How can we get rid of the wet-dog stench in our rented apartments?

Last year we rented out one of our apartments. The couple were both White, and maybe it was something they cooked, ate, or perhaps it was just the scent that they gave off - I honestly don't know what it was. But I remember that it smelled absolutely putrid each time that I had gone to pick up the check. Now they have stopped renting our apartment, but that wet-dog scent is still there. How can we get rid of the smell? Thanks.