A presumption, a strict rule, and a harsh defense
Under Courts and Judicial Proceedings §3-1901, evidence that a dog caused an injury creates a rebuttable presumption that its owner knew or should have known the dog had vicious or dangerous propensities, and a judge may not decide before the verdict that the presumption has been rebutted. Separately, an owner is liable for any injury caused by a dog running at large, unless the victim was trespassing or committing a crime on the owner's property, committing a crime against anyone, or teasing, tormenting, abusing or provoking the dog.
None of this displaces Maryland's contributory negligence rule, which bars recovery by a victim who contributed to the harm in any degree. Children under five are the exception — they can't be contributorily negligent. For anyone other than the owner, such as a landlord, the statute preserves the common law as it stood on April 1, 2012, regardless of breed.