Itself a symptom of many physical ailments, the most notable that stand out in my mind are Distemper, Parvovirus, and Coronavirus. Because of the potentially fatal nature of these diseases, you can understand why I am so adamant at having your pet checked by a vet. Sometimes depression is the only symptom you notice.
This can be caused by so many problems. Pain is frequently the cause of moodiness and irritability in a normally pleasant pet, but often locating the source of discomfort requires veterinary help, and often x-rays.
Rabies also cause seemingly unexplainable aggression. In this case, this disease is fatal, and also highly contagious. Aggression can also be caused by brain tumours, neurological disorders, or quite possibly it may not be aggression at all, but seizure activity.
Please rule out all physical ailments with the help of your veterinarian!
Introduction to Canine Mental Health
Do Dogs Get Depression?
When Anxiety Attacks
Biting the Hands That Feed You
Obsessive Compulsive Behaviors