The dog, a 6-month-old mixed-breed puppy, was living with a family in Chester County, Pa., when they noticed it began acting strangely. The dog was later euthanized, according to the Pennsylvania Department of Health. The rabies test, which can only be conducted by examining the brain after an animal is euthanized, showed the animal was positive with a canine rabies variant that is highly transmissible from dog to dog.