Experts: Late start to West Nile season not unexpected

An electron micrograph of the West Nile virus. West Nile virus is a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals.

West Nile virus has not yet been detected in Connecticut’s mosquitoes this season — a phenomenon that at least one local scientist said was somewhat anticipated.

Theodore Andreadis is director of the Connecticut Agricultural Experiment Station, which runs the state’s mosquito trapping and testing program. He said at the start of this season he expected West Nile might not be as rampant this year, as last season was the worst in the state’s history for the mosquito-borne illness.

“So many birds (which play a role in spreading West Nile) were exposed last year, so there may be some herd immunity,” Andreadis said.

Last year, 393 positive mosquito samples were collected from 65 sites in 53 municipalities. The state reported 23 human cases of West Nile, and one death — the first since 2006.

Andreadis said the number of Culex pipiens mosquitoes — the main mosquitoes that carry West Nile — has also been low this year, which might be another reason virus activity has been stagnant.

Another expert from the agricultural station, Philip Armstrong, echoed Andreadis — the low numbers of Culex mosquitoes could be responsible for what is likely just a late start to West Nile season — which he expects to pick up soon.

“In prior years, we have had a similar slow start, but virus activity can ramp up very quickly,” Armstrong, a virologist and medical entomologist, said in an email. “I expect the numbers of Culex pipiens to increase with the onset of hot weather in July and that could lead to an increase in WNV activity.”

West Nile is already increasing elsewhere in the country, including Massachusetts, New Jersey and New York. Eastern Equine Encephalitis, has also been spotted in those regions.

The latest test results from the state show that, during the week of July 15, 26,984 mosquitoes were trapped and tested. Of these, none tested positive for West Nile or Eastern Equine Encephalitis. Two mosquitoes, both in Hampton at the Hampton Reservoir, tested positive for Jamestown Canyon virus, an emerging mosquito-borne infection.

So far this season, 15 mosquitoes have tested positive for Jamestown Canyon virus. First spotted in Colorado in 1961, it is still fairly rare in humans, as 166 human cases of the illness have been reported between 2000 and 2018. That includes two in Connecticut.

Like West Nile and other mosquito-borne illness, Jamestown Canyon causes mild, flu-like symptoms in most people, but can lead to more serious illnesses, such as meningitis and encephalitis.

Connecticut Media Group