Matt Mitro of the Wisconsin Department Of Natural Resources "Trout Scientist" informed me that there has been many reports of Gill Lice on brook trout in South West Wisconsin. They are parasites that attach to brook trout and rainbows gills. They will not kill trout by themselves but in conjunction with another stressful event they can cause death to brook trout. They are NOT harmful to humans. Matt suggests a 1 percent bleach solution to your gear if you are changing streams or at the end of the day's outing. Some streams in Wisconsin have already seen a significant decrease in their brook trout population because of it.