Mosquito samples in 12 different Connecticut towns have turned up positive samples for a rare but potentially serious virus, according to the Connecticut Agricultural Experiment Station.

The Jamestown Canyon Virus, which is spread to people through infected mosquitoes, causes generally mild symptoms including fever, headache, and fatigue. But can in rare cases be more severe, causing encephalitis or inflammation of the brain, according to Philip Armstrong, an expert on mosquito-borne viruses. Cases generally occur from late spring through mid-fall, according to the Centers for Disease Control and Prevention.

“It’s something we do see here in Connecticut,” Armstrong said. “It’s spotted here every summer, but it is what we consider an emerging pathogen as more human cases have been discovered nationally throughout the country over the last few years. While we have increased our mosquito testing, the number of positive pools we are seeing with JCV is typical for what we see just about every year.”

Towns with confirmed positive samples include Cromwell, Danbury, Darien, East Haddam, Meriden, Monroe, North Haven, North Stonington, South Windsor, Wallingford, West Haven and Wethersfield, according to Armstrong. But although mosquitos have tested positive in several towns, there have only been two recorded human cases over the last 20 years in the state. There are no confirmed human cases of the disease this year.

“We don’t really see any human cases, I don’t want to alarm people,” Armstrong said. “But it is yet another virus mosquitoes carry that can cause human disease in rare instances.”

Armstrong said that a rainy summer has contributed to higher rates of mosquitos this year as standing and stagnant water increases breeding opportunities. Mosquitoes lay their eggs in standing water and the larvae need water to survive and mature. Armstrong recommends dumping any stagnant water and limiting exposure to mosquito bites.

The agricultural station, which handles the state’s mosquito testing, works with the state’s Department of Energy and Environmental Protection. DEEP handles mosquito control and management on state lands and advises local towns and homeowners about options on pest control, according to Armstrong. The agricultural station handles monitoring, including mosquito levels and confirmed viruses.

“West Nile Virus is by far a much more prevalent virus that does see hospitalizations across the state,” Armstrong said. “What we don’t see with JCV unlike West Nile is real outbreaks or clusters of the disease. It’s more sporadic cases you see pop up more infrequently.”

So far this year 26 towns across the state have tested positive for mosquitoes with West Nile Virus, according to Armstrong.

Mosquitoes trapped in Thompson on Aug. 24 by the Connecticut Mosquito Management Program tested positive for eastern equine encephalitis, the Connecticut Agricultural Experiment Station said. The five mosquitoes were of a species that predominately bites birds. Still, the station urged people to limit their exposure to the disease, which has caused 181 human cases including four fatalities since 2000.

“The recent detection of EEE virus and continued spread of West Nile virus is cause for concern as conditions are suitable for further build up of virus in the coming weeks,” said Dr. Philip Armstrong, medical entomologist at CAES. “We will continue to closely monitor mosquitoes for virus amplification, and we encourage everyone to take simple measures such as wearing mosquito repellent and covering bare skin, especially during dusk and dawn when mosquitoes are most active.”

Stephen Underwood can be reached at sunderwood@courant.com