State officials also pledged to redouble a statewide effort to monitor for the virus. But they cautioned against overreaction, saying the discovery was expected and pointing out there have been no reported cases of the virus in humans in Massachusetts, or even in mosquitoes. The disease is fatal in only a small minority of people who contract it. The West Nile virus can cause encephalitis, a dangerous swelling of the brain which can be deadly. Most who contract it experience flu-like symptoms and never realize they have been exposed to the virus.
More severe infections can cause tremors, convulsions and sometimes death.
About 7 percent of those sick enough to be hospitalized for the virus die from it, according to the state public health department. It wasn't clear whether the crow had picked up the disease outside of Massachusetts and flown here, or picked up the disease locally, said Timperi. Crows can fly up to 200 miles, and he confirmed this is the farthest the virus has been found in the U.S. from its epicenter in Queens, New York. In New York, where extensive efforts to combat the virus are under way, there have been no cases detected among humans this year, though birds and insects have tested positive. Dr. Alfred De Maria, director of the state Public Health Department's bureau of communicable diseases, said he expected there would eventually be human cases in Massachusetts.