The Faithful, by Lester del Rey (epub/Kindle/pdf)

Humans have surgically, genetically, and chemically bio-engineered dogs and apes until they are sentient, then gone ahead and destroyed their society with another world war. The story is told from the point of view of Hungor Beowulf IV, a descendent of the first sentient dog-person. Hungor leads his people in a nomadic lifestyle at first, but finally finds one surviving human, Roger Stren, whose experiments have extended the lifespan of the dog-people to 50+ years -- and allowed him to survive the bioengineered plague which has wiped out humanity..