Medical Causes of Aggression in Dogs

There are certain medical issues which contribute to behavior problems as well as others that are actually instrumental in causing them in dogs. Medical issues should be ruled out when certain factors are involved.  These include obvious problems with a dog’s physical condition (overweight, underweight, excessive shedding, alterations in thirst or appetite, etc.). Behavior problems that occur unusually early in life, especially if the dog is the runt of the litter or problems occurring seemingly out of the blue with no obvious environmental causes could point to a medical problem as well.

Hypothyroidism

Hypothyroidism is a condition in which the thyroid gland produces less thyroid hormone than normal.  Obvious signs are increased body weight, lethargy and hair loss.
 
Somewhere between normal (euthyroid) and hypothyroid, are dogs whose thyroid levels are lower than necessary for optimal function but whose levels are still technically within normal range.  In this situation, only one or two obvious signs of hypothyroidism may be present and only subtlety.  This can contribute to anxiety and aggression.

Congenital or Acquired Neurological Problems

If a dog is born with a neurological problem or develops one through injury or illness, its perception and judgment may be affected, causing inappropriate behavior.

               Hydrocephalus

Hydrocephalus is a congenital condition in which the fluid filled spaces in the brain (ventricles) become enlarged and the surrounding brain tissue subsequently becomes thinned or compressed. The term “water on the brain” has been used to describe this condition in humans.  The dog breeds most commonly affected are toys and brachycephalics (flat faced dogs).
 
There are few is any signs in mild cases.  However, more severe cases of Hydrocephalus are associated with a variety of neurological signs, sometimes including aggression.  The definite test for Hydrocephalus is a Computed Tomography (CT) scan or a Magnetic Resonance Imaging (MRI).

              Bacterial or Viral Encephalitis

Encephalitis is the inflammation of the brain with several causes and can have a variety of neurological signs which may include aggression.

• Idiopathic (unknown)
• Immune-medicated disorders
• Postvaccinal complications
• Viral Infections (canine distemper, rabies, parvovirus)
• Bacterial Infections (anaerobic and aerobic)
• Fungal Infections (aspergillosis, histoplasmosis, blastomycosis)
• Parasitic Infections (Rocky Mountain Spotted Fever, Ehrilchisis)
  
• Foreign Bodies

 
Diagnosis is made by observation of clinical signs and through evaluation of the Cerebrospinal fluid (CSF) within the brain and spinal cord.

              Brain Tumors

A tumor is an abnormal growth of cells and may be classified as primary or secondary.  A primary brain tumor originated from cells normally found within the brain and its surrounding membranes.  A secondary brain tumor, is either cancer that has spread to the brain (metastasis) from a primary tumor elsewhere in the body, or is a tumor that affects the brain by extending into brain tissue from an adjacent non-nervous system tissue, for instance bone or nasal cavity.
 
Brain tumors are diagnosed from clinical signs and by specific neurological testing, with or without ancillary diagnostic aids such as CT scans or MRIs.

               Behavioral Seizures

Partial seizures occurring in a region of the brain that controls aggression (hypothalamus or limbic system) can result in sudden unprovoked aggression. Breeds known for this sudden aggression are Springer Spaniels, Cocker Spaniels, Chesapeake Bay Retrievers, Bull Terriers, Poodles and Golden Retrievers.  Clinical signs of seizure related aggression are:
 

• A mood change just before the seizure
• Sudden violent aggression for trivial or no reason
• Signs of autonomic discharge (salivation, dilated pupils, and evacuation of anal sacs)
• Aggressive posturing, more or less continuous during an attach lasting several minutes, hours or even days

 
Following an attack, affected dogs often appear depressed and lethargic, unresponsive to commands and may stare at a wall or simply sleep.  Diagnosis is made by observation of clinical signs, Electroencephalogram (EEG) or MRI.