Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 15 years of experience. I look forward to working with you.
This sounds quite concerning for Harry. First, I must assume if the ear was cultured to know the type of bacteria present that a sensitive (extended panel) was done to know what antibiotic should work to clear this. Often with those two organisms we end up with a need to use something called amikacin. If sensitive to that antibiotic, then your vet can have an ear medication professionally compounded for him. Have you already tried this and it failed?
As well using a daily ear cleanser with TrizEDTA can be quite useful to change the pH in the ear canals and perhaps reduce the level of bacteria (it will not clear by itself).
In severe infections amoxicillin (either orally or topically) will not resolve this. By mouth this is not an antibiotic that will even penetrate into the ear canals (it doesn't get through skin) unless it is potentiated with clavulanic acid. Again, in these severe ear situations we can clear it but it takes time and finding that right antibiotic that the organisms are not resistant to (a good look at the extended sensitivity panel that is usually done with any culture is needed; if this was not done, then this is a first step to figure out how to treat this).
I wish there was an easy over-the-counter solution for these but these severe chronic infections can take a long time to clear (3-8 weeks) of medication, and then they are likely to recur unless you work with your vet (or a veterinary dermatologist -- you can ask for a referral to see the specialist) to determine the underlying cause which could be an allergy (food allergy, environmental allergy) or endocrine disease. In rare cases when there is so much inflammation and scarring in the ear, surgery is recommended. I find I rarely have to recommend this (usually cocker spaniels are the most common that need this) when we use corticosteroids (like prednisone) and find the right antibiotic to clear this; that right antibiotic may have to be compounded periodically throughout the year until that underlain tissue is discovered and can be addressed, though.
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