We’re in the middle of mosquito season, when humid conditions increase the risk of mosquito-borne diseases being transmitted to humans through itchy bites.
This week in Maine, officials warned residents of the town of Orono that mosquitoes in the area were carrying the Jamestown Canyon virus, which was first transmitted from a mosquito to a human in Maine in 2017, according to the state Department of Health and Human Services. It’s the first time this year that mosquitoes have tested positive for the virus, the Maine Center for Disease Control told USA TODAY.
“This gives us cause for concern and hope for perhaps a more active mosquito season this year,” said Megan Porter, a public health educator at the Maine CDC.
The virus, which scientists consider an emerging threat to human public health, is very rarely transmitted to humans, but when it is, it can cause serious neurological diseases such as meningitis and even death, according to Maine health officials.
“Severe symptoms can progress very quickly and people can become very ill,” Porter said..
West Nile virus and dengue fever are much more common, and the mosquitoes that carry them are more likely to transmit them to humans, experts say. Doctors and scientists are also watching this summer for St. Louis encephalitis, a mosquito-borne disease that can cause brain inflammation in severe cases.
More people will get these illnesses as we see more hot, humid days throughout the rest of the summer across much of the country, said Shannan Rossi, a professor of biology and immunology at the University of Texas Medical Branch at Galveston.
“As the summer goes on, we’re going to see more and more local cases, and that’s normal. We see it every year,” Rossi said.
Here’s what you need to know about the outlook for mosquito infestations in the coming months, according to experts.
Jamestown Canyon Virus in Maine Mosquitoes
Maine health officials said recent mosquito tests near the town of Orono in Penobscot County were the first positive results of the year for the virus in the state.
There have been no human cases of the virus this year, Porter said, and the last human case was in 2021.
The virus occurs naturally in mosquitoes in the northern U.S. and southern Canadian provinces, Rossi said. In Maine, mosquitoes known as “snowmelt mosquitoes” — named after the first to congregate around melting snow each spring — are the biggest contributors to the virus, Porter said. Overall, several species of mosquitoes carry the virus, but the number of humans infected by their bites is “incredibly small,” Rossi said.
In New York State, researchers found that infection rates of Jamestown Canyon virus in mosquitoes were “significantly higher” between 2012 and 2022 compared to 2001-2011.
“You don’t hear about human cases of the virus very often,” Rossi said. “The Jamestown Canyon virus is nebulous because it keeps showing up in mosquito tests.”
People infected with the virus may not have any symptoms. Those who do can experience vomiting, neck stiffness, loss of coordination and, in severe cases, brain swelling and meningitis, according to the Maine Department of Health and Human Services.
In 2018, a 56-year-old man suffering from chronic encephalitis caused by the virus died, according to the Maine CDC.
Tests conducted after the man’s death “revealed significant neuropathological abnormalities” in his spinal cord fluid, Harvard University researchers said in a report published in 2021.
In 2023, there were no cases of Jamestown Canyon virus transmission from mosquitoes to humans after three batches 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 we continue to see it in mosquitoes, that doesn’t necessarily mean it’s a huge risk to people,” Rossi said. “But it’s still alarming because we’re seeing something that’s mosquito-borne and could be in your backyard.”
Nationwide, no cases of St. Louis encephalitis have been reported in humans this year, according to the CDC.
West Nile virus widespread, dengue fever could spread further
After first spreading to the United States in 1999, West Nile virus is now present in mosquitoes across the 48 contiguous states, Rossi said.
According to the federal Centers for Disease Control and Prevention (CDC), so far in 2024, there have been 15 cases of mosquitoes transmitting the virus to humans in 11 states. 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 wetter than average, Rossi said, that could lead to more cases of West Nile virus.
“What worries me is that we have hotter and longer summers than usual, which means the mosquito season is also longer than before,” said Rossi, 43.
The number of dengue cases has reached 1,700 in Puerto Rico this year, prompting authorities there to declare a state of public health emergency. The spike comes as the CDC warned that countries in the Americas have recorded twice as many cases in the first half of 2024 as all of last year: 9.7 million cases compared to 4.6 million.
South Florida has historically been a hotbed of dengue fever in the United States, with nine locally acquired cases in 2024, according to CDC tracking. No other locally acquired dengue cases have been reported in the United States outside of Florida, while the U.S. Virgin Islands has reported 15 cases, according to the CDC.
According to the CDC, dengue cases in other U.S. states have been detected overseas, including a wave of dozens of cases reported in New York, Massachusetts, California and Illinois.
However, Rossi said, doctors are keeping an eye on local cases of the dengue virus that are spreading further north in Florida.
“As conditions get warmer, it’s easier for mosquitoes to move to areas they haven’t been before, and now we have to worry about places that are becoming potentially transmissible for dengue,” she said, referring to Georgia and the Carolinas, where 40 confirmed travel-related dengue cases have already been reported, according to the CDC.
How can I prevent West Nile virus, dengue fever, and Jamestown Canyon virus?
Mosquitoes that carry West Nile virus bite at dawn and dusk, Rossi said, so staying indoors during those times is key to avoiding transmission.
Dengue mosquitoes tend to congregate near standing water near homes, such as in a damp garden, Rossi said. Keeping windows closed if possible is a good protective measure to avoid contact with mosquitoes, she added.
Aside from that, insect repellent and proper clothing can go a long way in protecting you from bites, including those that can transmit viruses, including the Jamestown Canyon virus.
“It’s hard to say that in 35-degree weather,” Rossi said, “but wear longer sleeves.”