I haven't tested my dogs. They haven't had any preventatives this year either. I think I may test them (just to be 100% sure) as soon as I save up some extra money. HW isn't a huge concern here though. I heard from a holistic vet that there are only 2-3 reported cases in the Okanagan alone. A dog is more likely to react badly to HW preventative than get HW (in Vancouver).