I wormed Siggy last month with the normal Ivermectin - the vet has me worming him every three months, but I have not rotated wormers. I've just been using Ivermectin because it's worked (we did a stool "study" last October and showed that the wormers were working).
Anyway...he keeps rubbing his butt against the fence rail (not something he normally does), so I wonder if the Ivermectin is no longer effective. Can I worm again, now? What should I use this time?

