I havent heard of a natural heart worm preventative but would find one interesting. The first thing the vet and I talked about today was heart worm....my appointment was late and she had worked all day so she shut the door and sat down to relax a bit....she was telling me how she is having a bad day and diagnosed 2 heartworms cases today!!! She seemed concerned and wanted to make sure i was giving heartworm meds. Which I am. Diggy is on revolution and Bella interceptor she seemed pleased. SHe got out the model heart and showed the effects of heartworm....wow I never knew what they did to the heart and she explained they are treatable usually over the course of 4-5 years!!!
And the damage they do is irreversable.
I was glad she explained it all to me and I will NEVER go without heartworm protection....scared me!