Blood Filled Cyst Dog. My dog had a blood filled cyst filled with blood that burst. Sebaceous cysts are a common condition in dogs, comparable to a large pimple, and generally harmless.
False cysts are most commonly filled with either blood or serum (the technical terms for these are hematoma and seroma, respectively). We have tried epsom salt, peroxide, and bath soaks just to name a few. As pressure builds up inside a dog’s cyst, sometimes a weak spot develops.