Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your poor rescue German Shepherd is struggling with painful perianal fistulas.
Holistically means to take into account the entire patient when we treat a condition, and certainly that is important to do in a dog with perianal fistulas as often one modality of treatment, at least to get them under control initially, is often unsuccessful.
I suspect that you may have been inquiring whether it is possible to treat them in a naturopathic form, using diets and supplements from nature rather than drugs. In my experience naturopathy alone isn't enough. We are dealing with a severe immune system reaction that needs to be suppressed. After getting things under control it may be possible to use diet and supplements, and greatly reduce drug use. But for now we need aggressive treatment for an aggressive, painful disease process.
Of course the ideal situation is to get the fistula resolved and either wean him off drugs completely or at least get to a very low dose. In some dogs that is extremely difficult to do. I find in many cases it is helpful to use antibiotics too, as secondary bacterial infections in raw tissue around the anus are extremely common and will complicate the treatment. Possible antibiotics we can use include Cephalexin, Clavamox, and Metronidazole.
I also find that for some dogs rather than a single immunosuppressive drug like prednisolone or cyclosporine they may need a combination, such as ketoconazole and cyclosporine, or prednisone and cyclosporine orally. Or an oral immunosuppressive and/or topical cyclosporine or tacrolimus (protopic) as well as keeping the area clean with a gentle antibacterial cleaner like chlorhexidene solution or shampoo. Another immunosuppressive protocol involves prednisolone, azathioprine, and metronidazole. Antibiotics like metronidazole, or the others I mentioned above are to treat any secondary infections, so your veterinarian may wish to add one of those to his cyclosporine protocol now to clear his symptoms. Perhaps then you can try topical tacrolimus along with oral cyclosporine and an antibiotic.
He can also be fed a true hypoallergenic diet trial as food allergies may be a predisposing factor. This is where naturopathic treatment can be helpful. Grain free diets are probably not restrictive enough to control the disease process, I recommend Hills z/d or Purina Veterinary diets HA.
I would also check his anal glands to make sure that they are not infected, which can complicate treatment. As painful as he likely is now that will likely require sedation. At that point I would infuse his anal glands with an ointment such as Panalog, clip the hair and clean the area thoroughly and treat the area topically.
I understand that all of this will be expensive in a big dog. But when fistulas get out of control and severe we need aggressive therapy to get them well again, and then we can start to back off on the number of drugs and the number of times we need to administer them. This is not an easy or inexpensive disease process to treat. But these dogs are truly suffering, so they deserve that sort of treatment. Sometimes even with aggressive treatment the fistulas don't resolve or continue to come back. Perhaps that is why your veterinarian is recommending euthanasia now. But if you can use multimodal therapy it is worth trying.
Here is a link to an article to read more about perianal fistulas in dogs if you are interested in reading more: http://www.vcahospitals.com/main/pet-health-information/article/animal-health/perianal-fistula-in-dogs/1054
Please let me know if you have any further questions.