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Sometimes fleas can be hard to see so I would first want to ensure Bella is on an effective monthly flea prevention and run a flea comb through her coat to try and more accurately determine if fleas exist or not. This type of comb has fine close together teeth to help pull the fleas out of the coat.
Another common cause at scratching at the ears can be from the development of an ear infection (either with bacteria or yeast) which requires prescription medication to treat.
We also would want to rule out mites. These you can not see with the naked eye and can live in the skin or the ears. A microscope is used to help diagnose mites as a possible cause of itching.