It really isn't an issue coming back to Canada - I have to answer yes each and every time I go to Europe. Depending on the person they ask me a) what sort of farm, what type of animals (they tend to care far more about cows etc. than horses - types of diseases) b) whether I have mud on my boots / clothing (I clean everything before travel, so the answer is no).

During the foot and mouth crisis in the UK I was required to walk over sanitizing mats - given the fact I was wearing spotless ballet flats that had never walked around a farm it was a little absurd (especially considering the riding boots in my bag were left untouched) but as I knew I hadn't been on a livestock farm (just crops and horses) it wasn't an issue to me.