For improperly socialized dogs, aggressive behaviors can be a common problem, especially toward strangers. According to the American Society for the Prevention of Cruelty to Animals, as many as 60 to 70 percent of dogs show signs of aggression towards unfamiliar people, including barking or generally behaving in an unfriendly manner. Unfortunately, in some dogs this aggression can lead to nipping at strangers, a potentially serious situation. To prevent your dog from injuring someone else, take steps to keep him away from strangers or train your dog not to nip at them at all with positive, reward-based training techniques.
Bring your dog to a veterinarian if his nipping behavior is a recent occurrence or if touching some parts of his body elicits nipping behavior; certain diseases cause irritability or aggression in dogs. Your dog could be suffering from an underlying medical condition such as cognitive dysfunction, hyperthyroidism or have an injury that is causing him to nip at strangers.
Confine your dog or keep him away from strangers during situations that typically cause him to nip at them. Some dogs nip at strangers only while they're eating or being groomed, for example. If this is the case, anticipate the triggering event particular to your dog and avoid putting him in situations with strangers during these times. You can also put the dog on a leash during these times to control him and keep him from nipping at other people.
Get a friend who your dog doesn't know to help you discourage your dog's nipping behavior. Have her come over and spend time with your dog as a practice "stranger." If the dog attempts to nip her, have her say "ouch" immediately in a loud voice and walk away, ignoring him for 10 to 15 minutes, stopping any pleasant play behavior. Ignore your dog as well after he attempts to nip your friend, to send the message that nipping results in something unpleasant. Repeat this process with your friend every few days over the period of several weeks.
Expose your dog to a variety of strangers while he's on a leash. Keep him at a distance from the strangers, giving him praise and treats while he's calm in their presence, but not close enough for them to touch him or for him to nip them. Continue brief sessions of this exposure for 10 to 15 minutes at a time until your dog appears more comfortable in the presence of strangers. Move closer each time. At any point, if the dog attempts to nip at a stranger, immediately pull back on his leash to prevent him from making contact with the stranger, and ignore him for 3 seconds. If he remains calm after the initial attempt at nipping, click and treat him.
Dogs with fear aggression will sometimes lash out at strangers because these situations scare him; the idea is to get him to associate strangers with something positive, such as a food reward or play.
Place a canine head halter on your dog when you walk him. This will give you increased control over him and prevent him from nipping strangers while you are outside. Get him used to the halter by having him wear it for a few minutes at a time, praising him and treating him with it on until he can wear it for a longer period with no signs of discomfort.