Cat Open Sores

Question
My indoor/outdoor 14 or 15 year old cat recently developed bald patches and an open sore on her rear left hip. We thought that it was from one of the new full-grown dogs that joined our home within the last two months, but more sores have developed and they now ooze a blood tinged puss. The original sore has crusted over, however. The area is completely bald. It is bumpy - large .5cm bumps - the valleys being the sores. I haven't seen her scratch the area, just lick it. It doesn't look infected on the outside as she does keep it clean.

Neither of the other two cats (13 and 10 yo) show similar symptoms nor any of the three dogs - the new ones had a full vet checkout and were diagnosed with Lyme's disease and are being treated.

She has had varying food types for a very long time. The current food is not something new.

She does have symptoms of ear mites - balding patches on the ears. She does sleep a lot, but that isn't anything new as she is older. She was also hit a year or two ago and is blind in one eye - probably not relevant, but I was thinking about the allergy signs and symptoms. She seems weaker than normal and needier than normal - seeks me out when I am outside. It doesn't appear to hurt her too much and she doesn't whine. She clearly wants me to do something about it.

Suggestions?

Answer
Ask your vet if your cat can have a shot of DepoMedrol to determine if this is an inflammatory condition.  If it is, the long acting steroid shot will help.  If it is trauma from the other dog or from the cat rubbing that area, then you have to identify the cause and try to stop it. Cats rarely have skin infections, but that may need to also be considered in an older cat with bumps.