There is no documented resistance to Ivermectin. However, my vet said
that he saw foals who were wormed from birth with Ivermectin expel large
numbers of some visable worms when wormed with something else... like
strongid paste I believe. Ever since then he has recommended rotating
in Strongid 2x per year... once double and once single tube.
This is an anectdote only and not a researched occurance.
:) - kat myers
in San Mateo (No.) Ca. with Magnum the TB ex-racer