Please take your Shi-tzu to a veterinarian. There are numerous problems that can be causing skin issues. Allergies to over 100s of things, external parasites such as fleas and mites, and skin infections make up the most likely causes. To treat and control the problem correctly, she should be seen by a veterinarian.
There isn't much home care that can help until the underlying cause is found. Baths only work for about 10-30 minutes, so are not beneficial until the problem is known and can target the skin correctly. Baths may actually make problems worse pending the cause.
If you find fleas, please be aware that flea products not through your veterinarian work an average of 20%. They may work 100% the first day, but will not fix the issue. They may cause worse problems as they are the largest reason for EPA adverse reaction reports each year and it's rising. People have bought vet type products on-line and found that they didn't work either as they were tampered with and were not actually the correct product. Your veterinarian's product will work 100% guaranteed! No matter the product used, due to the lifecycle and common hibernation period that that cocoon stage goes through, your veterinary product will need to be used each month for 8 consecutive months on each pet.
If another problem is found as the culprit, of course the treatment is totally different.
You can do an accurate flea test at home when you buy one comfortis tablet from your veterinarian, then enclose your dog in the bathroom with a white sheet down for 4 hrs and come back and see dead fleas - should fleas be the culprit. If no fleas, then fleas are not the culprit - it's a test and treatment, as fleas are very very good at hiding.
The most common reasons for itchy skin in dogs include skin infections, microscopic skin parasites such as fleas or mites, food allergy, or pollen/dust allergies. Your veterinarian can take skin scrapings to examine under the microscope for parasites and for skin infection. If your dog is not already on a good monthly prescription flea preventative such as Frontline, Advantage or Revolution, then your veterinarian can prescribe an appropriate choice. The itch can be treated symptomatically with mild oatmeal shampoos, fatty acids, antihistamines or a short course of oral steroids, but steroids are not a good choice for long term treatment due to their many side effects. If your dog's itch persists or recurs despite parasite control and symptomatic medications, then I would suggest talking to your veterinarian about a prescription hypoallergenic diet. There is no accurate skin or blood test for food allergy; the test and the treatment are the strict hypoallergenic diet trial for 6-8 weeks with no other treats or foods. Lastly, if your dogs' symptoms persist despite all of the above, or if the symptoms only occur during certain seasons, then talk to your veterinarian about referral to a veterinary dermatologist for possible allergy skin testing and desensitization injections for pollen/dust allergies to identify and treat the underlying cause of the itch and recurrent infections, rather than just treating symptoms with medications (veterinary dermatologists can be found in your area by going to www.acvd.org and clicking on the "find a dermatologist" button). Hope that helps!
Kimberly Coyner, DVM
Diplomate, American College of Veterinary Dermatology
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