What happened to Dave in The Call of the Wild?

The Call of the Wild by Jack London is a short novel published in 1903 set in the Klondike Gold Rush. The story is told from the point of view of a dog, Buck, a domestic St. Bernard and Scottish shepherd mix, who is dognapped at the start of the novel to serve as a sled dog. The story is narrated in the third person, with Buck as the point of view character and the main theme of the story being how both men and animals revert to their primitive natures under duress. 

We are introduced to Dave in Chapter 2. He is part of the nine-dog team run by French Canadians, Perrault and Francois. We are introduced to the team through the eyes of Buck, who observes how the dogs of the team need to behave to survive and who gradually comes to understand the social and power relationships among the dogs. The dog Dave is described as older, hardworking and somewhat of a loner but acts as a mentor to Buck. Buck is normally harnessed just in front of Dave and Dave teaches him about life as a sled dog. In Chapter 3, one of the dogs, Dolly, goes mad and Buck wins a fight with the lead dog Spitz. When Spitz loses, he is killed by the other dogs and Buck becomes the team leader.

In Chapter 4, the team is sold and begins working on the mail run from Dawson to Skagway for an unnamed Scottish half-breed man. Given no rest between mail runs and heavy loads, the dogs begin to weaken and Dave begins whimpering to himself even though nothing is is obviously wrong. At  Cassiar Bar, Dave begins to sicken obviously, and has difficulty walking. The half-breed allows Dave to rest by letting him run beside the sled instead of being harnessed with the other dogs. After a brief rest, Dave catches up with the sled and insists on being let back into his harness despite his weakness:

[Dave] pleaded with his eyes to remain there...[the men] talked of how a dog could break its heart through being denied the work that killed it, and recalled instances they had known, where dogs, too old for the toil, or injured, had died because they were cut out of the traces. Also, they held it a mercy, since Dave was to die anyway, that he should die in the traces, heart-easy and content.

Dave struggles to keep up, but despite his efforts cannot really manage the pace. The team finishes its day's run and goes to sleep. The next morning, Dave is too weak to walk. The driver harnesses the other dogs and moves them and the sled a short distance ahead, leaving Dave behind to howl mournfully. Realizing that Dave will only die a lingering and painful death if abandoned, the driver walks backs and shoots Dave.