I"m sorry to hear of this with your Shih Tzu. Food intolerance/allergy is addressed with prescription hypoallergenic diets. These special foods contain just one novel (rabbit, duck, e.g.) animal protein or proteins that have been chemically altered (hydrolyzed) to the point that your Shi Tzu's immune system doesn't "see" anything to be allergic to. The over the counter hypoallergenic foods too often contain proteins not listed on the label - soy is a common one - and these proteins would confound our evaluation of the efficacy of the hypoallergenic diet. The prescription foods are available from your dog's vet. There are many novel protein foods and a prototypical hydrolyzed protein food is Hill’s Prescription Diet z/d ultra (a hydrolyzed protein diet is my preference because it avoids the possibility of my patient being intolerant to even a novel protein). A positive response is usually seen within a few weeks if we’ve eliminated the offending food allergen. Food intolerance can arise at any age and even after my patient has been eating the same food for quite some time.
There are other considerations in addition to food intolerance. I'll post a synopsis of them for you here...
Pruritic (itchy) dogs are suffering from an allergic dermatitis in the great majority of cases. Allergies to flea saliva, environmental allergens (atopic dermatitis) such as pollens, molds, dust and dust mites, and foods should be considered. (Paw and extremity licking indicates both atopy and a food intolerance and so it behooves vets to distinguish one from another.) In many instances, a concomitant pyoderma (bacterial skin infection), yeast infection (Malassezia), or mange mite (Demodex or Sarcoptes) might be contributory.
Your vet vet can check a sample of your dog's skin surface microscopically (a “cytology”) for abnormal numbers of bacteria and yeast and skin scrapings can be taken in an attempt to find mites. Pyoderma is treated with a minimum of 3-4 weeks of an antibiotic in the cephalosporin class such as cephalexin (Keflex) plus antimicrobial shampoos containing either chlorhexidine or benzoyl peroxide and yeast is addressed with ketoconazole plus shampoos containing either ketoconazole, miconazole, or clotrimazole for at least a month.
Our dermatologists tell us to provide one of the newer prescription products available from your vet even if fleas aren’t seen. Over the counter products containing imidocloprid (Advantage, e.g.) or fipronil (Frontline, e.g.) may be ineffective because many populations of fleas have developed resistance to those chemicals. Consider products containing a different class of insecticide such as Bravecto, NexGard, Comfortis, and Vectra. New prescription products are becoming available all of the time. Dogs can be such effective groomers so as to eliminate all evidence of flea infestation. Dogs who remain primarily indoors can contract fleas because we walk them in on us and flea eggs and larva can remain viable in your home for months. As the weather warms or you turn on heaters at this time of year, egg hatches are common. If the area between the edge of your dog's rib cage and tail (the “saddle” area) is particularly excoriated, a flea saliva allergy should be the most important differential diagnosis. In severe cases, an anti-allergenic prescription glucocorticosteroidid such as prednisone will work wonders for dogs allergic to the saliva of the flea. If you have other pets they may have fleas too but may not be allergic to the flea’s saliva.
Environmental allergies (atopy) are usually initially addressed with prednisone as well. In some dogs an over the counter antihistamine such as clemastine (Tavist) at a dose of 0.025 - 0.75mg/lb twice daily or diphenhydramine (Benadryl) dosed at 1-2mg/lb twice daily (maximum dose of 50 mg at any one time) may be effective. Antihistamines, however, aren’t reliably effective. Adding fish oil to the diet at a dose of 20mg/lb daily of the EPA in the fish oil might synergize with antihistamines to provide better anti-pruritic action. The omega-3 fatty acids in fish oil are antiinflammatory but may take 8-12 weeks to kick in. The new cytokine antagonist oclacitinib (Apoquel) is likely to revolutionize how we address atopic dogs and should be discussed with your dog's vet. Oclacitinib works as well as a steroid without a steroid's adverse effects. Please note that atopy, at least initially, should have a seasonality to it while a food intolerance should cause pruritis regardless of the season. Chronically atopic dogs may be pruritic year round.
Please respond with further questions or concerns if you wish.