black scaly rash on my dog

Question
My 6 year old havanese (Abby)has a rash on her head and tail for about two years.  It first appeared in a circle about the size of a quarter. It is very crusted and when you pick it off, the skin is very red and irritated with some hair loss. My vet has examined it several times and done testing and is stumped.  She was on and antobiotic for several weeks and it seemed to get better.  But when we stopped the meds, it came back.  She was also prescribed a steroid medication which also helped but did not make it go away totally,  After that, I changed her diet to lamb and rice and did not give her any poultry because I heard that it could be related to the chicken.  After a couple of months, the black rash on her head was gone.  She had only a little spot on her tail.  About two months, the whole thing came back with a vengeance.  

In addition to this she has a thick gunmy substance that builds in her ears.  It can come out of the ear into the hair and around the perimeter of the whole ear.Need less to say, I have to clean her ears constantly.  The vet does not why she gets this either.  She is now taking Zyrtec to see if it is allergy related.  

I would love to know if you have any thoughts on this as it is driving us crazy not to mention making my beautiful white dog look very funny.  I should mention that none of the rashes seem to bother Abby.

Answer
Your dog is most likely allergic.  Zyrtec will do nothing for allergic skin in the dog.  Dogs really have little histamine in their skin. This dog needs to be on a steroid medication and long term antibiotics for the pyoderma you are describing.  Cephalexin is the choice of antibiotics for skin disease in the dog.  The ears need to be cleaned out and depending on what type of infection, your vet will need to prescribe the right medication. Ear disease is almost always associated with allergic skin disease in the dogs.