My daughter’s mare has had a tail rubbing issue which was resolved by daily washing with generic “Listerine” type mouthwash.
Kathy
It may have nothing to do with Ivermectin or worming. My vet explained
that some mares are prone to build up "crud" between their legs and
udders made up of sweat, dirt, and microscopic critters that thrive
in such an environment. It itches and so they rub their tails. It is also
why they don't rub their tails in the winter.
Weekly washing with warm water and mild soap, then rinsing well
went a long way toward clearing up the problem. Not totally, but
a *lot* better.
Linda Marins