AUGUSTA – State health officials said Thursday that six dead birds found in Maine have tested positive for West Nile virus during the past week, raising the state’s total for the year to 13.

“The intensity of West Nile virus has increased in the past week,” said Dr. Kathleen Gensheimer, the state epidemiologist. “This is not an unexpected development given the expansion of the virus in New England and the United States over the summer.”

West Nile virus has been detected this year in birds, mosquitoes, horses and people in 41 states and the District of Columbia.

The latest Maine birds to test positive included two crows in Portland, one crow each in Arundel, Cape Elizabeth and Lewiston, and a blue jay in South Portland.

“This is the time of year that we would expect the most heightened virus transmission,” Gensheimer said, “and that is evidenced by the fact that we have identified nearly half of the positive birds for the entire summer in the past week.”

West Nile virus is a disease that can lead to an infection called encephalitis, or swelling of the brain. It is spread through mosquito bites, and dead birds are an indication of its presence.

Most people who contract West Nile virus show no symptoms or mild symptoms, but some people – especially the elderly – can become ill and even die. Nationwide, there have been 1,086 confirmed human cases of the virus, including 45 fatalities, in 29 states.

No person has tested positive for the virus in Maine, but Gensheimer said the state could see its first human cases before the end of summer.

“We should not be naive about the possibility that a case of this virus may occur in a person in Maine,” she said, “given that the virus has been documented in people in other states.”

In Lewiston, Kathy Lawrence said she found the dead crow in her back yard several weeks ago and sent it to the Bureau of Health to be tested. Lawrence, who is the president of the Stanton Bird Club, said she knew that the state was looking for test specimens, but didn’t imagine the bird she found would be infected.

“It’s a sad situation, but I guess it’s nature,” she said. “I guess it’s one of those things you have to look out for now.”

Birds confirmed to be infected earlier this year included three from Cumberland County, two from Penobscot County and one each from Hancock and Androscoggin counties.

Seven birds tested positive for the virus in all of last year.