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.
Thank you. I appreciate the reply. I will keep him on Iverhart while watching for symptoms.