Whirling disease has been verified in trout in the Watauga River in Watauga County North Carolina. While the disease apparently will not harm people or pets, in other parts of the country it has had significant impact to other trout fisheries by killing off up to 90% of the trout in some areas. It is caused by the Myxobolus cerebralis parasite which causes neurological damage to the trout or salmon it infects causing it to ‘whirl’ or swim in a circle. The spread of the disease is cause when an infected fish dies and spores are released into the water to infect other potential hosts.
More information can be found at the NC DNR website >here<
Here is a video I found of the fish behavior: