I'd mostly be concerned about tapeworm, which infects dogs when the dog eats an infected flea, because moles can definitely have fleas. Flea preventative might help some, but it's far from 100% since preventatives tend to kill fleas after they're already on the dog, not repel them. My poodle got tapeworm while on a flea and tick treatment and heartworm preventative years ago, despite never having had a flea infestation in his life, probably from the dog park I was going to at the time (one of the reasons I've stopped using them).
Definitely ask your vet about the risks. Certain heartworm preventatives may also include a more broad-spectrum dewormer (but not all do), or you may already be deworming monthly with something that covers the biggest risks. I imagine hunting moles is enormously fun and rewarding for dogs, so the only real long-term solution might be looking into how to better control them on your property in an eco-friendly, dog-friendly way.