Dr. Orla Keane:

That very much depends on the parasite species. Within the stomach and gut worms, it is quite widespread now. Some farms have resistance to all of the three active items. Many farms have ivermectin resistance.

In fluke, it is limited to the triclabendazoles and in some other species, such as nematodirus, there have been no reported cases of resistance. There have been a few in lice. The level of resistance depends on the parasite species and the drug involved.