Large black cyst rupture on penis

History:

• He has a skin condition and allergies that he takes steroids daily for
• he’s about 10 years old
• has food allergy to grains and birds
• has had similar lumps before that were like blood blisters but would go away with proper diet

this lump on his penis developed a few months ago and just recently grew from a regular blister size to a cherry tomato. It has ruptured once before and the swelling went down but then a few days later it was full again and got even bigger. Yesterday I caught him chewing and licking the area and the whole thing is torn open and it smells like rotten sewage. It’s brownish green tissue inside and leaking dark red thick foamy stinky liquid. I’ve cleaned it out with peroxide and then put antibiotic ointment on it with a gauze pad and tegaderm wound covering so he can’t lixk it. I’ve been changing it every 4-6 hours and the gauze is soaked. What do I do

Hello!   I'm afraid that this doesn't sound like a cyst.  Cysts are usually closed-off cavities that produce a sterile liquid on the inside.  This liquid sounds somewhat unpleasant and has some of the hall-marks of necrosis or infection.  Your description made me wonder about neoplasia (cancer), abscessation or infectious diseases, some as venereal warts. When I first graduated there were some old vets who used still used Hydrogen Peroxide for a lot of minor ailments, but even back then it was contraindicated on the inside of the flesh, including raw wounds.  It is a known carcinogen and corrosive, which means that it can irritate and burn sensitive skin.  Especially given that the structure sounds to have ruptured and that the tissue beneath is exposed, there may now be much better treatments to choose from. Here in the UK, vets would want to consider from several options, potentially including biopsy, surgical removal or medical treatment if applicable.  The treatment depends on the likely identity of the lesion, so we would strongly advise that they examine your dog. Please do let us know how you get on.