Aerial spraying in 14 counties to combat eastern equine encephalitis (EEE) was delayed Sunday due to weekend rainfall. The spraying was supposed to begin Sunday, but the Michigan Department of Health and Human Services said inclement weather had forced the plans to be postponed. The agency said it would begin spraying Monday night, depending on weather.

The operation will be the first since 1989,  after the rare mosquito-borne virus has killed three people in Michigan and has been detected across the southern half of the state. The aerial spraying will include 14 counties where EEE has been confirmed in humans or animals. The counties to be targeted are Allegan, Barry, Berrien, Branch, Calhoun, Cass, Jackson, Kalamazoo, Kent, Lapeer, Montcalm, Newaygo, St. Joseph and Van Buren.

Aerial spraying has recently been done in several states, including Massachusetts and Rhode Island. Michigan is encouraging officials in affected counties to consider postponing or rescheduling evening outdoor events until there’s a hard frost.

The number of U.S. deaths and illnesses from EEE have been higher than usual this year.