Pigment loss?

Hi, My 18 months old patterdale has lost pigment on one side of her mouth, she's been checked over by vet her gums are fine,  vet not said what it is and just said to keep an eye on it. My girl is fine eating/drinking full of energy. I want to know why she's lost pigment could this be an allergy thing? Has anyone else experience this with their dog? I've uploaded a photo of her below. Be grateful for any answers or advice. Thanks

Hello!   This looks unusual.  I wonder whether your vet thinks that Vitiligo might be a possibility? - although patterdales are not the first breed that I would associate with it.  I have never seen a case, but it involves the destruction of Melanocytes (the pigmented skin cells) without skin inflammation.  It may need a biopsy before it can be diagnosed.  Vitiligo may come about following a change in the immune system, so sometimes periods of stress or illness may set it off;  vets often look for concurrent problems.  There is also a syndrome (collection of symptoms occurring together) called Uveodermatologic syndrome, where skin depigmentation may occur with eye changes, so it may be worth having the eyes examined with a scope.  Other possibilities include some unpleasant causes of skin depigmentation such as lymphoma, although often these cases are less tidy looking.  As a vet, I always find it useful to chat to the pathologist at the time of taking a biopsy because they are so knowledgeable.  A biopsy may help to turn speculation into a more definite answer.  Best of luck - and please do let us know how you get on.