And for fleas too (topically applied). We have Guinea Fowl, which, as the story goes, help keep down flea and tick populations: we aren't cursed with many ticks like elsewhere, though I've run across a couple (in six years).
According to
this article
Myth 9: There's No Effective Natural Prevention
According to Dr. Gerald Wessner of the Holistic Veterinary Clinic in Summerfield, FL, holistic pet parents do have an alternative to traditional heartworm preventive drugs. He has documented success over an 8-year period using heartworm nosodes (a homeopathic vaccine) in conjunction with Paratox (a multi-remedy of homeopathics) and including diatomaceous earth in pets' food.
We only do the diatomaceous earth. Either it is working on its own or we've just been lucky. I just struggle with the thought of injecting various crap into animals: I'm not against it, just that I think sometimes nature has to prevail on its own.