The one we use is Ivomec. NOT THE PLUS....the regular Ivomec.Our vet gave us the equine form of liquid ivermectin, to give to our newfs. It's called EQVALAN. One 100 mL bottle cost under $100, and contains enough product to treat all three newfs for about three years. Quite a savings over the usual heart worm preventative.
Once the dogs had the blood tests to be sure they're clear of heartworms, we started giving them one mL a month each. Works great! Ask your vet about it.