The Vermont Department of Health said Friday that the person was bitten by an infected mosquito in late August in Essex County.
In the first case, a Chittenden County adult was hospitalized with the virus in September.
The Health Department also says two alpacas from Highgate and Morrisville have died of the disease. Neither of the animals had been vaccinated for the virus.
State epidemiologist Patsy Kelso says people should continue to take precautions to avoid mosquito bites until the state has two or three hard frosts.