Jennifer White, a New York state epidemiologist, has a reminder as summer’s end approaches: “It may be pumpkin spice season, but it’s not the end of mosquito season.”
August and September are the peak months for mosquito-borne illnesses in the U.S. That’s because populations of the insects — which prefer warm temperatures and high humidity — have had time to grow and transmit more virus by the end of the summer.
Indeed, new cases of the three most common diseases spread locally by mosquitoes within the U.S. — dengue fever, eastern equine encephalitis and West Nile virus — are still being reported in many states. This week, California health officials issued a warning about three locally acquired dengue cases in Los Angeles County, two of which were announced Wednesday.
“This is an unprecedented cluster of locally acquired dengue for a region where dengue has not previously been transmitted by mosquitoes,” Barbara Ferrer, director of the Los Angeles County Department of Public Health, said at a news conference.
In the U.S. overall this year, a concerning, though not unprecedented, number of dengue, EEE and West Nile cases have been reported. Last month, New Hampshire saw its first case and death from EEE since 2014, and Vermont reported its first case since 2012. In perhaps the most public case of West Nile this year, Dr. Anthony Fauci, former director of the National Institute of Allergy and Infectious Diseases, was hospitalized in August then sent home to recover.
Although the spread of mosquito-borne illnesses usually declines in October as the weather cools, rising global temperatures are leading to longer summers and shorter winters, creating more opportunities for mosquitoes to breed.
“With climate change, we’re basically extending the mosquito season,” said Chantal Vogels, an assistant professor of epidemiology at the Yale School of Public Health.
Dengue’s expanding threat
Cases of mosquito-borne illnesses are not necessarily spread evenly throughout the country. Dengue fever, which can cause fever, aches, nausea and rashes, has been found locally in Arizona, California, Florida, Hawaii and Texas, as well as Puerto Rico and the U.S. Virgin Islands.
This year, dengue cases are abnormally high both nationally and globally. In the U.S., cases have outpaced those of West Nile virus, which is typically more prevalent. An outbreak in Puerto Rico makes up the majority of the cases: Of the roughly 3,085 locally acquired cases reported this year, 2,960 were in Puerto Rico and 23 in Florida, according to the Centers for Disease Control and Prevention.
