Itchy Pitt

[demoura]

hello
First I like to say that this site is wonderful!
Me and my Pitt bull Huslter need some help, he is 1 year and 2 months old and since the age of 5 months began developing some icthiness which turned down the road to worse, so I took him to the Vet to have it checked out and they did skin scrapping to check under the mircoscope to see if it was parasites and nothing, they also removed skin tissue to have it examined by a lab and nothing so they said I would have to live with this and play it by year.
Since the first visit to the vet he has been on medication on and off such as Novolexin , Vanectyl ,which seems to control some of the icthiness but not put an end to it, and Bayer Advantage flea adulticide and, special diet food.
His living space is washed twice daily, but despite alll of this he still continues with this problem, he has lost 50% of the fur on his head.
So my question is, Does anyone know what it is and how to treat it.
Thank You
Robert