How did Junior's dog die?

Life on the reservation is hard. Poverty, alcoholism, and drug abuse are widespread. Life's especially hard for the children, and Junior's no exception. It's bad enough that he was born with "water on the brain" and that most kids at school think he's weird, but growing up poor makes it even worse.
One of the few bright spots in Junior's life is his little dog, Oscar. So we can imagine just how devastated he is when his father shoots his beloved pet. Oscar, like Junior, has become a victim of poverty. Junior's dad kills the dog because he can't afford to pay the vet's bills. Taking proper care of a dog costs money and the Spirits just don't have enough. Junior's so heartbroken at the death of his dearest and closest friend that he says it's worse than going hungry. That gives you some idea of just how close they were.