Negligence per se is a legal doctrine which holds that the violation of a statute or ordinance intended wholly or partially to prevent harm may constitute the basis of a claim for compensation for harm that approximately results from the violation. In some states, negligence per se establishes a cause of action, while in others it constitutes only a presumption of negligence. The difference is substantial: if the violation establishes the cause of action, there is no defense; if the violation is only evidence establishing a presumption of negligence, then the defendant can try to explain away the violation as being more safe under the circumstances. These concepts are further discussed at Legal Rights of Dog Bite Victims in the USA

Negligence per se has been defined as "[c]onduct, whether of action or omission, which may be declared and treated as negligence without any argument or proof as to the particular surrounding circumstances, either because it is in violation of a statute or valid municipal ordinance, or because it is so palpably opposed to the dictates of common prudence that it can be said without hesitation or doubt that no careful person would have been guilty of it." Black's Law Dictionary, p. 933 (5th ed. 1979).

Restatement of Torts, Second, section 286, defines it as follows:

§ 286 When Standard of Conduct Defined by Legislation or Regulation Will Be Adopted

The court may adopt as the standard of conduct of a reasonable man the requirements of a legislative enactment or an administrative regulation whose purpose is found to be exclusively or in part -
(a) to protect a class of persons which includes the one whose interest is invaded, and
(b) to protect the particular interest which is invaded, and
(c) to protect that interest against the kind of harm which has resulted, and
(d) to protect that interest against the particular hazard from which the harm results.

Here is a sample of cases that hold that violation of dog laws constitutes negligence per se:

Miller v. Hurst, 302 Pa. Super. 235; 448 A.2d 614; 1982 Pa. Super. LEXIS 4695 (1982)

Pigman v. Nott, 305 Minn. 512, 233 N.W.2d 287 (1975) [doctrine of negligence per se caused owner of horse held liable for damages to automobile, without showing of negligence, because horse was running at large in violation of statute]

Alex v. Armstrong, 215 Tenn. 276, 385 S.W.2d 110 (1964) [doctrine of negligence per se caused owner of dog "at large" to be liable for knocking plaintiff to ground, because of violation against prohibition against dog running at large]

Butler v. Frieden, 208 Va. 352, 158 S.E.2d 121 (1967) [doctrine of negligence per se caused owner of unleashed and unattended dog held liable for violation of combination at large law and leash law]

Eigner v. Race, 54 Cal.App.2d 506 (1942) [doctrine caused owner of Great Dane held liable for knocking down plaintiff, because dog, even though leashed, was not under effective control of minor girl, as required by local ordinance]

Brotemarkle v. Snyder, 99 Cal. App. 2d 388 (1950) [doctrine caused owner of dog held liable for injuries to person riding motor scooter, when at large dog ran into the scooter and knocked it down]

Duffy v. Gebhart, 52 Del. 312, 157 A.2d 585 (1960) [doctrine caused owner of dog held liable for injuries to person who was walking her own dog, where defendant's dog was running loose in its owner's front yard with the owner present, ran onto the public sidewalk, attacked the victim's dog, and the victim was injured while attempting to protect her dog; also held that plaintiff was not contributorily negligent when she tried to get her dog out of the attack.]