Fall Temps Extend Mosquito Season

After August, September is the peak time for mosquito-borne illnesses in the U.S. Mosquitos prefer warm temperatures and high humidity, and by this time of the year the insects have had more time to grow and transmit viruses. In turn, new cases of the three most common diseases spread by mosquitoes in the U.S.—dengue fever, eastern equine encephalitis (EEE), and West Nile virus—are still being reported in many states.

While the number of cases is concerning, an unprecedented number of EEE and West Nile virus are not being reported. Surprisingly, the most prevalent of the mosquito-spread infections this year in the U.S. is dengue, which is also abnormally high globally. An outbreak in Puerto Rico makes up most of the cases. Of the 3,277 locally acquired cases reported as of Aug. 28 by the U.S. Centers of Disease Control and Prevention (CDC), 3,147 were in Puerto Rico, 102 in the U.S. Virgin Islands, 27 in Florida, and one in California. This number is expected to continue to grow as California Health officials have reported three locally acquired dengue cases this year so far. It is also only the second year in which locally acquired cases have been reported in California.

As of Sept. 24, 748 cases of West Nile virus disease have been reported in the U.S., which led to 515 West Nile virus neuroinvasive disease cases this year, according to preliminary data from the CDC.  Traditionally the most prevalent of the mosquito-spread infections in the U.S., 43 states have reported cases of West Nile virus disease this year. At least 18 West Nile virus deaths have occurred this year.

EEE human cases in the U.S. total 10 as of Sept. 24, with the number of states reporting the virus up to six. All 10 cases have led to neuroinvasive disease cases, and one death has been recorded. Late last month, New Hampshire saw its first case and death from EEE since 2014. The state has since reported another case of EEE. Vermont also reported its first case since 2012. Massachusetts has reported four human cases of EEE this year, and the state’s first cases since 2020. New Jersey, Rhode Island and Wisconsin have also reported cases this year.

An epidemiologist said the risk for mosquito-borne illnesses will continue this fall until a hard frost occurs that kills the mosquitos.