I realise that if I forget that the garage door is open and my dog gets out and bites someone I'm in big trouble. What would be the case if I'm sleeping in my house and someone breaks in and my dog kills the intruder before I'm even fully aware of what's happening? Can I be held accountable if it is considered not to be self-defense? Let's just assume for argument's sake that the dog in question has not received any attack training.
The law on such matters within the US varies from state to state. In general a dog owner is responsible for taking "reasonable" measures to control the dog and prevent harm. What measures are reasonable will depend on the detailed facts. particularly important will be the previous history and actions of the dog. If a dog has a history of attacking people, then the owner is expected to know this, and to take measures which are reasonable in light of that history.
The page "Strict Liability Dog-Bite Laws" from Nolo documents states in which an owner may be responsible for any bite. As that page says:
When a dog hurts someone, the injured person is likely to sue the animal's owner for medical costs and other damages. In more than half of the states, those lawsuits may be based on laws that make the owner automatically liable for most dog-bite injuries. These laws are often called "strict liability" statutes, because the injured person doesn't have to prove that the animal's owner knew the dog was dangerous (often called the "one-bite rule") or that the dog owner was negligent (legalese for "careless").
That page goes on to say:
Also, dog owners may have legal defenses to avoid liability for dog bites. Most dog-bite laws include exceptions—typically when the injured person was trespassing or provoked the dog. And many of the laws don't apply if the dog was performing its duties as a police dog when the bite happened.
The page lists details on the laws for holding dog owners liable in the listed states under a "strict liability" theory. The page notes that even when one of the listed exceptions applies, so a strict-liability suit cannot be brought, a suit under a negligence theory (failure to take reasonable precautions) may still be possible. A plaintiff may cite both theories in a single suit where the law permits. The page lists 36 states and the District of Columbia as having some sort of strict liability statute for dog attacks as of December 2020.
In a number of states that the person attacked was trespassing is considered an exception to the strict-liability rule.
The page "Brief Summary of Dog Bite Laws" published by Michigan State University in 2004 says:
Liability can be based on a common law theory of negligence of the owner where recovery is based on the action or lack of action by an animal owner /keeper. Common law is the law derived from court decisions and historical traditions rather than explicit statutory provisions. Recovery at law under this concept requires a showing by the injured party that there was a legal duty owed to the injured party by the animal owner /keeper and that the injury arose because of a breach of that duty. This duty can arise from a failing to properly secure an animal or entrusting the animal with someone unfit to restrain the animal. It can also arise independently from violating a local ordinance, such as those ordinances that prohibit dogs running at large or mandatory muzzling provisions. It has been said that one who keeps a vicious dog, with knowledge of its savage and vicious nature, is presumed to be negligent if he or she does not keep the animal secure from injuring others.
States may also impose a more stringent standard of fault called strict liability . In those states, liability automatically arises when a domestic animal has a known vicious propensity attacks, bites, injures, or even chases someone. There is no necessity to show that the owner was negligent in his or her actions in proving liability. ( Click here for a summary of states that impose strict liability). In addition, owners may face liability based on the legal category of the dog him or herself.
Nearly all states have some laws that govern what can be termed “dangerous dog” or “vicious dog” laws. These laws outline what constitutes a “dangerous dog” or even a “potentially dangerous dog” and under what circumstances an owner will be liable for the actions of such dogs. Moreover, some states impose what is called “strict liability” on dog owners for any injury resulting from a dog deemed dangerous regardless of any knowledge of the dog’s tendencies. These strict liability laws may also limit the ability of the owner to claim a defense to the action. In other words, the law may state that the owner is liable regardless of whether the person who was bitten was trespassing on the owner’s property or whether the owner knew the dog was vicious.
Regardless of whether you are dealing with a strict liability state or not, provocation will be an important factual issue. Provocation simply refers to a situation where a dog is incited, encouraged, or provoked into biting a person. In states where there is strict liability, an owner may use provocation as a defense to the dog bite. This will either reduce the owner's liability based in part (comparative or contributory negligence on the part of the victim) or bar the victim's claim completely. A court will either determine provocation from the perspective of the injured party (i.e., did the person intend to provoke the animal or have knowledge that his or her actions would provoke the animal) or from the perspective of the dog.
The page "State Dog Bite Laws: "One-Bite" vs. Strict Liability" from AllLaw says:
Prior to the twentieth century, a dog owner was usually only held liable for injuries when their dog bit someone if the owner had reason to know the dog might bite. This was known as the "one bite" rule because it generally meant that a dog was allowed "one free bite" before the owner would face legal trouble.
In modern times, the one bite rule does not necessarily allow a dog one free bite. If an owner knows the particular breed is dangerous, or if the particular dog might be prone to biting because of its general character or some other factor, he or she could face liability for the dog's first bite.
The focus is on whether the owner knew or should have known that the dog might bite, and whether the owner took reasonable precautions based on that knowledge. For example, if a relatively aggressive dog recently underwent surgery and the owner did not warn a house guest not to pet the dog, the owner might be held liable if the house guest is subsequently bitten.
As a fault concept, the "one bite" rule has much in common with (and often overlaps) the personal injury liability rule of negligence.
Many states have enacted "dog bite" statutes that create a form of "strict liability" for dog bites. Strict liability means that the defendant is held liable if a certain event occurs, regardless of whether the defendant could have done anything to prevent the event.
The typical strict liability dog bite statute says that a dog owner is liable if his or her dog bites someone, regardless of whether the owner did anything wrong, as long as the injured person:
- was not trespassing or otherwise breaking the law at the time of the incident, and
- did not provoke the dog.
If a strict liability dog bite statute applies, what the owner did or did not know about the dog prior to the bite is usually irrelevant. Note that strict liability dog bite statutes are not the same in every state that has one of these laws on the books. ...
In short, the particular law of the state or locality, and the particular facts of the case will both be important in the case of any dog attack.
By the way, nothing an animal does on its own can be classed as self-defense, although where a human orders an animal to attack that might be self-defense by the human (using the animal), depending on the facts. Actions that would have been self-defense by a human will probably not be grounds for finding an animal's owner liable.