Under Pennsylvania law, dog owners can be found guilty of harboring a dangerous dog and the dog can be declared dangerous.  The only way this can be done is if an injured victim, a police officer or other official files a complaint in the local magisterial district court. That party must then prove that the dog is dangerous.

If a magisterial district justice finds a dog dangerous and finds the owner guilty of the summary offense of harboring a dangerous dog, then the owner is subject to specific requirements for the duration of the life of the dog.  For instance, the owner must register the dog each year, must muzzle and leash the dog when out of the owner's home, must spay/neuter the dog and must carry $50,000 of liability coverage. 

My article on Pennsylvania's Dangerous Dog Statute explains the procedure, law and owner requirements in more detail.