Allergy & Food: I would try a single novel protein/novel carb source diet, including restricting all treats (can only be that protein/carb).
This needs to be something your dog has never eaten before, ideally both the meat/protein source and the carb(s). You feed this a long while and other monitoring (you and your vet), then slowly add back in one protein at a time. You see what makes the symptoms disappear in your dog.
Heartworm Preventative: There are only so many active ingredients effective in heartworm prevention. Heartguard would have the same ingredient as Iverhart, Ivermectin. Your other option is the main active ingredient in the formerly made Interceptor, which now only is available with other anti-parastitics in the formula.
You do need to use one of them, as heartworm is very deadly & brutal on a dog.
You can also have the main active ingredient compounded for your dog at a compounding pharmacy, in whatever form and flavoring/inactive ingredients (in case of severe allergy to what you’ve already tried) and exactly custom to the weight of your dog.
Dry skin & shampoo/conditioner — Please see my thread I already started on that topic here, asking for help.
