The yellow part in the middle is curious, makes me think bruise. If it is a staph infection, that is an easy cure too. Whatever you do, don't take her to that er vet again, LOL! No telling what he'd suggest
found this on bruising:
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Go over the animals body slowly, moving fur aside every inch or so. Look for any changes in color. Specifically look for blue, brown or yellow areas.
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3
Press on the spot with your hand. See if the animal pulls away in pain. This verifies that it's a bruised area.
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Determine if the bruise is hard or raised. If so, it might not be a bruise but a cyst or tumor.
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Look to see if the fur is thinning in that area. That would also indicate a bigger problem.
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Figure out how old the bruise is. There are essentially two types--bruises that are new and bruises that are several weeks old. You can tell because older bruises are yellowed in color.
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Treat the bruise by placing a cold compress on it. Hold it there for 10 minutes, if possible.