There is quite a lot of acquired resistance to ivermectin in horse strongyles. Just not in S. vulgaris that was causing infarction. Now, the recommendations f

There is quite a lot of acquired resistance to ivermectin in horse strongyles. Just not in S. vulgaris that was causing infarction. Now, the recommendations for horses are changing to a quantitatively driven approach based on fecal egg count rather than routine annual deworming. Pasture rotation is also useful. In sheep, the barber-pole worm Haemonchus has become resistant to most approved parasiticides. Resistance is a definite problem.