I have a pet, she is two, not spayed yet. She has what seems to look like mud in her nipple pores. She hasn't been anywhere near mud. Could it be parasites?
They do get ground-in dirt which sticks to their bellies. And this can get on the nipples too. Mine gets that, especially if it has been damp weather. But be careful this isn't some kind of scab, from irritation etc. If you feel at any stage that this isn't normal, it's not dirt or anything, then take her straight away to the vet.
Hard to tell. I assume there is no irritation? It could be dirt. It could be from a slight discharge from the nipple. It could be flea dirt.
I would take some warm water and a mild soap and a wash cloth and clean them individually to see if that helps.