Other conditions that can mimic the symptoms of feline colon cancer include gastric blockage, viral, bacterial and fungal infection, colitis and parasitic infestation. The symptoms of colon cancer in cats include difficulty defecating and straining to defecate. However, cats suffering from colon cancer will defecate more often than usually. There may be mucous or brightly colored, red blood in your cat's feces. Your cat may lose weight and seem to be lethargic and depressed.