Should Diesel die because of something he might do in the future? How difficult would it be for you to use euthanasia to break a bond with a beloved pet in the belief that this choice is better than the alternative?

Diesel, a 14-month-old, 50-pound neutered male German Shepherd mix was rescued from certain shelter death by Rob and Melissa Belaver. When greeting people he knew, Diesel was so friendly he wagged his entire back end and leaked a little urine.

When Diesel began showing aggressive threats, the Belavers called a dog trainer. At one session the trainer was with Diesel in the back yard, kneeling next to him petting him when he suddenly bit her face. Not surprisingly, the trainer quit the case. Rob then went to his veterinarian, Dr. Moreno for help. In turn, Dr. Moreno asked for my help on the case as a veterinary behavior consultant.

Diesel's medical history revealed a normal urinalysis, but an ongoing tendency to eat grass resulting in frequent vomiting. At my request, Dr. Moreno did a new physical exam, lab work, and a behavior assessment using tests that I provided.

At the beginning of the behavioral assessment Dr. Moreno gave food treats, which Diesel happily ate while wagging his tail. Diesel watched Dr. Moreno's closed hand with the food indicating quick learning. He was willing to move away from the owners to obtain the food, suggesting confidence. Dr. Moreno and an assistant applied a muzzle and continued the behavioral assessment in an attempt to learn Diesel's full responses. During the "please roll him over to look for fleas" test, Diesel suddenly lunged at the owner (while still wearing the muzzle) — but did not connect — and immediately pulled back, indicating he has some control over his aggression.

Further behavioral assessment tests showed a normal response to sudden noises, but exaggerated whining and pacing when the owner was asked to leave the room for 30 seconds. The neurological evaluation was normal. The dog's basic obedience ranged from good for SIT, to poor for DOWN. Without a food lure, he stayed hidden cowering behind the owner. The "menace" test (pretend to face slap, but don't touch) was deemed normal reflex blinking. The most common body language was tense muscle tone with ears back and tail down, all suggesting fear. The veterinarian's impression was that during the evaluation Diesel was nervous, tense, fearful, and reactive except when food treats were offered.

The results of Dr. Moreno's and my behavioral consultation with the Belavers revealed neophobia (fear of anything new), territorial aggression, fear aggression, possessive aggression, separation anxiety, excitatory urination, destruction, pulling on leash, hyper-reactivity, and overall unruliness in the home.

It turned out that the Belavers also had a 7-year-old son, Shawn. The boy and the dog had played together fine so far, but Diesel had threatened Shawn when the boy came near his food. Because of the aggression toward unknown people, I felt there was an even greater risk to Shawn's friends.

I learned there had been other times that Diesel had bitten people, but the only skin puncture was with the trainer. When aggressive, he did not give any warning, and the context was unpredictable. The severity of the bites was escalating.

My report to the owner and the veterinarian contained some positives. The fact that Diesel frequently ate grass and vomited suggested there may be an underlying medical condition resulting in frustration that influenced his aggression threshold. The temperament testing done in the vet setting may have been affected by previous negative experiences in that room. Diesel appeared to have a normal peripheral nervous system, did not have evidence of beatings (because of his normal menace test), and he quickly learned to follow a closed hand with treats. His motivation for food treats represented good potential for reward-based learning.

With medical treatment of his vomiting, professional training, plus medication for his anxiety I felt we could manage his neophobia, separation anxiety, excitatory urination, pulling on leash, territorial displays, destructiveness, and overall unruly behavior. However, Diesel had crossed a line — he had unpredictably punctured human skin without warning.

In my lexicon, aggression is defined as "violence, or the threat of violence." This means that lip-lifting, growling, lunging, and snapping are all part of aggression, even if there is no contact. My definition of a bite is "tooth touches skin." For every dog bite I ask these questions: Was there any warning? Did the tooth touch the skin? Did it leave a mark? Was there a skin puncture? Were there multiple bites? Multiple episodes? Was there a puncture and hold with shaking? Was the human face or throat a target? Was human medical treatment necessary? Was there any permanent damage or fatality?

In this case, I feel it is an unacceptable risk having a young boy in a house with a strong dog of any breed, that attacks without warning, and punctures skin. I advised the veterinarian that there might be someone who would work with this dog, but the risk of injury was high, and the prognosis guarded. I encouraged a second opinion, but gave mine. I felt that with treatment there was a 25 percent chance this dog could be made into a reasonably safe pet, and a 75 percent chance that either Shawn, or one of his friends, or someone else might be severely injured by this dog. The family decided to have Diesel humanely euthanized.

Some might blame the breeder, who bred a dog with genetic problems like submissive urination, and an unstable personality. We might blame the first owner, who did not socialize Diesel by taking him to puppy classes, and teaching him bite inhibition — by stopping play every time he bit too hard in play. We might even indict the former veterinarian for not advising the owners on aggression prevention techniques for breeds with a reputation for aggression.

However there are some heroes here as well. I applaud Dr. Moreno for a proper evaluation, and I have high respect for the Belaver family. First they attempted to give a home to an itinerant dog. They gave food, shelter, and love and attempted to train the dog while tolerating more than many families would accept. Before giving up they gathered multiple professional opinions, and in the final analysis, made a difficult and painful decision that probably saved their own child or another human from a tragic moment.

Dr. Rolan Tripp is a veterinarian and animal-behavior consultant. He is the founder of www.AnimalBehavior.Net.