I'm sorry to hear of this with your Shih Tzu. If many vets haven't been able to diagnose her properly, please consider having a specialist veterinary dermatologist attend to her as can be found here: rvc.ac.uk/small-animal-referrals/dermatology
I'm going to post my synopsis of the pruritic (itchy) dog for you so you have a broad overview of all the conditions we need to consider in such a dog as yours. Please take your time perusing it and then return to our conversation with additional questions or concerns.
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 Shih Tzu's vet can check a sample of her 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 apply an effective over the counter flea spot-on such as Advantage/Advocate, a fipronil-containing product such as Frontline or one of the newer prescription products available from her vet even if fleas aren’t seen. 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 her 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 glucocorticoid 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 her 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.
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 her 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 her vet. There are many novel protein foods and a prototypical hydrolyzed protein food is Hill’s Prescription Diet z/d ultra. (I prefer the hydrolyzed protein diets 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 our patient has been eating the same food for quite some time.
We need to consider seborrhea in such a patient as well. This is skin disorder of keratinization and maturation. It's a diagnosis of exclusion of the above mentioned skin disorders and can be suggested by skin biopsy.
As mentioned above, you also have the option of having a specialist veterinary dermatologist attend to her. You can expect some combination of skin scrapings, cytology, bacterial culture and sensitivity, fungal culture, skin biopsy, intradermal or blood allergy testing, or presumptive hypoallergenic diet trials to be performed.
Please respond with further questions or concerns if you wish.