At the moment, our understanding of the biologic behavior is still rudimentary. In some dogs, the multiple nodules undergo spontaneous and apparently permanent remission over the several weeks or months after the diagnosis. In other cases, there is temporary remission followed by reappearance of the nodules, which then remain until euthanasia. In the majority of cases, the nodules are progressive, becoming larger and more numerous. At the moment, I suggest a very guarded prognosis because the majority of cases seem to eventually progress to involve hundreds of unsightly cutaneous nodules.
Visceral involvement appears to be uncommon but few cases have had necropsy follow-up.
The microscopic lesions can be distinguished from the syndrome of multiple benign cutaneous histiocytosis that occurs sporadically in various breeds by the greater cellular pleomorphism and the presence of atypical, large hyperchromatic histiocytes. Whether this is a genuine trend or just a transient blip remains to be seen. One can imagine things like less frequent and more selective vaccination as possible explanations. They may occur as quickly as 3 weeks after the very first vaccination. We still do not know what the carcinogen is, or whether there is a single shared carcinogen among all the incriminated vaccines.