Severe itching in a pet is typically due to one of several things: Fleas, Food Allergies, Inhalant Allergies or infectious disease due to mites, fungal organisms or bacteria.
You could be dealing with several things so I will briefly mention each of them.
If your pet goes to the grooming shop, park, or doggie daycare alot, you will always need to be on the lookout for Ringworm or Dermatophytosis. Ringworm can cause small multi focal lesions of crust and scale with hair loss and minimal itching. Ringworm is a fungal organism and it is present in the spore form everywhere. It is in highest concentrations where multiple carriers shed their hair...so grooming shops, breeders homes, pet shops, vet offices, etc..... Diagnosis of ringworm requires that two tests be done. First a simple Woods Light exam by your vet will fluoresce 50 % of active Microsporum Canis cases a bright green color. If this test is negative, it means nothing and you go on to the second step! Next, several of the affected hair shafts along the perimeter of an active lesion are collected and placed in a culture medium vial. If ringworm or other fungal organisms are present they will start to grow within a few days. It can take up to 14 days for a positive culture, but this is typically the diagnostic tool of choice (DTM FUNGASSAY). In the interim, I always go ahead and start treating for ringworm due to it's unsightly appearance and CONTAGIOUS potential to humans and other pets. Treatment involves shampoo therapy, antifungals, and topical sprays and ointments.
If Ringworm is not the culprit , than my next suspicion would be a staph infection or bacterial infection of some nature of the skin. Usually staph is quite itchy, but your pet just may not be that sensitive to the toxins YET. To diagnose staph requires visualization of the pimple like pustules commonly seen on physical exam and also the epidermal collarettes that form after the pustules rupture. A sterile culture from the skin can be taken and cultured to confirm this pathogen. Your vet will likely put your pet on an antibiotic like Clavamox or Cephalexin for an initial period of 3 weeks. Rechecks are necessary every 3 weeks to assess the state of disease. Antibiotics cannot be discontinued until TWO WEEKS after a clinical cure!! Shampoo therapy with a >2% Chlorhexiderm product is advised 2 x week for a 15 minute lather each time. I also like to put my patients on antihistamines, too, b/c I do feel they are itchy!! Staph is not contagious to other pets or people!!
Bacterial skin infections can be extremely itchy and uncomfortable for your pet. Staph dermatitis usually presents as tiny pimple like "pustules" on the skin. They have an increased occurence on the abdomen, thighs, and perigenital regions of a pet's body. After the pustules come to a "head", they rupture and can form lesions called epidermal collarettes that can easily be identified on the skin.
Food allergies are a biggee on Shihtzus. I have to admit I actually cringed when you mentioned both Kibbles N Bits and Pupparoni's because these are two of the items on my top ten list of THINGS NOT TO FEED YOUR PET!! Why? Because they are full of fat,sugar, preservatives, and an assortment of dyes and colorings that can cause many different problems for your pet. When a pet develops food allergies, it is typically to a protein, dye or preservative in it's pet food. Allergies take time to develop, so it does not matter that your pet has been on this diet for months or years....this actually supports the diagnosis of food allergy in many cases. Once the body identifies a foreign substance as an "invader" than the body will respond by forming an inflammatory reaction against the foreign food substance and try to eliminate it from the system. Food allergen particles can stay in the bloodstream for up to 30 days after ingestion, so your pet is likely continually itchy and reactive if food allergies are the culprit.
For even more info, please check out this site: http://www.europeananimalwelfare.com/Canine_Allergies.html
Allergic dermatitis is practically always itchy and uncomfortable. Allergies to shampoos, new foods or treats (and old ones!), inhalant allergens, and contact allergies can all manifest in several areas of the body. Treatment of choice after ruling out fungal/yeast/bacterial infections is corticosteroids and antihistamines...+/- antibiotics.
Lastly, a mite infection with either scabies or demodex mites could be present also. Demodex and scabies like to be around the face, ears, and muzzle, so I would definitely expect your vet to do a proper skin scrape to be sure one of these two parasites isn't at hand. If a scrape is negative for scabies ( they are extremely difficult to get on a sample), I often treat for scabies anyway with Revolution once a month, just to be sure!!
An underactive thyroid is common in dogs older than 7 years of age. Typically, pets with HT are overweight, are heavy despite not getting fed much, shed excessively, have poor haircoat, have alot of dandruff, have areas of patchy hair loss and increased pigmentation, experience increased thirst & increased urinations. Hypothyroid is not typically itchy at all....BUT having an allergic component mixed in with HT could be the problem!
A blood test to measure both the Thyroid stimulating Hormone and Free thyroid hormone in your pet's bloodstream is available. Turnaround time on test results is 2-4 business days. Treatment is simple....you give a hormone supplement every 12 hours for the life of your pet. Thyroid meds are relatively inexpensive and safe with twice a year blood monitoring.
So there you have it....there are many possibilities!
I believe what YOU need to do to save your pet discomfort and to save time, is to try to stop treating your pet yourself, and actually take your pet into the vet for a proper veterinary examination. Odds are , your pet may need antibiotics, antiinflammatories, antihistamines, and a specially formulated medicated shampoo.
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Best wishes to your and your pet, Dr. Smith