What is Candy's dog a symbol of in John Steinbeck's Of Mice and Men?

The old dog and Candy himself are symbolic of what happens when someone outlives his usefulness. The dog has lived a long life as Candy's companion but no longer serves any purpose. He smells bad, is blind and has a bad coat. Carlson, a static character whose main purpose in the novel is to kill the dog, complains:

“Well, I can’t stand him in here,” said Carlson. “That stink hangs around even after he’s gone.” He walked over with his heavy-legged stride and looked down at the dog. “Got no teeth,” he said. “He’s all stiff with rheumatism. He ain’t no good to you, Candy. An’ he ain’t no good to himself. Why’n’t you shoot him, Candy?” 

Candy can't bring himself to put the dog down, but Carlson volunteers, and Slim, whose "opinions were law," thinks it's the best thing to do. Slim reiterates Carlson's claim that the dog is no longer useful:

“Carl’s right, Candy. That dog ain’t no good to himself. I wisht somebody’d shoot me if I get old an’ a cripple.” 

Symbolically, the dog is similar to Candy. The old swamper, who lost his hand in a ranch accident, has also outlived his purpose. He fears that he will soon be fired from the ranch since he can no longer work as hard as the other men. He expresses this fear after he offers to contribute his money for the farm that George wants to buy. He says,

Maybe if I give you guys my money, you’ll let me hoe in the garden even after I ain’t no good at it. An’ I’ll wash dishes an’ little chicken stuff like that. But I’ll be on our own place, an’ I’ll be let to work on our own place.” He said miserably, “You seen what they done to my dog tonight? They says he wasn’t no good to himself nor nobody else. When they can me here I wisht somebody’d shoot me. But they won’t do nothing like that. I won’t have no place to go, an’ I can’t get no more jobs. 

Unfortunately for Candy, the dream of the farm is shattered after Lennie accidentally kills Curley's wife. In Chapter Five, George is no longer interested in buying the farm because he knows he will have to kill Lennie. Lennie, too, has outlived his purpose. He can no longer live in society because of his actions. The dog, Candy and Lennie become castaways in a society which cannot be burdened by the old, the crippled or the mentally challenged.