For a collie, the dose of ivermectin that you're concerned about causing problems in one that has the MDR mutation is 100 mcg/kg and over (it actually takes more so this is conservative). The dose in Heartgard is around 6 mcg/kg. So collies are "sensitive" to ivermectin when given at a high dose but are not sensitive to Heartgard when given in the usual prophylactic dose.