South Dakota is headed toward its lowest summer count of human West Nile virus cases since the disease was found in the state seven years ago.

Only 17 human cases have been reported this summer and the threat is over because a freeze has killed mosquitoes that transmit the disease.

There's a lag of a few weeks before cases are reported so the total could increase by one or two, but Lon Kightlinger in the state Health Department is certain it will be the lowest total yet.

The West Nile count has ranged from 37 human cases in 2002 to 1,039 in 2003.

Researchers think this year's low number might be linked to cooler temperatures in spring and early summer.