Interdog Aggression: When to Intervene?
World Small Animal Veterinary Association World Congress Proceedings, 2007
Diane Frank, DMV, DACVB
Université de Montréal, Faculté de Médecine Vétérinaire, Centre Hospitalier Universitaire Vétérinaire
St-Hyacinthe, QC, Canada

Are all cases of inter-dog aggression normal interactions? Is obedience training sufficient or should we medicate? If so, how long should we medicate? Is it the client's fault or the dog's "fault"?

Body Language and Behaviour Sequence

Behaviour is always a sequence. Important questions include asking whether aggression was triggered by specific behaviours on the part of the receiving "victim" dog. Was communication between the two dogs clear and appropriate? Description of the body language is important to assess relative roles played by fear, anxiety or predatory behaviour during aggressive events.


Context is also important to determine if behaviour is normal or illness-related. A dog attacking another dog without prior interaction (or communication) is ill. A patient that is unable to make the distinction between threat and non-threat is ill. Generally adult dogs are very tolerant of puppies. An adult dog attacking a puppy that was not interacting with the former is generally not normal!

Intensity or Severity

Dogs can bite without contact (snap) or bite with contact (some may or may not control pressure applied at the time of the bite). Severity of the bite has to be appropriate given the context. A dog causing serious injuries to another dog that had submitted is likely ill or presenting predatory behaviour.


A dog challenging another dog expects a response. If the receiver submits or defers, one does not expect the challenger to repeat the same threat or sequence. If a dog repeatedly challenges another dog and the recipient is always submitting, we are dealing with a sick dog. A normal dog will not repeat the exact same behaviour over and over again to always obtain the same and only response (submission).

Normal Behaviour (Household Dogs)

Several scenarios are possible: The first involves a young animal reaching social maturity (18-36 months) and challenging the older dog. In this scenario, a dog that has been challenged can respond 1) by immediately deferring or submitting, 2) by not submitting initially but eventually doing so if the challenge is intensified, 3) by fighting, with one dog winning and both dogs accepting the outcome or 4) by fighting, and neither dog yielding to the other. These dogs are following the rules of social canine communication and exhibiting complete unaltered behaviour sequences. These dogs are behaving normally and owner intervention will perhaps only be necessary in the last example. These cases are rarely seen in referral practice.

Abnormal Behaviour (Household Dogs)

A different scenario is an older dog perceiving the younger dog as a threat even though the younger dog has not challenged or threatened the older one. Another scenario could involve one dog challenging the other with the latter deferring but the challenger still attacking and injuring the victim. An anxious dog "worried about everything and anything" and simply redirecting the aggression to another household dog is another possibility. These last three scenarios are examples of illness-related behaviours of at least one of the dogs involved. In these cases, clients eventually consult because the fights become more frequent and severe. Treatment will differ in cases involving patients that are normal and patients that are ill.

Most inter-dog aggression cases seen at the Veterinary Hospital of the University of Pennsylvania (VHUP) Behavior Clinic occurred between housemates. Some dogs were aggressive both to familiar and unfamiliar dogs.

Predatory aggression is occasionally a cause "of inter-dog aggression" between housemates or towards an unfamiliar dogs. The behavioural sequence and the history allow us to confirm that diagnosis.

Aggression to Unfamiliar Dogs

Aggression to unknown (unfamiliar) dogs can occur if one dog is anxious (or fearful) or because it perceives a threat or social challenge. This challenge or threat may be real or perceived. The fearful dog may react spontaneously or may have had a traumatic previous experience. It may have been attacked by an unfamiliar dog or may have had inadequate socialization. Again one must detail the behaviour sequence of our patient. Was he responding to a real threat? Or was it a perceived threat that is compatible with illness? General treatment recommendations are similar to those described for fighting between familiar dogs. An anxious-ill dog may be overtly aggressive to non-familiar dogs and may be threatening the other household dogs without the client's knowledge... Clients often miss silent signs of threats and challenge. These can include blocking access to a location, stealing objects, posturing in a ritualized display where the challenger approaches the other dog's shoulders in a perpendicular manner (T-challenge). Video clips will illustrate some of these silent threats. The situation may in fact be worse than what the client has perceived.


Testosterone stimulates dogs to roam and urine mark and facilitates fighting. Although castration greatly decreases roaming, urine marking and fighting between dogs and appears to be effective in about 60 % of dogs, it is not a cure for all cases of inter-dog aggression. In fact most of our patients presented as a referral for inter-dog aggression have already been neutered. So how do we treat them?

Traditionally, emphasis was placed on identifying the dominant dog and reinforcing the dominant status. But ill dogs no longer communicate normally. They also perceive or anticipate dangers or threats that are real or imaginary. Not all inter-dog aggression cases are due to hierarchical conflicts. The ideal situation is to be able to visualize the behavioural sequence (directly or on video). Is each dog signalling appropriately and is each dog reading the signals properly?

Treatment of inter-dog aggression with normal dogs that are fighting is easily achieved by establishing "hierarchy". The dog that is physically most likely to win will be fed, walked, or given attention first. If the fighting only occurs over special food items or toys, these triggers can simply be removed from the environment.

Treatment of anxious (ill) dogs includes pharmacological intervention, behaviour modification, and a systemic approach. Clients are instructed not to use body parts to separate fighting dogs. We are no longer talking about a dominant or submissive dog but instead we are considering behaviours that are normal or illness-related. Dogs are separated when supervision is impossible. The dog behaving appropriately (and normally) is reinforced. This dog receives everything first. He gets attention, food, treats, first. Each dog must sit before any interaction with any given person takes place. They can be fitted with head collars (Gentle Leaders®). Eventually the dogs are taught to sit in each other's presence and are rewarded for being non-aggressive in each other's presence. The aggressor can be medicated with clomipramine, fluoxetine, or fluvoxamine. None of these drugs are labeled for use for aggression in dogs. Complete blood cell counts and biochemistry panels are always done prior to prescribing medication for behavioural conditions. Occasionally victims also require medication. Duration of pharmacological therapy varies from case to case.


This approach distinguishing normal patients from ill dogs (anxiety-related disorders) can simplify decisions in terms of treatment. Questions such as which dog should be favoured or which dog should be medicated are easily answered.


1.  Overall KL, (1997): Clinical Behavioral Medicine for Small Animals. St. Louis, Mosby-Year Book Inc.

2.  Shyan MR, Fortune KA, King C., (2003): "Bark parks"--A Study on Interdog Aggression in a Limited-Control Environment. Journal of Applied Animal Welfare Science, 6 (1), 25-32.

Speaker Information
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Diane Frank, DMV, DACVB
University of Montreal
QC, Canada

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