Cleanse the forest of 20 Rabid Thistle Bears and return to Tharnariun Treetender in Darkshore.
As I had feared, the cure does not work.
It saddens me greatly to order the death of any of nature's beasts, but the sick and tainted thistle bears must be put down.
Return to the forest and destroy 20 rabid thistle bears, [player]. It will most certainly not put a stop to the plague but it will slow down the damage the animals are doing to our forest and its inhabitants for a short time.