There are no holistic treatments to prevent heartworms. I have had many pups and have never had any issues with Heartgard or Sentinel....those are the ones I use and have been for many years. it is highly unlikely your pup is having issues due to any heartworm prevention....you need to look at other possible causes such as food and/or treats.....or chews.
Here is a link that will give you the information you need regarding the risk of heartworms...definitely not a good idea to leave your pup unprotected. The treatment if they contract heartworms is horrible and expensive....and it can kill them.
https://www.heartwormsociety.org/
It only takes one mosquito bite.
