Mosquitoes collected in Joliet last week have tested positive for West Nile Virus, the Will County Health Department said Wednesday.

This is the first batch of mosquitoes in Joliet to test positive for the virus this season, officials said. In the south suburbs, mosquito samplings from two sites in Bolingbrook have tested positive 12 times. Two samplings in Frankfort have tested positive this season.

No human cases of West Nile Virus have been reported in Will County for about three years, said Vic Reato, spokesman for the Will County Health Department. Four human cases have also been reported this year; two from DuPage and one each from Kane and Cook counties.

The Joliet mosquito sampling was taken Friday on the city’s east side near Interstate 80 and Richards Street.

The number of mosquitoes with the virus in Illinois has increased dramatically since Aug. 1, Reato said, and the majority of activity continues to occur in the Chicago area.

As of Tuesday, 27 Illinois counties had reported at least 1,400 West Nile Virus-positive mosquito batches. The numbers represent an increase of more than 246 percent over 2009 activity levels, Reato said.

For information about West Nile Virus activity in Illinois, visit www.idph.state.il.us.