A dog can live without a spleen, but the organ plays an important role in recycling old red blood cells. The spleen is a leaf-shaped, elongated organ with long-winding, narrow vessels. When old red blood cells enter the organ, quite a few of them are not able to make it to through the narrow vessels and rupture in the process. The spleen thus recycles the captured iron of these cells to make new red blood cells. The other function that the spleen performs is to bite off cells that have been marked by the immune system, removing red blood cell parasites to restore general cell health. As such, it is involved in the production of cells involved in immune responses.
Occasionally, the spleen develops tumors. The spleen consists of red and white pulps. The red pulp performs the functions detailed above and the white pulp is somewhat like a lymph node and is part of the lymphatic system. Spleen tumors may be benign (hemangiomas) or malignant (hemangiosarcomas). Hemangiosarcoma is a blood-fed sarcoma, a rare but a highly invasive type of cancer. In dogs, it arises mostly from the red pulp. Feline cancer of the spleen mostly arises from the white pulp.
Invariably, both hemangiomas and hemangiosarcomas amount to a grouping of rapidly growing blood vessels that eventually rupture, leading to excessive bleeding. This can be a life-threatening situation, because even if the bleeding stops on its own, the spleen is certain to bleed again. If the spleen is not removed, the dog will bleed to death.
The ideal situation is to remove the spleen as soon as a tumor (benign or malignant) is detected, before the bleeding starts or when the spleen is not actively bleeding. An actively bleeding spleen, however, is a cause of concern for the surgeon and can end in a catch 22 situation. It is not appropriate to wait until the bleeding has stopped and the dog may have lost so much blood that it may not be possible for him to survive surgery.
A spleen removal surgery may not prove to be the end of it, either. Dog cancer of the spleen is highly aggressive and metastatic. Surgery may save the dog from death from bleeding, only to be fatally overwhelmed with cancer.