Problems of canine aggression towards non-familiar people and other dogs are relatively frequent. When aggression occurs between dogs in the same household, the most likely diagnosis is competitive aggression. If aggression is directed towards unfamiliar dogs the clinician must differentiate between three categories: intrasexual, fear related and territorial aggression. The last two diagnoses are also the commonest types of aggression towards unfamiliar people. In fact, etiology, diagnosis and general treatment principles for fear and territorial aggression are similar no matter if the animal becomes aggressive towards people or in front of other dogs.
Aggression between dogs living in the same household
The first step in the diagnostic protocol is to perform a thorough physical and neurological examination to detect any physical condition that can cause aggression. Also, measures of serum TSH and T4 should be obtained to rule out hypothyroidism.
If no physical conditions can be detected the most reasonable diagnosis is dominance aggression between dogs. Although the vast majority of cases aggression occurs between dogs of the same sex, hierarchical aggression problems between males and females can be sporadically found. Dominance aggression appears when the hierarchical relationship between both dogs is poorly defined or it is somehow being modified. The onset of the problem is usually coincident with either, puberty of the younger dog, or when the dominant dog becomes weaker because of aging or any debilitating disease.
Careful history taking usually reveals that aggression episodes appear in competitive situations, like mealtimes. Frequently, aggression is only shown when the owners are present, as they become an object to compete for.
Competitive aggression between dogs often resolves itself spontaneously. However, in some cases the owner's attitude interferes with the natural resolution of the conflict. This happens when the owner favors the subordinate dog upon the one that is supposed to be the leader. As a result, a stable dominant/subordinate relationship could not be firmly established and aggression prevails.
Therefore, it is essential to persuade the owner to always favor the dominant dog upon the one that should be in a second position. In some cases this recommendation is enough to resolve the situation, whereas in others castration and drug therapy should also be used. Castration is reserved only for aggression problems between males and it should be done to the most submissive individual. If severe fighting is present both dogs can be neutered. It is important not to castrate the dominant dog, since that could lower the hierarchical distance between both animals. Drug therapy is reserved for cases of severe fighting, especially is impulsivity is present. Fluoxetine is the first line drug and should be administered for a long period.
Aggression between unfamiliar dogs
First, it is important to rule out any physical condition that can cause aggression. Once this has been done three different diagnosis should be considered: intrasexual aggression, fear aggression and territorial aggression.
Intrasexual aggression is the easiest to identify. It is typical of males that have reached puberty and is characterized because aggression is selectively directed towards other males. Dog's body posture is clearly offensive. More than 65% of affected dogs respond to castration, which is indeed highly recommended. It is also important to remember that effects of castration could not be immediate and take a few weeks to be observable. If the dog is unresponsive to castration, a head collar should be used to increase owner's control during walks. Best results are obtained when castration is combined with obedience training during walks. Response to training alone is usually poor.
Fear aggression can be observed in both males and females at any age. There are two main causes for defense aggression to appear: socialization problems and traumatic experiences. Prognosis is poor for the first and usually good for the second of these categories. Also, a genetic predisposition to manifest fear aggression appears to exist. In some cases, aggression is directed towards any kind of dog, whereas in others the dog reacts aggressively only in front of dogs of a particular size, color or breed.
Dog's posture is the most reliable criteria to identify fear aggression. During aggression episodes the dog adopts a typical defensive posture and tries to hide from the other dog. However, in certain situations the dog's body language can be confusing, trying to jump over the other dog. Even then, a careful observation of dog's behavior often reveals the true motivation of aggression. The dog often moves back and forward, showing a state of motivation conflict. Some authors suggest that barking is more frequent in fear related than in other forms of aggression.
Territorial aggression could also be observed in dogs of both sexes, although it seems to be more frequent in males. Similarly to fear, aggression can be directed towards any or a specific kind of dog. Although aggression is expected to occur when the dog is in its territory, some dogs show this behavior when approached by another dog no matter where they are. Dog's body posture is clearly offensive.
Treatment of fear aggression
Treatment of fear related aggression is based on an especial training program, sometimes together with the use of psychotropic drugs. Castration has no effect in reducing fear aggression.
The first step of the training program is to make the dog respond to basic commands, especially "sit," "stay" and "heel." The use of head collars is extremely useful to control the behavior of the dog during training sessions. Once achieved this step the dog can be exposed to increasing levels of the stimuli that provokes aggression, a technique called desensitization. The owner has to distract dog's attention with food or play. This way the since that moment unpleasant situation should be progressively associated with positive events. This behavior modification technique is called counterconditioning. In some cases if the dog becomes aggressive the owner can punish the behavior by pulling on the leash. If the dog relaxes the owner must reward this alternative response. As already said, this process must be gradual, avoiding early exposure to high intensity stimuli. It is also important to teach the owner not to calm down the dog when aggressive, since that would in fact reinforce the undesired behavior.
Although not strictly necessary, drug therapy could be useful in cases where a strong fear response is present. Amitriptyline and Paroxetine are two of the more frequent drugs prescribed for this problem. The use of benzodiazepines should be avoided, since they can cause an increase in aggression.
Treatment of territorial aggression
Castration is not useful to control territorial aggression. Drug therapy is not also currently recommended, although in some cases the use of fluoxetine could help the dog to assimilate the training program. Such training should start by increasing the owner's control of the dog, especially during walks. Once achieved this objective a desensitization and counterconditioning program should be undertaken. The way to perform this program share the same principles already described for fear aggression. It could be a good idea to start training the dog away from its territory, since that could increase the aggression threshold.
References
1. Beaver BV (1999) Canine Behavior: A Guide for Veterinarians. W B Saunders Company, Philadelphia.
2. Lindsay SR (2001) Handbook of Applied Dog Behavior and Training: etiology and assessment of behavior problems (vol. 2). Iowa State University Press, Iowa.
3. O'Farrell V (1992) Manual of Canine Behaviour 2nd ed. British Small Animal Veterinary Association, Gloucestershire, UK.