(HealthDay) -- Pain may be the cause of sudden, unexplained aggression in dogs, a new study says.
Spanish researchers studied aggression problems in 12 dogs -- giant schnauzer, Irish setter, pit bull, Dalmatian, two German shepherds, Neapolitan mastiff, Shih Tzu, bobtail, Catalan sheepdog, chow-chow and Doberman -- whose owners brought them to a veterinary clinic.
All 11 males and one female were diagnosed as having aggression caused by pain, and eight of the dogs had hip dysplasia, according to the team at the Autonomous University of Barcelona.
"Dogs that had never been aggressive before the onset of pain began to behave in this way in situations where an attempt is made to control them," lead researcher Tomas Camps, a researcher at the UAB's Animal Nutrition and Welfare Service, said in a university news release.
Hip dysplasia is a hereditary and degenerative bone disorder that affects the joint connecting the hip and the head of the thigh bone, the release notes. It can affect any breed of large dog but is less common in smaller breeds.
These findings suggest that hip dysplasia-related pain is a key factor in the risk of large dogs becoming aggressive, the researchers said.
The study was published in a recent issue of the Journal of Veterinary Behavior-Clinical Applications and Research.
Explore further: Gold treatment relieves pain in dogs
More information: The ASPCA has more about hip dysplasia in dogs.