Rising global temperatures are resulting in an increase in mosquito “disease danger days” across much of the United States, including Minnesota, according to a new report by Climate Central, a nonprofit news organization run by scientists and journalists.
As the report points out, human-caused climate change has significantly affected the pattern, incidence and location of illnesses spread by mosquitoes, ticks and fleas (known as vector-borne diseases), creating a growing public health challenge.
Indeed, last spring, the Centers for Disease Control and Prevention (CDC), reported that the number of vector-borne illnesses has tripled in recent years, rising from 27,388 in 2004 to 96,075 in 2016.
The total number of cases reported during that 13-year period topped 640,000.
Two mosquito species, nine diseases
For their report, the Climate Central researchers focused on the role temperature is playing in the transmission of diseases from two species of mosquitoes: Culex and Aedes (specifically, Aedes aegypti and Aedes albopictus).
Together, these mosquitoes can spread nine diseases, seven of which are already known to have been transmitted in the U.S.: West Nile virus, dengue, Zika, chikungunya, yellow fever, St. Louis encephalitis and eastern equine encephalitis.
(The primary mosquito vector in Minnesota is Culex tarsalis, which carries West Nile virus, St. Louis encephalitis and eastern equine encephalitis. Aedes aegypti mosquitoes are not currently found in Minnesota, and Aedes albopictus mosquitoes have appeared in the state only sporadically in recent years.)
The Climate Control researchers analyzed the daily temperatures of 244 U.S. cities between 1970 and 2017. They looked to see how many days in the spring, summer and fall of those years that the cities had an average temperature between 61 degrees and 93 degrees Fahrenheit. When temperatures are within that range, the risk of transmission of a Culex or Aedes mosquito-borne disease climbs considerably, the researchers point out.
The data revealed that 94 percent of the cities have experienced an increase since 1970 in the number of days within that temperature range.
Here are the 10 cities with the biggest increases:
- Reno, Nevada (52 more days)
- San Francisco, California (47)
- Santa Maria, California (39)
- Las Cruces, New Mexico (34)
- El Paso, Texas (33)
- Tucson, Arizona (29)
- Helena, Montana (28)
- Erie, Pennsylvania (28)
- Fresno, California (27)
- Bluefield, Virginia (27)
The three Minnesota cities included in the report weren’t too far behind. Duluth has 24 more days of “disease danger days” than in it did in 1970, while Minneapolis-St. Paul has 18 more days and Mankato has 17.
But climate change has also lowered the number of “disease danger days” in some cities, such as Phoenix. That’s most likely, the report says, because those locations have more days that are above 95 degrees Fahrenheit than in 1970. Such temperatures are too hot for mosquitoes to survive and/or for disease to be transmitted.
An expanding problem
“Overall, the number of mosquito disease danger days is increasing across much of the U.S., representing a greater risk for transmission of mosquito-borne diseases,” the report concludes. “Dengue, Zika, chikungunya and West Nile viruses all represent significant health threats globally, and while they are currently limited in the U.S., could become more major problems if climate change is not abated.”
How much of a problem? The report notes that the land area of the U.S. most suitable for just one of the disease-carrying mosquitoes — Aedes albopictus — is projected to increase from 5 percent to about 50 percent by 2100.