What you know is a little dangerous
Americans are traditionally comfortable with laws that prohibit them from taking certain actions such as expectorating in public or conducting insider trading in private. There's often much more concern over the prospect of the government putting specific obligations on us as citizens. A good example of this distinction was highlighted in one of the final episodes of TV's Seinfeld, in which the main characters were arrested for failing to act in a situation where Massachusetts law obligated a witness to act.
A similar legal obligation is quickly working its way to the U.S. Supreme Court for interpretation of its constitutionality. The legislation in question is that portion of the new federal health-care law that requires Americans to purchase health insurance or be subject to a monetary fine. Many legal scholars consider this affirmative mandate to be a violation of the Constitution in that it puts demands on citizens merely as a result of their birth.
Now, contrast this debate with the legal concept of taking an action and thereby causing detrimental reliance on the part of another. This age-old legal principle provides that even though there may be no obligation to act, should a person choose to act, they must do so in a way that does not lead another into failing to receive other assistance they could have obtained absent the first person's act.The drowning problem
The example traditionally used in law school is the scenario where a person is drowning and there are several people on the beach. One big guy swims out to help then discovers that the drowning person is a person he doesn't care for. The rescuer simply turns around and swims back to shore.
By the time others on the beach swam out to help after hearing from the potential rescuer that he refused to try to help, the one in peril had drowned.
Ordinarily, there is no affirmative obligation on the part of anyone who is not a working lifeguard to make every effort to save a drowning person. And if the rescuer were the only one on the beach, he would be pretty much entitled to let the swimmer drown before or after swimming out and recognizing him. However, in this situation, the rescuer took action that led others on the beach to not call emergency services and not try to render help themselves.
To his detriment, the drowning swimmer relied on the act of another.
Therefore, civil, and possibly criminal, liability could follow the rescuer who refused to rescue.
Your patient problem
With these principles in mind, consider the position you're placed in when providing a physical exam or other service to a dog that appears aggressive or otherwise capable of causing injury to its owner or others.
Technically, we are, I suppose, the dog's physician, and our primary responsibility in most circumstances is the care and treatment of our patient, not its handler or owner. It would be easy to ignore the potential impact of disease, personality or abnormal mental state of that patient as it might affect its owner or the public. However, the situation is not that simple, because the general public and the individual pet owner rely on our advice (potentially to their detriment, if we're not thorough). And we may have legal obligations to act when we observe dangerous circumstances which we are trained to recognize.
By way of analogy, consider the obligation of human physicians when they encounter a patient who is diagnosed with a dangerous communicable disease or a mental illness that could pose a danger to the patient himself or to society. Members of the general public have the luxury of ignoring public dangers if they encounter them. Doctors do not share this luxury.
When a physician recognizes a threat such as a positive diagnosis of tuberculosis or plague, the law requires action beyond mere treatment of the affected patient. In the same way, doctors cannot dismiss clear evidence of spousal abuse or child abuse.
The failure to act in such circumstances would probably amount to malpractice, and in some cases, would likely constitute criminal malfeasance. In addition, such a failure to act could also give rise to an actionable claim for damages. Such a claim might be brought by a third party member of the public; it might arise from his or her detrimental reliance on the practitioner who was supposed to behave in his professional capacity as protector of public health and safety.