When a normally mild-mannered dog suddenly displays aggressive behaviour, what are the causes?

"Sudden onset of aggressive behaviour in dogs is the result of interactions between many complex factors caused by genetic predispositions, hormones, age, sex, reproductive status, breed, environment, early experiences, later learning, behavioural issues or medical conditions," says Rosemarie Tang, a certified professional animal trainer and behaviour counsellor.

Getting a vet to eliminate possible and potentially serious medical conditions as the cause is the first step, as there is an urgency to treat a health issue sooner than later. If medical causes are eliminated, the next step is to look at the most common behavioural causes.

One of those is pain-elicited aggression. Tang says: "Even a friendly, well-socialised animal may bite if injured or in pain. Inflicting pain [unintentionally, such as when treating a wound] always creates the serious risk of making the animal aggressive."

She adds that it is one of the most common reasons for sudden onset of aggressive behaviour in dogs that have not previously displayed any signs of aggression. Depending on the cause, the pain may be treatable or at least manageable.

Fear-motivated aggression occurs when a dog becomes intimidated and is defending itself. "Identifying and managing the triggers of your dog's fearfulness may be helpful in reducing aggression. When it's not practical to do so, then systematic desensitisation will be an excellent method to help make the dog feel less fearful," Tang says. Desensitisation involves exposing the dog to whatever it is that previously evoked its fear or anxiety, but at a distance and intensity that does not produce a response. In this case, it is best to work with an experienced trainer such as Tang. Your vet may be able to provide some recommendations.

Another cause is possessive aggression, or resource guarding, which occurs when a dog is defending its resources such as a toy, a bone or food. The best way to treat this behaviour, Tang says, is to use a combination of management and behaviour modification. "Management means preventing a situation from happening so that the dog cannot repeat inappropriate behaviour. Also, use a behaviour-modification programme of systematic desensitisation and counter-conditioning."

Counter-conditioning is a process in which a dog's aggressive reaction is replaced with a more desirable response. "For example, pair up the trigger of the aggressive behaviour, such as your approach, with a tasty treat. This methodology has been proven to work, and is relatively easy and pleasant for both human and dog."

Similar to possessive aggression is territorial aggression, which occurs when a dog is defending its perceived territory, such as a sleeping area or even a regularly walked path. "Dogs also generally consider vehicles in which they are travelling to be part of their territory. Desensitisation and counter-conditioning will help treat this behaviour," says Tang.

There is also maternal aggression, which is hormonal behaviour that occurs when a mother dog is protective of her litter, or of her toys and possessions during a false pregnancy. "This type of aggression lasts around two months during the false pregnancy, and longer if there's a real pregnancy," Tang says.

In next week's article, Tang will discuss additional causes for aggressive behaviour, what to do when a dog first shows such behaviour before the cause is identified, what to do about an aggressive puppy, and what to do if you encounter an aggressive dog while out. 

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