There are about 2 dozen things I can think of off the top of my head that can cause itching, hair loss, and scabbing on a pet.
Most vets will try the food trials first because it's one of the easiest, cheapest things to try and a common causes of allergies. For a food trial to be effective you have to feed the new food EXCLUSIVELY for 6-8 weeks. That means no treats, no people food, no goodies, no other foods during that time.
Flea bite allergy is another common cause of itching and another relatively easy/cheap one to eliminate. It can take as few as ONE flea bite to cause an allergic reaction that may take a week or more to recover from. You don't necessarily have to see the fleas to have a single one sneak on, have a bite, then hop off to lay eggs leaving behind an itchy pet.
Pets can also suffer from pollen and environmental allergies like people do which can cause alot of itching.
Dry skin can also cause a fair amount of itching.
In addition, regardless of original cause, constant scratching by an itchy pet can lead to skin infections that can make the whole situation worse.
I hope the dermatologist can help shed some light on things for you. Too bad they couldn't get you in sooner though.
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