Medical Causes of Aggressive Behavior in Dogs

Aggressive behavior in dogs can be traced to many causes. Frightened or frustrated dogs and dogs in significant pain can all exhibit aggressive tendencies. When you address the problem of your dog's aggressive behavior your first priority must be to look for a possible medical origin for the difficulty. Once a medical problem is ruled out as the possible cause; you can consult with your vet about techniques of behavioral modification to eliminate the inappropriate aggression. Here are some medical problems you should consider as possible causes if your dog's aggression has become an issue:


Hypothyroidism has been linked with aggressive or anxious behavior in dogs. Hypothyroidism is a disorder that appears when your dog's thyroid gland produces less than the normal amount of thyroid hormone. Look for signs of the other symptoms associated with hypothyroidism such as; weight gain, hair loss or lethargy. If you see these symptoms take your dog to the vet for a blood test of thyroid hormone levels.


This congenital problem is mostly seen in small dogs, and brachycephalics; the short nosed breeds, such as bulldogs and boxers. Hydrocepphalus occurs when fluid filled spaces in the brain called ventricles become enlarged which then compresses or thins the brain tissue around those spaces. Increased aggression is just one of the many neurological symptoms associated with Hydrocephalus. EEG testing or brain scans (MRI, CT) will be necessary for a definitive diagnosis.


Encephalitis is an inflammation of brain tissues. The swelling is usually caused by one of several types of infection which may enter the brain in various ways such as insect bite, or bacterial infection starting in the neck and/or nasal cavities. Herpesvirus has been found to cause Viral Encephalitis but it most often begins with viral canine distemper. Aggression is one of the symptoms observed along with coma, seizures, and clumsy gait.


Epilepsy can be passed on to offspring, and it can be caused by ingestion or exposure to toxic substances like lead paint, fertilizer, pesticides or chocolate. Epilepsy is also associated with trauma and brain tumors. Epileptic seizures symptoms include compulsive ‘chewing gum' motions; violent tremor and long periods of unconsciousness or semiconscious staring. Seizures can also be preceded by periods of disorientation or alarm. Seizures are often followed by a period of what looks like ‘drunken' behavior. Unusual aggression may occur anywhere in this continuum.

Brain Tumor

Aggressive behavior can be one of the symptoms associated with a brain tumor. Other symptoms you may see re seizures, disorientation, and forgetting trained habits or learned behaviors. Your dog may form a new habit of pressing its head against hard surfaces like walls. Walking in circles and other kinds of compulsive walking are commonly reported. Watch for any signs of tremors; wobbling, or loss of sensory faculties. Be particularly alert for any signs of loss of the sense of smell or loss of eyesight.