First let me reassure you that I think it’s highly unlikely that your daughter’s impetigo has caused the scabs on you cat’s ears or vice versa. Impetigo is the term used to describe a skin bacterial infection (usually Staph aureus) primarily occurring in children. There are equivalent conditions in animals but the strain of bacteria in animals is different to that in humans and is not likely to spread from one to the other. None the less we should always practice good hygiene when handling our pets. So washing hands and avoiding face contact etc are good rules to follow.
“Scabby ears” is a common problem in cats and there can be a variety of causes. In particular we need to consider:
It’s very unlikely that the problem relates to the feline acne. The latter is a very special type of infection that only affects the chin in cats because of the rich concentration of sebaceous glands. However the fact that the scabs didn’t clear up when the acne was treated strongly suggests that the ear problem is not a bacterial infection. It’s interesting the antibiotic ointment seems to be helping….but I’ll come back to that.
To determine if this is a mite infection or a fungal infection your vet would need to do a scraping, microscopic examination and/or special culture and if things don’t resolve that may be the next step. In addition if the condition progresses and in particular if it starts to severely affect just the edges of the pinna (ear flaps) and spreads to the nose or toes then we might need to consider an autoimmune disease called pemphigus foliaceus. So keep a check on that and if it is spreading I’d suggest a revisit to your vet for a biopsy. If this was pemphigus then high doses of cortisone are needed (orally) and the treatment should be started early for a better response. However if the scabs tend to be restricted to and spread across the face of the pinnae then it’s highly unlikely that this is pemphigus.
Now I feel that the most likely cause is allergy. You’ve changed his diet which is a good start although it’s better to actually switch to a hypoallergenic diet, available through vets to be sure. Hill’s Feline z/d diet is worth trying.
A very common cause of raised scabby sores on cats’ pinnae is a hypersensitivity to mosquito bites. If your cat goes outdoors and you are in a mosquito area it’s a good possibility. The only effective treatment if that’s the case is protecting the ears with ointments (caution with repellents in cats!) or keeping him indoors at night. Otherwise we need to use cortisone to suppress the allergy.
Now you’ve noticed some improvement with the “antibiotic ointment”. Apart from the obvious (antibiotic) this may be because of the protective properties of the ointment (keeps the mosquitoes off), because the ointment also contains cortisone (read the contents) or because of the chemical base in the ointment. I’d suggest you try Vitamin E cream or Aloe Vera cream. I get very good results using either of those on this type of problem. They are both totally safe in cats.
So consider what I’ve discussed. Rule out mosquitoes, diet allergy and try the creams. If there’s no response and particularly if it starts to progress you should get your vet to run further tests.
I hope I’ve been of assistance but please contact me back if I can help further.
Kindest regards, Peter