updated 6/26/2012 9:47:58 PM ET 2012-06-27T01:47:58

Sudden changes in a dog's temperament, for example episodes of aggression, could be related to some internal pain they are feeling, which sets them on edge if they are touched, new research indicates.

"If the pet is handled when in pain, it will quickly act aggressively to avoid more discomfort without the owner being able to prevent it," study researcher Tomàs Camps, of the Autonomous University of Barcelona, in Spain, said in a statement. "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."

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Irritability from pain can make otherwise affectionate dogs violent and already aggressive dogs even more aggressive. As such, the researchers say, their findings support the importance of the diagnosis and treatment of pain in dogs.

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Aggressive Animals

Several factors can explain a barking, aggressive dog: The conditions of the mother during gestation, the handling of the puppy in the neonatal phase, the age at weaning, the experiences of the animal during the socializing phase, diet, exercise, genetics and learning techniques based on active punishment during adulthood.

Focusing only on sudden increases in aggression, the researchers studied 12 dogs (Giant schnauzer, Irish setter, Pit-bull, Dalmatian, two German shepherds, Neapolitan Mastiff, Shih-tzu, Bobtail, Catalan Sheepdog, Chow-chow and Doberman) that were brought to the university's veterinary hospital by their owners in 2010 and 2011.

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Scientists asked dog owners the who, what, where and when of the attacks on their owners: They identified the most frequent circumstances in which dogs were aggressive, the most characteristic positions, the most frequent target of attacks (usually the owners) and if there were warning signals before an imminent attack.

Pained pups

"All (eleven males and one female) were diagnosed as having aggression caused by pain. Out of the 12 studied, eight had suffered a hip dysplasia," Camps said. Hip dysplasia is a painful condition common in larger dogs, in which abnormal formation of the hip socket can cause crippling lameness and painful arthritis of the joints.

Canine hip dysplasia is hereditary and affects more than 40 percent of Golden retrievers, Labradors and Rottweilers. The problem begins slowly, so it's important to catch it early. Random aggressive episodes brought on by pain could help vets diagnose the disease.

The study was published February 23 in the Journal of Veterinary Behavior -- Clinical Applications and Research.

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