Aggression in dogs can have a medical base. Medically-induced aggression can result in growling, snapping, showing teeth and positioning the body in a dominant stance. Medical causes of aggression in dogs include hypothyroidism, hypoglycemia and brain inflammation.
Brain inflammation that occurs in conditions such as encephalitis is one of the most serious medical causes of aggression in dogs. Rabies and distemper are both types of viral encephalitis. In cases of brain inflammation due to diseases such as encephalitis, the aggression is usually a neurological reaction to the affect of the inflammation on the brain.
Hypoglycemia, or low blood sugar, is another possible cause of aggression and may or may not be related to diabetes. Dogs with low blood sugar may feel disoriented and strange. If they feel vulnerable, they may behave aggressively in order to avoid appearing weak. Dogs with low blood sugar may appear glassy-eyed with a staring look. Weakness or collapse may occur. A veterinarian can perform a simple blood test to diagnose hypoglycemia.
Hypothyroidism is one of the most common medical causes of aggression in dogs. Very low thyroid levels affect mood. Thyroid replacement therapy has worked to relieve aggression in dogs with hypothyroidism in some cases. Weight gain, lethargy and the loss of a large amount of hair are other symptoms of hypothyroidism.
Over 50 different breeds and cross breeds are susceptible to hypothyroidism, and it is one of the most studied causes of dog aggression. Behavioral changes, especially aggression, are often the earliest signs of this condition. Even a dog that was once friendly to strangers may become aggressive if it has extremely low thyroid levels. A veterinarian can conduct a blood test to diagnose hypothyroidism.
Medical causes of aggression in dogs may sometimes be more subtle than growling and snapping. A dog may suddenly begin to stand on people's feet or block doorways as a way of exerting dominance. Cognitive Dysfunction Syndrome (CDS), sometimes called "dog's Alzheimer's disease," is thought to be another cause of aggression. CDS may lead to behavioral changes due to the loss of brain cells caused by brain degeneration.