The most common cause for this in any itchy dog is "salivary staining". If the dog licks repeatedly (and he may been doing it when you are not home), the saliva will stain the fur.
There is no remedy to remove the stain other than to stop the licking.
At this time of year, treat the dog for fleas with Frontline or Advantage, give 1/2 a Benadryl twice a day for itchiness, and talk to your vet about allergies. VERY common in Shih Tzus....
Dr. T
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