I'm thinking a bad case of worms. The saliva is probably from the pup having a seizure before it died... seen it happen before, not a pretty site.
see mike that's why me and my stepdad thought hook worms because he had a heeler who got them a few years ago, he got real anemic, and seized in the yard,he got him to the vet but it was too late, though they did confirm it was hook worms