Not heartworm no, it was probably a clot, and no theres nothing you can feed, the only way to prevent them is to discover them early and put your dog on blood thinners, but if you do this then any scratch could become as life threatening as the clot itself. This is a horrible thing to have had happen and I'm very sorry, its a tramatising event to have to have gone through, and my thoughts are with you in this horrible time.
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