I think the type of bacteria and AB resistence profile should be established by bacteriological testing on the pus.
If it is a deep anaerobic infection hospitalisation and a drip with intravenous AB (after establishing WHICH AB the bacteria respond to) would probably be in order. I don't think oral AB are getting there as well as they should do, considering that the infection seems to be spreading.

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Jiskefet & the mognificent Dutch bunch

My little angels at the rainbow bridge

I too think a bacteria screen is done as I have heard of a similar thing in a dog that had a knee op and the wound would not heal. It turned out the dog had MRSA in the wound and needed intense treatment!

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@Kittenfostermummy, @Jiskefet
Thanks for this. I will let the vet know abt this.