Yikes!
While my pure-bread Ausie black-tri was alive, I had him on IverHart Max (ivermectin / pyrantel pamoate / praziquantel). Never had a problem while he was alive (lived to 17!).
I have since adopted another dog that had been picked up as a stray, so the lineage is unknown. The adoption counselor thought he might be a Catahoula Leopard Dog, or a mix. He has the blue merle gene. I’ve had him on Iverhard Max as well this summer. So far, no problem, but with an unknown breed, which obviously shares a gene with other breeds which are sensitive to ivermectin, I’m now concerned.
Should I proceed with a MDR1 test?
Here’s some pictures of him:
https://secure.flickr.com/photos/michael-litscher/sets/72157622814251321/
I think you are fine to use Iverhart for your Catahoula. I have two clients with Catahoulas who have their dogs on Iverhart. One of them has been on it for almost a decade without problems. If your dog were going to have a problem with the ivermectin it would have most likely happened with the first dose. Also, the dose of ivermectin in the packaged heartworm preventives is unlikely to be enough to cause a problem. But if it’s going to worry you, yeah, you can do the test.