My 10 year old black lab has had a bite on his paw for over a month. He has been seen by his vet, but it has not gotten any better. He has been given salve, antibiotics, and epsom salt soaking daily. It seems to not be getting any better. No one seems to know what it is.

Has it been cultured?

The vet extracted cells from the site but doesn't know what it is

Has it been cultured?
Yes

Have they surgically explored the area to see if there is anything foreign inside of the hole? Often times a foreign body will stop an area from healing.

Have you tried wrapping it and keeping your dog from licking? Sometimes, when dogs think they are trying to take care of their own boo-boos... they prolong the healing effect.. Put a sock on his foot and possibly an e-collar around her neck and see if it doesn't heal.