If your dog becomes aggressive, don’t assume it’s a behavioral problem or that he’s just being “bad”. It could mean he’s ill or in pain.

Jake was a cheerful, loving dog. The Shih tzu cross was friendly with everyone and enjoyed romps at the local dog park. Then one day, without warning, he became aggressive and bit his person, Meg, when she tried to pet him. Hurt and shocked, she took Jake to the vet where she learned he had a painful ear infection that made him sensitive to touch. With the proper treatment, Jake was soon back to his sociable and affectionate self.

Not all dogs are as fortunate as Jake. Every year, thousands of aggressive dogs find themselves in shelters because their families assume they’ve developed behavioral problems that can’t be fixed. Many of these dogs are euthanized because they are deemed untrainable.

Whether a dog’s aggression occurs suddenly or develops gradually over time, it’s important to consider the possibility that the cause might be physical rather than behavioral. In fact, more than 50 medical conditions can turn Fido into Cujo. They include injury, arthritis, congential defects, oral problems, ear infections, diminishing eyesight and more. Behaviors arising from such physical problems can include “growling, baring of teeth, and tail tucked between the legs if the dog is fearful,” according to veterinarian Dr. Mark Newkirk. “The tail may also be up or straight out in a dominant position.” Snapping and biting may also occur, depending on the problem.

Because we usually associate these postures with anger or fear, it’s natural to assume they spring from behavorial rather than physical causes, especially if there are no other visible symptoms. But before seeking the help of a trainer or behavior specialist, have your dog seen by a veterinarian to rule out a physical condition that might be causing him pain or discomfort.

Brain effects

Along with conditions that cause discomfort leading to aggression, there are many others, such as the following, that actually have a physical or neurological effect on the brain. With some of these diseases, the accompanying symptoms make it obvious the dog is sick and not just acting out; with others, though, the signs may be more subtle.


Encephalitis is a brain inflammation that can result in aggressive or fearful behavior. It can be either viral or bacterial, and can arise from diseases such as distemper or rabies. The causes of this condition are usually associated with infection, says Dr. Newkirk. “The behavior depends on the location of the infection,” he says.


Hypothyroidism is caused by an abnormally low level of thyroid hormones and affects more than 50 purebreds and crossbreeds. Symptoms include weight gain, hair loss and behavioral changes, including fear and aggression. Dr. Newkirk states that aggressive behavior in dogs with hypothyroidism is due to a special form of thyroid disease, called autoimmune thyroiditis.

Cognitive dysfunction syndrome

Cognitive dysfunction syndrome can arise as an animal grows older, and is essentially the equivalent of Alzheimer’s in humans. Over the age of ten, 50% of canines will display symptoms such as memory loss and disorientation in familiar surroundings. This decline in mental faculties may cause your old friend to become aggressive at times. “These dogs are treated with diet, nutritional support and medication such as seglinine,” says Dr. Newkirk.


Hypoglycemia is low blood sugar. Symptoms include muscle weakness, glassy eyes, collapse, disorientation and tremors. The dog may become fearful or snappy. If he exhibits any of these symptoms, get him to a vet. Natural, long-term care for a hypoglycemic canine involves feeding smaller meals throughout the day, rather than one large meal in the morning and at night.


Hydrocephalus basically means water on the brain, though the “water” is actually excessive quantities of cerebrospinal fluid caused by overproduction, obstruction or reabsorption. Dogs who suffer from this condition will show signs of irritability, knuckling of the paws, will sleep all the time, suffer seizures and have a clumsy gait. Hydrocephalus occurs most commonly in brachycephalic breeds such as pugs bulldogs, boxers and Chihuahuas – breeds known to have dome-shaped skulls or short noses. Toy breeds can be afflicted as well.

Epilepsy and seizures

Epilepsy and seizures can be difficult to diagnose because their cause can arise from a variety of problems, such as diabetes or hypoglycemia, liver and kidney problems, infections, tumors and brain damage. Dr. Newkirk explains that aggression brought on by seizures occurs just before or after the attack. Diagnosis is done with both physical and neurological exams. Further testing such as blood work and radiographs can also be used. While there is no cure for epilepsy, medication can help minimize the frequency, duration and severity of the seizures.

Brain tumors

Brain tumors not only lead to aggression, but can alter your dog’s mental status with symptoms such as irritability, confusion, increased barking or whining, trouble seeing, excessive circling and a decreased appetite. “Herbals, homeopathy, neoplase and Poly-MVA can be used to treat brain tumors holistically,” says Dr. Newkirk.

Head trauma

Head trauma is fairly common when it comes to our sometimes accident-prone companions, whether they’ve been hit by a car or have fallen from a height. Head trauma of any kind can cause swelling or excessive bleeding, which interferes with normal brain function and can cause your dog to become aggressive. Additional symptoms to look for are loss of consciousness, change in pupil size, bleeding from different orifices, seizures and a tilted head. Veterinary attention should be sought immediately.

Rage syndrome

Rage syndrome, also known as behavioral seizures, causes partial seizures in the part of the brain that controls aggression. A sign of this condition is when a dog suddenly becomes aggressive, but in the next moment is back to his calm normal self. A veterinarian who specializes in neurology is best equipped to diagnose this condition, using an EEG and genetic testing.

Any unexplained aggression in your canine companion should be evaluated by a veterinarian. Chances are it’ll turn out to be something simple that’s making him cranky, like an ear or gum infection, but it’s best to be sure he’s not suffering from something more serious. Either way, he’s not just being a “bad” dog.