What is this black stuff around my cat’s nail bed?

This is definitely NOT dirt. It grows from inside of the nail bed towards out.   My 1st female cat has been getting this half year ago and been through two vets: The 1st vet said it’s dirty and after clean it would been gone. And everything came back after his “professional” cleaning. 2nd vet prescribed “Animax” Ointment (nystatin-neomycin sulfate-thiostrepton-triamcinolone acetinide ointment) and said this med can treat all yeast/fungal/bacterial infection. I have applied it twice per day for two week following the vet’s instructions. Because I have to manually scraped the black stuff off before I apply the medication, after two weeks, it “looked” like it’s gone and vet verified so. But after two months, it all CAME BACK! And unfortunately, since my cat always licks her paws after got this infection, her paws passed on the black stuff into her ear canal on Month 3, and vet prescribed“Tresaderm” for her ears. I, again, applied it twice per day for two weeks. Because the ear drops temporarily “washed” the black stuff out, the vet confirmed it’s “cleared” again, then after two months, it all grew back inside her ears.   Now on Month 6, not only my female cat is still suffering from this, but also my five year-old male indoor cat who is extremely clean and never has this in his entire life, is getting this on his every single nail bed as well. I am extremely frustrated and angry about the mistreatment of this from the previous two vets. It’s financially hard and mentally suffering for me to trust another new vet who might possibly give the wrong treatment again and it’s uncomfortable for my cats wearing collar nonstop to prevent licking. I am also extremely worried that my 3rd cat might get it any time since two cats out of three are already battling it daily.   I just want to know for those of you who has SUCCESS in treating this disease- WHAT IS IT and what medication indeed WORK and can CURE this? Or is there something that can never be gone once infected?! Please also share what you have tried that DID NOT WORK so that I can avoid possibly be wasting the treatment time again. Thank you!!

Thankyou for your message, photos and description.  Given that this crusting repeatedly responds to anti- yeast / fungal / bacterial treatment, I suspect that the black is in association with at least one of those, possibly a yeast called malassezia.   What's interesting is that it is affecting multiple cats, because such organisms often live on healthy skin and only sometimes build up - for example, when the immune system is compromised.   I do wonder whether there could be underlying viral diseases present, such as affect the immune system for example FIV or FeLV - or whether the cats could be related and have skin disease that is causing over-grooming or something else.  I also wonder whether treatment is being given for long enough to truly clear it up on microscopic level - although eeven if it is, if the immune system is and remains compromised then it is likely to recurr.  Your vet might propose more tests in order to understand, but it is possible that financial constraints have been preventing this; it may lead to a conversation about what should most practically be done to make the most difference to your cats.

Finally, it may be worth asking your vet whether they would recommend any over the counter treatments for this purpose.  Im not sure what's on the market in this area at the moment - it would have to be licensed specifically for cats.