My vet uses interceptor/sentinel over hartgard (which uses ivermectin as the active ingredient to prevent heartworms) because the interceptor/sentinel will take care of far more intestinal parasites than the hartgard. If she's going to be using ivermectin alone as heartworm preventative (like the pour on or something) rather than an oral heartworm preventative tablet that has ivermectin as the active ingredient she needs to be very very careful. Border collies and cattle dogs are not as sensitive to ivermectin as some of the other types of collies, but they are still at a higher risk for having problems with it than, say, a labrador retriever. Even most ivermectin-sensitive breeds do find on products such as hartgard because the dose used to prevent heartworms is WAY lower than the dose used to treat other parasites and ivermectin toxicity is dependent on dosage (ie, the dose used to treat ear mites or roundworms may be enough to cause problems, but the dose used to prevent heartworm is not).
Just a side note, she may have a little bit of difficulty finding interceptor or sentinel. The Novartis plant that makes sentinel, interceptor, deramaxx, and clomicalm was shut down by the FDA earlier this year for some problems that they were experiencing on the human side of the production lines and have only just started back up. There weren't any issues with the veterinary products, but the entire plant was shut down and it was the only plant that produced these products. So there was a shortage of the veterinary drugs that the company produced and many of the products are still on backorder (with no solid date for when they will become available again).
