, average annual temperatures (including spring, summer and fall temperatures, specifically) are projected to rise, with the amount of increase depending on how rapidly we curb our greenhouse gas emissions. In most parts of the U.S., the warming will likely increase the total number of days each year when temperatures fall between 50-95°F. In some places, more extreme heat could shorten the mosquito season, though it is unlikely to remove the risk of mosquitos entirely. 

In addition, projections for more heavy downpours in the U.S. and globally may also increase the threat from Zika. Intense rainfall can create pools of standing water on the ground, which are ideal for mosquito breeding.

Studies investigating how climate change will influence other mosquito-borne diseases, like Dengue fever and the West Nile virus, suggest that the range and season for mosquitoes is projected to increase in much of the U.S., though hotter temperatures could force mosquitoes out of some areas. 

Analysis by Alyson Kenward, PhD, and Jennifer Brady. 

Methodology:

The formula for mosquito-suitable days is based on National Institute of Health studies incorporating temperature and humidity. This analysis examines conditions on individual days rather than multi-day heat events which can affect mosquito populations. Temperature and humidity data (calculated from vapor pressure) for the largest 198 metropolitan areas in the U.S. from 1980-2015 was accessed from Daymet.