Oh my. I better provide you with my entire synopsis of the itchy dog.
Pruritic (itchy) dogs are suffering from 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. (Anus, paw and extremity licking indicate both atopy and 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.
Izzy's vet can check a sample of representative 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) and yeast is addressed with ketoconazole for at least a month. Mites are addressed topically, orally, and by injection.
Our dermatologists tell us to apply an over the counter flea spot-on such as Advantage/Advocate, a fipronil-containing product such as Frontline or, preferably, one of the newer prescription products available from Izzy's vet to which fleas are less likely to have become resistant 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 at this time of year, egg hatches are common. If the area between the edge of his rib cage and tail (the “saddle” area) is particularly excoriated, a flea saliva allergy should be an important differential diagnosis. In severe cases, an anti-allergenic prescription glucocorticoid (steroid) 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. Be sure to treat your premises with a product such as over the counter Siphotrol Area Treatment spray which contains the insect growth regulator (IGR) methoprene which doesn't allow flea eggs and larvae to develop into adult fleas thus breaking up the life cycle of the flea. Avoid inefficient foggers that can't reach under furniture or into cracks and crevices where flea eggs and larvae are found.
Environmental allergies (atopy) are usually initially addressed with prednisone as well. In some dogs an over the counter antihistamine such as chlorpheniramine (many generics) at a dose of 0.1 - 0.25 mg/lb (maximum) every 8-12 hours or diphenhydramine (Benadryl) dosed at 1-2 mg/lb every 8-12 hours (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 anti-inflammatory but may take 8-12 weeks to kick in. The cytokine antagonist oclacitinib (Apoquel) is likely to revolutionize how we address atopic dogs and should be discussed with Izzy's vet. Oclacitinib works as well as a steroid but without a steroid's adverse effects. The injectable Cytopoint is another good immune mediator to consider, particularly if Apoquel may be causing adverse effects.
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 Izzy's immune system doesn't "see" anything to be allergic to. 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 his vet. There are many novel protein foods and a prototypical hydrolyzed protein food is Hill’s Prescription Diet z/d (I prefer the hydrolyzed protein diets because they lessen 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 had been eating the same food for quite some time.
We need to consider seborrhea in such a patient as well. This is a 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.
You also have the option of having a specialist veterinary dermatologist (please see here: https://www.rvc.ac.uk/small-animal-vet/specialist-referrals/clinical-services/veterinary-dermatology) attend to Izzy. 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.