You REALLY need to get an accurate diagnosis.
"Contagious" mange equals sarcoptic mange and is not only contagious to other animals but to HUMANS.
Demodectic mange isn't "contagious" per say but could have come from the mother (you mentioned teething so I suspect she is a pup). The shelter *may*have been saying "contagious" in that respect. Just a thought.
Hard to believe that one vet offered a diagnosis of CANCER with no further means of treatment. What type of cancer?
You say losing roughly 90% of her hair. This would mean that she would be nearly BALD all over??
Any allergy symptoms? Itching, scratching, licking, discharge from eyes, nose?
Again, assuming she is a pup......you have tried far too many food changes and add-ins in a very short period of time. She is more than likely on her way to training you that she will hold out for something more tasty.
Is she gaining weight? Losing?
If she is losing weight then she isn't getting enough calories. If she is gaining/maintaining then she IS eating enough, perhaps just not what you think she should be eating.
How is her health aside from the hair loss?
Stools normal? Energy levels normal?
The shelter was treating her for sarcoptic mange with medicated rinses and antibiotics. The first vet said demodectic mange, no known origin. Gave her antibiotics first, and when that did nothing, she started getting weekly injections of stronger antibiotics. Second vet said food allergies, possibly corn and wheat, but said nothing about changing her food. Third vet said cancer. I was out of town for both the second and third vet visits, so I'm just going by what my bf told me.
Layla is roughly 2 years old. The shelter got her as a voluntary owner relinquish, and the owner gave only partial records on her. She has a 5-6 inch scar on her tummy, but nobody seems to know if she's been spayed, or if the scar was due to surgery as a puppy. She was estimated to be 7 months old when my bf adopted her, and she had the scar then.
She seems to be stable as far as weight loss or gain. She has watery/slimy discharge from her eyes, and a vet said it was normal for Boxers. Her stools are normal, urine output is normal, she's VERY active, but seems itchy because she wiggles and rubs on the carpet when laying on her back. The thickest patches of fur she has is between her shoulderblades and on the top of her head. Her skin is pink.
According to the dog food bag, we're supposed to feed her twice per day, and give her 5 3/4 cups of food for 90 pounds of body weight, and add an additional 1/2 cup per 15 pounds. If that's the case, with her being about 153 pounds, we would have to feed her 7 3/4 cups of food twice per day. That's a LOT of food...it would completely fill her bowl!
We soaked her food in hot water until it was softened this morning, but she didn't bother with eating it until this evening. We just don't know what else to do with her. My bf doesn't get paid until the beginning of the month, and he's decided to try to find a vet nearby who will work on a payment plan so we can find out what's going on with her. She does also have what looks like a tumor on the edge of her left ear, so we're getting that checked out too. I would think if Layla DOES have cancer, she would be on some sort of treatment for it, but she's not.