VAN HORNE, Iowa (AP) -- Some lettuce recalled over fears of listeria contamination may have been served in the Benton County school district in eastern Iowa.

The Waterloo-Cedar Falls Courier reports school officials told parents and others about the possibility on Wednesday. The district learned about the problem from its food vendor and the Black Hawk County health department.

California lettuce grower, River Ranch Fresh Foods, on Monday expanded a voluntary recall nationwide. No illnesses have been reported. Symptoms of listeria infection include high fever, headache and neck stiffness.

Benton County school officials say the products could have been served between May 7th and May 18th. The district includes Atkins, Blairstown, Elberon, Keystone, Newhall, Norway, Van Horne and Watkins.