The moment of a pet's euthanasia nearly always is difficult for the owner and is troubling for the veterinarian as well. While the doctor is placed in a role in which he or she must demonstrate compassion and share to an extent in the grief of the moment, often there are a number of other concerns swirling in the doctor's head.

The veterinarian must rapidly and thoughtfully review whether he/she provided sufficient information about possible further treatments. There are documents to be signed and, in some cases, an account receivable arising from prior treatment to be considered. And of course, there are the matters surrounding public-health compliance and the credibility of the owner presenting the pet for euthanasia.

Often these issues are more straightforward if the pet is being put to sleep after a long course of medical or surgical therapy. In that instance, the doctor and the staff know the circumstances leading to the decision.

'Constructive knowledge'

As anyone who watches "Nancy Grace" or "Law and Order" probably is aware, lawyers are always throwing around the phrase, "known or should have known." This term implies the application of a concept referred to as "constructive knowledge."

Legal liability often attaches to an act or omission based on whether the person involved knew certain circumstances existed (such as faulty brakes on a car) prior to an outcome (such as a collision). If a new car fails to stop properly and kills someone, it probably will amount to an "accident" (as far as the driver is concerned) if the factory left off one of the brake pads. There was no way for the driver to have known.

On the other hand, if a person buys a junky "fixer-upper" and the same fatality occurs due to faulty brakes, that event would probably not be deemed an "accident." Rather, the death might be considered a negligent homicide on the part of the car's owner/driver because he "knew" the car had been sitting in a field for years and therefore "should have known" that the brakes likely were unreliable.

In short, the doctrine says that you can't avoid liability for acting imprudently simply by closing your eyes and hoping for the best.

Keeping this doctrine in mind, consider the circumstances where a client you have never seen before comes in to have his dog euthanized. He tells you that the dog has become aggressive with his kids but hasn't bitten anyone. The euthanasia form is signed, the bill paid and off goes the owner.

So here is a situation where the owner signed a form stating that the pet has not bitten anyone in the last two weeks. Yet he is in what seems to be a real hurry to put the dog down. Could there be more to the story? How much more questioning is the veterinarian obligated to undertake in the face of a signed authorization?

Is there something the vet should have "constructively known?" Was the owner wearing a bandage on his hand? Might there be a bite victim at home that Dad doesn't want to mention because of potential hassles from the health department or Child-Protective Services?

Maybe the owner was lying about some recent bite because he is violating a court-ordered pet confinement by having the dog at his home in the first place.

The constructive-knowledge doctrine is a slippery concept and at times can be hard to comply with.

In the example, perhaps more should be known about that dog's prior medical treatment. Has the dog's regular vet been bypassed because she knows that there has been a recent bite requiring reporting and/or confinement?