The North Shore Mosquito Abatement District said the batch was collected on Monday in a trap and tested positive for the virus on Thursday.

Illinois has confirmed its first human case of West Nile virus this year, health officials say. The person, who lives in southern Illinois, was hospitalized with complications from the mosquito-borne virus.

Most Missouri counties aren't testing mosquitoes for West Nile, state officials said, which makes experts worry.