Thin ice.

I think it's a stretch to argue for a "placebo" effect in pets, and certainly not in the general animal population that has been used in animal studies. Placebo, as understood on a common-sense level, is the effect of "believing" that some ritual, whether it be taking a pill, or any act or series of actions, indeed, has the power to effect a positive change in bodily function, or dysfunction. There are enough studies in humans to show such a link in humans. The placebo effect, aside from the "fear" of it affecting scientific data, long predated any studies to confirm it, formally. It has been a common criticism of any treatment outside the mainstream of modern medical treatments and has also been used by doctors with some patients; especially those patients considered to have conditions that existed only in their minds.

I seriously doubt that rats and mice, commonly used in medical testing, were stroked and comforted as a pet would be. Even the idea, as argued here, would need to be avoided for the very reasons implied here. One such study for the, still, controversial treatment, acupuncture, (http://www.sciencedaily.com/releases/2011/12/111219150915.htm) doesn't conjure up visions of technicians petting and comforting the rats during the testing. One might contact the authors and find out if any such comfort was given to the rats in the study.

Submitting the idea of a "placebo-like" effect, is just an extension of what "seems" logical. (The stroking could be a whole other study in fact -- mechanical vs. human or another animal, even.) I would argue, that, at best, the stroking is a transference of the emotion of love, not a transference of the owner's belief in the treatment. Even a creation of some kind of "faith" on the animals part would still not be connected to any "belief" by the animal in the efficacy of the treatment.

Considering the vast number of studies with animals, offering comfort as part of the interaction with the animals, during monitoring physiological parameters, is extremely doubtful. If so, the authors should mention it as part of the methodology.

Most medical practitioners, whether physicians for humans OR animals, still, feel a need to show that everything must be explainable by "science" or scientific thinking -- or that, that is their guide. Actually, there are holes in that adherence, such as, the study of vitamin C -- or lack thereof. (See http://orthomolecular.org/library/jom/1991/pdf/1991-v06n02-p099.pdf)

Physical contact affecting an animal isn't the same as placebo in the classical sense. I don't doubt the effect at all. But it isn't placebo.

The whole issue of placebo effect in animals could be cleared up by finding a competent animal psychic. ;)