In the "Little House" books, Jack got old and slow and finally the family found him dead one morning.
In "Cast-offs", Jack seemed perfectly fine - save the foxtails in his ears. He was running away from Laura and wanting to play with toys and gave no indication that there was a problem. I'm no dog expert, but I've had dogs in the past and notice that they slow down, seem listless, don't eat, etc. when there is something wrong. Jack didn't seem to have any of those issues, even though Pa said he was getting old and slowing down.
So, did the foxtails kill him?