The best possible action is to take it to a vet. If that's not possible here are some things I found on the web that may be of help, but I have no way of knowing for sure. What is for sure is that antibiotics don't heal fungal infections, so gut loading a mouse with tetracycline isn't going to help. And, from what I can read treatment needs to be both topical (on the skin) and internal (oral/injectable). These sections of text are from several articles on the web, again I don't know if they'll help or they may even harm. You could call a vet and ask about these. Good luck.
http://www.reptilerooms.com/Sections+index-req-viewarticle-artid-35-page-1.html
Skin Disease
These are generally caused by fungi (mycosis). Symptoms include brown splotches that can be observed to grow. These patches can eventually break, releasing pus and then spread rapidly. They can infect an entire snake in only a few weeks.
It is important to treat these skin disorders as soon as possible. Your vet can prescribe general antifungal treatments, which must be consistently applied to your herp.
There are many types of fungus that can look/act alike, the only way to know for sure is a culture.....
Miconazole has been tried with little success but Tioconazole has a more successful record. If your vet doesn't have Tioconazole cream, go to the drug store and buy some Monistat1. The real threat with this infection is that yeast attacks the organs and more often than not, it reaches fatal stages before a successful diagnoses and treatment is derived. For this reason, you need to give oral meds as well as treating the skin.
I have been having some good results using lamisil and anitifungal cream for human use. I tried lotrimin, mycelex and several others. I spoke with the pharmacist at my local drugstore and she explained that there are many different types of antifungals that all work in a different way attacking and killing different types of fungi. The active antifungal in lamisil is terbinafine hydrochloride 1%, it is the only one that has turned the fungus black (I am assuming this means is it dying) and the yellow has not returned. This is my own personal experience and I am not certain it is a long term fix, I just know it seems to be working.