The Indiana Department of Natural Resources said most Indiana counties could resume feeding birds after investigators narrowed the investigation of a mysterious illness affecting songbirds.

Since May, DNR investigators have attempted to find the source of the illness, which was found to give birds neurological problems, eye swelling and crusty discharge around the eyes.

DNR investigators confirmed over 500 possible cases in 72 counties, but data showed the illness consistently affected specific areas — Allen, Carroll, Hamilton, Hancock, Hendricks, Johnson, Lake, Marion, Monroe, Porter, St. Joseph, Tippecanoe and Whitley counties.

The agency said Hoosiers from all but those 13 counties could resume putting up bird feeders, but recommends seed and suet feeders be cleaned at least once every two weeks.

It recommends scrubbing feeders with soap and water, followed by a short soak in a 10% bleach solution to keep birds healthy and limit the spread of the disease. Hummingbird feeders should be cleaned at least once a week with a 10% bleach solution and rinsed thoroughly.

DNR and federal agencies have not yet found the source of the illness, but all birds have tested negative for avian influenza, West Nile virus and several other viruses and parasites.

Hoosiers can report sick or dying birds at DNR’s wildlife reporting tool.