The issue of dog aggression comes up time and time again. If you are dealing with aggression in a dog, there are many sources out there to help you.
In a nutshell, the first step is to discover if this is a behavioral or physical issue.
Some dogs reach a certain age and try to change their place in the pack by challenging others. Or by challenging the humans. It is important to maintain the pack structure with the human as the pack leader to keep everyone in balance and the family in harmony. Being a pack leader doesn't mean that the human yells, shouts or hits. They maintain structure by teaching the pack members the rules of the pack (sit politely, respect boundaries, etc.). If there is not a human who adequately guides the pack, then a dog will rise into that position. They might not even want to be alpha, but they realize that the structure is missing and someone has to provide it.
A class with a qualified behaviorologist will help to determine a course of action for obedience training which would also give helpful pointers to the human so that the human better understands his/her role. Another great resource is the NILIF program easily found on the internet. It is based on the premise that nothing in life is free. You must sit politely as I prepare your food and only eat when I say ok. You will wait behind me as I go through the door and only exit with my permission. There will be time spent up front as you and your dog go through these exercises, but once the roles are firmly established, it will make for a smoothly run and happy household for many years to come. I would say we spent the first year with our springer spaniel working on the basics, but we had a well-behaved and happy dog for the next 13 years who earned compliments everywhere she went. We didn't know about NILIF at the time, but we worked with basic obedience and the rules that worked well for our family.
There are also internet groups through forums such as Yahoo and MSN where you can find qualified trainers and behaviorologists who will answer your questions online and help point you in the right direction.
Sometimes aggressive behavior is exhibited when the physical body is not right. A dog in pain, for instance, will lash out at anyone trying to touch him because the touch results in more pain and he's trying to warn the person away. This is an obvious case of physical problems, but what about something a bit more deep-seated and harder to determine?
Many people who have studied dog health and nutrition have found that there are two things which can help spur a sudden and unexplained display of aggressive behavior.
Thyroid disfunction can be a cause of aggression in dogs. You might even see other thyroid issues (unexplained fears, poor quality of fur, etc). Request that the veterinarian either run a full 6-panel thyroid test to determine if this is the issue, or have the blood drawn and send the sample to Dr. Jean Dodd's lab for anaylsis. Dr. Dodds is the leading expert on thyroid issues in canines and has an extensive background of knowledge as well as a database used for tracking trends in specific breeds.
Another major culprit in aggressive behavior is vaccinosis from over-vaccinating our animals. Most often vaccinosis displayed as aggression is associated with the rabies vaccine. Resolving this will require the assistance of someone skilled in healing. Most frequently homeopathy seems to have better results, although in most cases no matter the therapy, the aggression is minimalized and never thoroughly gone. This is why so many people are very upset that rabies is mandatory even when titer tests indicate that the immunity level for the virus is still very effective. In an effort to minimize the risk to our animals from these terrible side effects, the rabies challenge fund has been established to document and prove the effectiveness and long-lasting protection of the shots.
There are many resources out there and I strongly encourage a person to exhaust all efforts at attempting to work with their dogs in these issues. Sometimes we've done all we can and it is not enough. I read a story of a large powerful dog who was rescued and worked with, yet attacked and bit the elderly mother of the woman who took the dog in. They were forced to euthenize and found out later through an animal communicator that the dog had been kicked by someone in the head before it came to them. The pain in the dog's head was intolerable (like something wasn't attached right anymore) and it felt like it could not live with it. It appreciated the love from the family and was trying hard to fit in, but just couldn't take the pain and bit the mother, knowing what the result would be. Sometimes there can be a higher purpose in the events surrounding the experience and all we can do is our best to help those in our care.