The actual cause(s) of death in veterinary cancer patients have rarely been well-documented. Deliberate euthanasia by the veterinarian for the animal with advanced debilitating disease often spares the animal the fate of death directly attributed to a complication of the neoplasm. If the disease follows its natural course, like in the human patient, death from cancer will be associated with complications such as hemorrhage, intercurrent infections, or organ failure. A small number of patients die as a consequence of treatment, including operative mortality, mortality following radiotherapy or chemotherapy. The classic cause for death in cancer patients is cachexia, a term used to encompass a myriad of metabolic derangements characterized by tissue catabolism.  

Cancer cachexia is characterized by anorexia and nausea, weight loss, anemia and muscle weakness. Weight loss associated with cancer has marked adverse effects on survival, and weight loss exceeding 30% may be fatal. Cancer cachexia is a complex process attributed to metabolic abnormalities produced by the tumor in addition to anorexia. The energy balance in the cancer patient may be negative due to decreased caloric intake and/or increased expenditure of energy.  

Increased mobilization of host lipids may occur and loss of adipose tissue constitutes the major proportion of the weight loss in cancer. Skeletal muscle mass depletion, however, is probably most important in the overall survival of the cancer patient.  

The mediators of cancer cachexia have been partially elucidated and include hormone-like substances and lipid mobilizing factors produced by the tumor. Inappropriate release of cytokines by host tissues might also mediate changes in host metabolism which accompany tumor growth.  

In particular, the cytokines TNF-alpha, interleukin-6 and interferon-gamma produce in experimental animals some of the features of cancer cachexia.