Testing for rabies is often done once the animal is dead. It requires a sample of brain tissue. I was bitten by my own dog (around age 15) who was suffering some serious physical distress at the time ( I believe she had a slipped disc in her back). Rather than treat the problem, my mother decided to just put her down (that's a topic for another therapy session) and they tested her for rabies because I had been bitten. I was informed by my mother how that is done with use of the brain tissue. (P.S. I would have been MUCH better off not knowing).