Denis S,
Any chance that your dog was diagnosed as having perianal fustula? My friends shepherd had this many many years ago and at that time it had just been identified that adverse food reaction could be a component of the problem. Her dog was placed on a vet therapeutic diet along with medication and eventually was maintained on diet alone.
Here is some information on that condition https://www.ncbi.nlm.nih.gov/pubmed/20536692
https://veterinarypartner.vin.com/default.aspx?pid=19239&id=4952922
Only the vet therapeutic diets are specifically made for treating adverse food reactions as OTC foods often have proteins in them that are not on the ingredient list. Home cooking is a alternative using items your dog hasn’t eaten in the past. Ask your vet for advice.
