St. Paul, MN (KROC-AM News) - The source of the highly virulent bird flu that hit poultry farmers in Minnesota and other Midwestern states this spring remains a mystery.

The State Department of Natural Resources says researchers have collected and tested over 3,300 samples from wild birds, and have only found one positive for the H5N2 virus strain that has resulted in the deaths of over 9 million turkeys and chickens in Minnesota and tens of millions of chickens in Iowa and other states. The positive test result was from a Cooper’s hawk in Yellow Medicine County in late April.

The testing has found less virulent strains of avian flu in about 3-percent of the wild bird samples, but DNR wildlife research manager Lou Cornicelli says that is not surprising because waterfowl serve as reservoirs for avian influenza. He says what is not known is what role wildlife may have played in the recent outbreaks of the deadly bird flu strain in commercial poultry flocks.

Researchers are planning to conduct expanded surveillance through the summer and fall by testing ducks and geese in an effort to learn how the H5N2 virus was introduced and spread across a wide area of southern and western Minnesota.

At last report, state officials had confirmed 108 bird flu outbreaks in 23 Minnesota counties.