What's the Jamestown Canyon virus, the virus found in some Maine mosquitoes?

We're right in the middle of mosquito season, when humid conditions increase the risk of mosquito-borne diseases being spread to humans through the bugs' itchy bites.

This week in Maine, officials warned residents in the town of Orono that mosquitoes in the area are carrying the Jamestown Canyon virus, which was transmitted from a mosquito to a human for the first time in Maine in 2017, according to the state's Department of Health and Human Services. This is the earliest in the year tests for the virus in mosquitoes have ever come back positive, the Maine Center for Disease Control told USA TODAY.

"This is perking our ears up and making us definitely look forward to perhaps a more active mosquito season this year," said Megan Porter, a public health educator with the Maine CDC.

The virus, which scientists consider an emerging public health threat to humans, is very rarely transmitted to people, but when it is, it can lead to severe neurological diseases like meningitis and even cause death, according to Maine health officials.

"Severe symptoms can progress very rapidly, and people can get very sick," Porter said.

West Nile virus and dengue virus are far 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.

More people will come down with these illnesses as we see more hot and humid days the rest of this summer across much of the country, said Shannan Rossi, a biology and immunology professor at the University of Texas Medical Branch in Galveston.

“As the summer continues, you’re going to see more and more of these local cases, and that’s expected. We see this every single year," Rossi said.

Here's what to know about the mosquito outlook for the next couple of months, according to experts.

Jamestown Canyon virus in Maine mosquitoes

Health officials in Maine said mosquito tests recently taken near the town of Orono in Penobscot County were the state's first positive test results of the year for the virus.

There have been no cases of the virus in humans this year, Porter said, and the last human case was in 2021.

The virus is occurs naturally in mosquitoes in the northern U.S. states and Canada's southern provinces, Rossi said. In Maine, mosquitoes referred to as "snowmelt mosquitoes" − named for the first to gather around melted snow each spring − contribute the most to the virus, Porter said. Overall, several species of mosquitoes carry the virus, but the number of humans infected from their bites is "incredibly low," Rossi said.

In New York State, researchers found infection rates of Jamestown Canyon virus in mosquitoes were "significantly greater" in 2012 to 2022 compared with 2001 to 2011.

Cases of the virus in humans "is not something we hear about a lot," Rossi said. "Jamestown Canyon virus is nebulous in that we continually see it being picked up in mosquito testing if you're looking for it."

People infected with the virus may not show symptoms. Those who do may experience vomiting, neck stiffness, loss of coordination and, in severe cases, brain swelling and meningitis, according to Maine Department of Health and Human Services.

In 2018, a 56-year-old man experiencing chronic encephalitis caused by the virus died, according to the Maine CDC.

Testing after the man's death "revealed extensive neuropathologic abnormalities" in his spinal cord fluid, Harvard University researchers said in a report published in 2021.

In 2023, there were no cases of transmission of Jamestown Canyon virus from mosquitoes to humans after three batches of mosquito testing in the state came back positive for the virus, the Maine health and human services department said in a news release.

"Even though you can continue to see it in mosquitoes, that doesn't necessarily mean it poses a huge risk to people," Rossi said. "But it's still alarming, because you see something that's transmissible by mosquitoes and it could be in your backyard."

Nationally, there are also no reported cases of St. Louis encephalitis in humans this year, according to the CDC.

West Nile Virus is widespread, Dengue could spread more

After first spreading to the U.S. in 1999, West Nile Virus is now found in mosquitoes in all 48 contiguous states, according to Rossi.

So far in 2024, there have been 15 cases of mosquitoes transmitting the virus to humans across 11 states, according to the federal Centers for Disease Control and Prevention. The 15 cases are evenly spread across Texas, Mississippi, Arkansas, Tennessee, North Dakota, Nebraska, Kansas, Arizona, Michigan and Maryland, the CDC says.

Because this summer is forecast to be hotter and more humid than average, Rossi said, that could lead to more cases of West Nile virus.

"What I’m concerned about is that we’re having hotter and longer summers than we have traditionally, and what that means is the mosquito season also lasts longer than it did," said Rossi, 43.

Cases of dengue fever have spiked to 1,700 in Puerto Rico this year, triggering officials on the island to declare a health emergency. The rise comes as the CDC warned countries in the Americas saw twice as many cases during the first half of 2024 compared with all cases from last year: 9.7 million cases versus 4.6 million.

South Florida has historically been a hot spot for dengue fever in the United States, with nine cases locally acquired in 2024, according to the CDC's tracker. No other cases of locally acquired dengue fever have been reported in the U.S. outside Florida, while the U.S. Virgin Islands have reported 15 cases, the CDC says.

Cases of dengue fever in other U.S. states were caught abroad, according to the CDC, including a spate of dozens of reported cases in New York, Massachusetts, California and Illinois.

However, Rossi said, doctors are keeping an eye out for locally acquired cases of the dengue virus spreading farther north of Florida.

“As the conditions get warmer and hotter, it makes it easier for the mosquitoes to move into areas where they haven’t been before, and now we have to worry about places becoming potentially transmissible for dengue," she said, referring to Georgia and the Carolinas, where 40 travel-related cases of confirmed dengue fever have already been reported, according to the CDC.

How can I prevent West Nile, dengue, Jamestown Canyon virus?

The mosquitoes that carry West Nile virus bite around dawn and dusk, Rossi said, so staying indoors during those times is key to avoiding transmission.

Mosquitoes that carry dengue are more likely to gather near standing water closer to people's homes, like in a wet backyard, Rossi said. Keeping windows closed if possible is a good protective measure to avoid contact with mosquitoes, she said.

Other than that, insect repellant and proper clothing can go a long way toward saving you from bites, including possibly virus-transmitting ones, including the Jamestown Canyon virus.

“It’s hard to say this in 95-degree weather," Rossi said, "but wear longer sleeves."