My 3 1/2 Golden Retriever began having difficulty breathing. The vet did a nasal cavity search looking for mast cell tumors; none was found and he placed her on predisone 20mg. daily. Two weeks later, the breathing was more labored and she sounded like she was gargling/growling. I took her back in and the vet found mass growth around the face, right nasal cavity and muscle loss around the right side of the face. The vet took 3 biopsies for mast cell cancer and they came back negative although some yeast had been discoverd. She was placed on ketaconizole and an antibiotic. Within a week, she began walking in circles and seems to be unable to "see" from the left side. She only walks in cirlces to the right. The vet stated he thought she had cancer and that it had developed into the brain. But, her biobsy was negative and another vet suggested possible Histoplasmosis. What do you think should be done as the biobsies are negative? What other tests should she have?