Mosquito populations in the region are starting to shrink but the threat posed by insects infected with the deadly eastern equine encephalitis virus could continue well into October, the head of the state’s mosquito-management program said Friday.

Infected mosquitoes have been found this summer at trapping sites in a dozen Connecticut towns, including, Groton, Ledyard, North Stonington and Stonington, where school and recreation officials have taken steps to limit outdoor activities around dusk and dawn. On Friday, Ledyard Libraries postponed a hike it had scheduled for 6 p.m. Wednesday in Gales Ferry while Norwich Free Academy announced it was rescheduling team practices and games. Earlier, Groton Parks and Recreation canceled any previously approved use of its fields after 6:30 p.m., and Stonington High School moved up the start time of two Friday night football games to 4 p.m.

Horses infected with the virus have been found in two additional towns, including Colchester.

“Some infected mosquitoes will linger on over the course of the next two weeks, so we feel actions being taken by local officials — curtailing school activities and rescheduling football games on Friday nights — makes a lot of sense,” said Theodore Andreadis, director of the New Haven-based Connecticut Agricultural Experiment Station.

The first “hard” frost of the year, which typically occurs by mid-October, will put an end to the threat — until next year, he said.

CAES maintains 92 strategically located trapping stations in 72 municipalities in the state, and sets mosquito traps at the sites every 10 days on a rotating basis. The sites in southeastern Connecticut are in Groton; Ledyard; Lyme; North Stonington, where there are four; Old Lyme; Stonington, which has three; and Waterford.

Residents of towns near those where infected mosquitoes have been trapped also should be taking precautions, Andreadis said.

The prevalence of infected mosquitoes has spiked throughout the Northeast this year, resulting in nine human cases, including two fatalities — one in Massachusetts and one in Rhode Island. The virus also has been detected in Maine, New Jersey and New York.

“We haven’t seen this level of activity in such a wide geographic area,” Andreadis said.

In Connecticut, it’s the worst year since 2013, when the state’s only confirmed human case of the virus occurred. The victim, a Killingly resident in his or her 40s, died after being infected during the first week of October that year, according to Andreadis.

Health officials in Rhode Island, where a West Warwick resident over 50 years of age died of the virus Sunday, have begun releasing aerial mosquito treatments in parts of the state, including Westerly.

Andreadis said Connecticut has no plans to spray.

“We’ve elected not to,” he said. “We are not seeing the level (of mosquito activity) they’re seeing in Rhode Island. We don’t believe broad-scale spraying is warranted at this time. ... If we had seen human cases in mid-July or early-August, we would consider it.”

Instead, CAES will continue trapping and testing mosquitoes. As of Wednesday, it had trapped 222,552 mosquitoes, 84 of which had tested positive for EEE.

Outbreaks of the virus are driven by a particular species of mosquito that thrives in red maple and white cedar swamps, Andreadis said.

“These mosquitoes do well after mild winters and very wet springs and wet summers, which is exactly what we experienced this year,” he said. “Going into the season, we knew it was going to be a high (mosquito) population.”

The virus is introduced by infected birds that migrate in the spring from Florida, where the virus circulates year-round. Mosquitoes pick up the virus when they feed on infected birds and can transmit it to horses and humans. It cannot be transmitted from horses to humans or from one human to another.

Horses are highly susceptible to the virus, with the mortality rate near 100 percent. Horses that contract the disease usually are euthanized, which was the case with the two horses infected this year in Connecticut, Andreadis said. An effective vaccine for horses exists.

For humans, the disease can have serious consequences.

About a third of those who contract it die. Of those who survive, about three-quarters experience neurological problems for the rest of their lives. No vaccine exists to protect against it and no antibiotics fight it. Symptoms can include high fever, headaches, nausea and vomiting and can progress to convulsions and coma.

b.hallenbeck@theday.com

Reduce the risk

To reduce the risk of being bitten by mosquitoes, the Connecticut Agricultural Experiment Station recommends the following precautions:

• Minimize time spent outdoors between dusk and dawn when mosquitoes are most active.

• Be sure door and window screens are tight-fitting and in good repair.

• Wear shoes, socks, long pants and a long-sleeve shirt when outdoors for long periods of time or when mosquitoes are more active. Clothing should be light-colored and made of tightly woven materials that keep mosquitoes away from the skin.

• Use mosquito netting when sleeping outdoors or in an unscreened structure and to protect babies when outdoors.

• Consider the use of mosquito repellent.