Panacur (fenbendazole) does not treat nor kill external parasites, it'll kill internal parasites, a good wormer. The goat wormer is given orally.I use 1 ml of panacur goat wormer externally, on the neck to control internal and external parasites. And yes chickens will eventually secumb to parasites. Knocks their immune system for a 6. " an ounce of prevention , is worth a pound of cure ".