Thanks for the additional information. Based on your description, I would be worried about mites (Sarcoptic Mange in particular), in addition to trauma from an unresolved ear infection, a vasculitis (possibly from a vaccine reaction), or possibly allergies.
Based on your description, I agree that mange is my top concern. Unfortunately, skin scrapings don't rule out Sarcoptic Mange, they only rule out Demodectic Mange
. In general, you can do 20 skin scrapings on a dog with Sarcoptic Mange and maybe only find one mite in a dog with severe clinical disease. For this exact reason, if I am even suspicious of sarcoptic mange, I will often start the dog on Revolution as a precaution. Revolution is a good once a month preventative for them to be on anyway (prevents against fleas and heartworms) and it just so happens to treat sarcoptic mange if present. I would ask your vet about starting him on Revolution as a precaution.
If you would like to read more about sarcoptic mange, I recommend visiting the following link: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=616
As far as the other possible causes, if your dog hasn't recently received a vaccine, and has no other abnormal bruising, vasculitis is less likely. If your vet hasn't noticed an ear infection recently from his/her exam, trauma from otitis externa
is less likely.
If your dog also has trouble with anal glands and itching all over, food allergies is a possibility. (However, based on your description, I would be most worried about sarcoptic mange.) If you would like to read more about food allergies and food allergy diet trials, I am going to recommend that you click on the following link:
Hopefully that answered your question. Please let me know if you have any additional questions or concerns and I will be happy to try to answer them further.
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Thanks and best of luck with your boxer