Samples have been sent for testing.
Up to 200 cows have mysteriously been found dead in Wisconsin in the latest mass animal death.
Dozens of birds were found dead in Romania last week, 300 grackles dropped from the sky in Alabama and thousands of fish have been discovered dead in Maryland.
However, veterinarians believe the cows were wiped out by an infectious disease - although the exact cause remains unknown and samples were being sent to laboratories for testing.
The owner of the cattle found his animals dead in Stockton, WI, and immediately alerted police.
It was thought they may have had a respiratory condition known as red nose - infectious bovine rhinotracheitis - or Bovine Virus Diarrhea (BVD).
Police said there is no risk of any illness spreading to other animals or humans after the farmer made the grim discovery on Friday.
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