We are in the middle of mosquito season. The humid conditions increase the chance of mosquito diseases being transmitted to humans through the itchy bites of mosquitoes.
This week, Maine officials warned residents of the town of Orono that mosquitoes in the area are carrying the Jamestown Canyon virus, which was first transmitted from a mosquito to a human in Maine in 2017, the state Department of Health and Human Services said. This is the first time all year that tests for the virus in mosquitoes have come back positive, the Maine Center for Disease Control told USA TODAY.
“This puts our ears on edge and makes us look forward to a more active mosquito season this year,” said Megan Porter, a public health educator at the Maine CDC.
Maine health officials say the virus, which scientists consider an emerging public health threat, rarely spreads to humans. When it does, it can lead to serious neurological conditions such as meningitis and even death.
“Severe symptoms can develop very quickly and people can become very ill,” Porter said.
West Nile virus and dengue virus are much more widespread, and the mosquitoes that carry them are more likely to transmit them to humans, experts say. Doctors and scientists are also keeping an eye out this summer for St. Louis encephalitis, a mosquito-borne disease that can cause brain inflammation in severe cases.
According to Shannan Rossi, a professor of biology and immunology at the University of Texas Medical Branch in Galveston, more people will suffer from these diseases as we experience more hot and humid days for the rest of the summer in much of the country.
“As the summer progresses, you’re going to see more and more of these local cases, and that’s to be expected. We see this every year,” Rossi said.
Here’s what you need to know about the mosquito outlook in the coming months, according to experts.
Jamestown Canyon virus in Maine mosquitoes
Maine health officials reported that recent mosquito testing near the Penobscot County town of Orono was the state’s first positive test results for the virus this year.
According to Porter, there have been no human cases of the virus this year. The last human case was in 2021.
The virus is naturally found in mosquitoes in northern U.S. states and southern Canadian provinces, Rossi said. In Maine, mosquitoes known as “snow melt mosquitoes” — named for the mosquitoes that first gather around melted snow each spring — carry the most virus, Porter said. Overall, several species of mosquitoes carry the virus, but the number of people infected by their bites is “incredibly low,” Rossi said.
In New York State, researchers found that Jamestown Canyon virus infection rates among mosquitoes were “significantly higher” during 2012-2022 than during 2001-2011.
Cases of the virus in humans “are not something we hear a lot about,” Rossi said. “The Jamestown Canyon virus is vague, because we see it pop up all the time in mosquito tests if you look for it.”
People infected with the virus may not show any symptoms. Those who do may experience vomiting, neck stiffness, loss of coordination and, in severe cases, swelling of the brain and meningitis, according to the Maine Department of Health and Human Services.
According to the Maine CDC, a 56-year-old man died in 2018 from chronic encephalitis caused by the virus.
Post-mortem examinations revealed “extensive neuropathological abnormalities” in the man’s spinal fluid, Harvard University researchers reported in a report published in 2021.
There were no cases of mosquito-to-human transmission of the Jamestown Canyon virus in 2023 after three sets of mosquito tests in the state tested positive for the virus, the Maine Department of Health and Human Services said in a news release.
“Even though you can still see it in mosquitoes, that doesn’t necessarily mean it’s a big risk to people,” Rossi said. “But it’s still alarming, because you’re seeing something that’s transmittable by mosquitoes and it could be in your backyard.”
According to the CDC, there have been no cases of St. Louis encephalitis in humans reported nationwide this year.
West Nile virus is widespread, Dengue could spread further
According to Rossi, West Nile virus first spread to the U.S. in 1999, but is now found in mosquitoes in all 48 contiguous states.
So far in 2024, there have been 15 cases of mosquitoes transmitting the virus to humans in 11 states, according to the federal Centers for Disease Control and Prevention. The 15 cases are evenly distributed among Texas, Mississippi, Arkansas, Tennessee, North Dakota, Nebraska, Kansas, Arizona, Michigan and Maryland, the CDC said.
Because this summer is expected to be warmer and more humid than average, that could lead to more cases of West Nile virus, Rossi said.
“What I’m worried about is that summers are warmer and longer than usual, and that means the mosquito season is also longer than before,” said Rossi, 43.
The number of dengue fever cases in Puerto Rico has risen to 1,700 this year, prompting officials on the island to declare a health emergency. The surge comes after the CDC warned that countries in the Americas could see twice as many cases in the first half of 2024 compared to all of last year’s cases: 9.7 million cases versus 4.6 million.
South Florida has historically been a hotspot for dengue fever in the United States, with nine locally acquired cases in 2024, according to the CDC’s tracker. There have been no other cases of locally acquired dengue fever reported in the U.S. outside of Florida, while the U.S. Virgin Islands has reported 15 cases, the CDC says.
According to the CDC, cases of dengue have also been identified abroad in other U.S. states, including dozens of reported cases in New York, Massachusetts, California and Illinois.
However, Rossi said doctors are on alert for locally acquired cases of the dengue virus spreading further north from Florida.
“As conditions get warmer and hotter, it becomes easier for mosquitoes to move into areas where they weren’t before, and now we have to worry about places where dengue is potentially transmissible,” she said, referring to Georgia and the Carolinas, where the CDC says 40 travel-related cases of confirmed dengue fever have already been reported.
How can I prevent West Nile virus, dengue virus, and Jamestown Canyon virus?
The mosquitoes that transmit West Nile virus bite around dawn and dusk, Rossi said, so it’s important to stay indoors during that time to prevent transmission.
Mosquitoes that transmit dengue are more likely to congregate near standing water closer to people’s homes, such as in a wet backyard, Rossi said. Keeping windows closed when possible is a good protective measure to avoid contact with mosquitoes, she said.
Additionally, insect repellent and the right clothing can go a long way in protecting you from bites, including those that potentially transmit viruses such as the Jamestown Canyon virus.
“It’s difficult to say at 35 degrees,” Rossi said, “but wear longer sleeves.”