See statewide data on West Nile virus by county here. In 2014-15-16, Chester County had a total of 3 known human cases and no known WNV-related deaths.
In 2016, 4 Chesco releases concerned mosquitoes and WNV, including 2 about spraying. See here.
Statewide WNV data by county here. So far: one known human case in Chesco as of 8/10/17, no deaths.
One Chesco spraying event so far (see here).
Chesco health Department News Releases (including about mosquito spraying) here.
See spraying (“adulticide events”) in Chester County here, beginning 8/10/17.
WNV “hot zones” in PA, for the preceding 3 weeks here.