My old girl used to get thrush a lot, and someone once told me that once they get a good case of it, they're always susceptible. My vet had packed her frogs with penicillin soaked gauze when it got bad and didn't respond to other treatments. I've heard of people using bleach/water combo, but then I've heard of horses nearly going through the roof if the ratio wasn't right. Might want to ask you vet or maybe think about changing the bedding or checking the bedding to see if it's staying dry in the winter.