Hello Customer (name blocked for privacy),

There is some danger in giving heartworm preventative to heartworm positive dogs in that if there are lots of microfilaria (baby worms) in the bloodstream they can cause reactions and blockages as they die. For this reason heartworm preventative is usually not recommended for dogs with heavy worm burdens and microfilaria. However, in dogs with a low worm burden and few or no microfilaria - heartworm preventative is often recommended. It is getting more and more popular to just put dogs with not too many worms and no symptoms on heartworm preventative year-round for a couple of years and just wait for the adult worms to die. This usually has very good results and few complications. In dogs with lots of microfilaria, though, heartworm preventative can actually be fatal. So, to sum up, I would work with your veterinarian to decide the best course of action for your dog. This may include treatment or just heartworm preventative and periodic testing.

I hope that this helps.

Donna P.

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thank you.