A common reason people give as "proof" that various questionable therapies work is that "animals do not get placebo effects, because they don't know they are supposed to feel better".
This seems to make sense-animals do not "know" that a given treatment is supposed to do something, and I have been known to say this myself in years past. However, there are several reasons why this view may not be accurate.
One reason is conditioned responses, otherwise known as a Pavlovian response. Animals learn to respond in certain ways to actions or treatments. This response becomes automatic or subconscious. Humans are susceptible to conditioned responses just as animals are, and a conditioned response may affect our expectations or the way we perceive a given situation.
It does take time for a conditioned response to develop, but it may happen fairly quickly, sometimes over the course of a few treatments. When combined with effective treatments, regression to the mean and/or the normal healing process, the effect can seem dramatic. We (both owners and veterinarians) have a tendency to think that what we are doing is helpful because we want it to be helpful. This is a normal part of human nature, and is why randomized, blinded and controlled trials have been devised to help eliminate sources of error and bias, and why such trials are considered stronger, more reliable evidence than case reports, anecdotes, and unblinded, uncontrolled trials.
Another thing that has been demonstrated to affect both animals and humans is expectation.
It is not unusual for dogs to be more active and lively even after a simple, quick treatment like a vaccination. The dog has learned that a trip to the vet involves a brief, mildly unpleasant episode, then often a treat or car ride that the dog enjoys. The same expectation on the human owner's or veterinarian's part of improvement can also affect the way we perceive how a treatment is working.
What is the evidence that this expectation on the part of a caregiver can create a "placebo effect" in the caregiver as well as the patient? There have not really been any studies done in the veterinary field, but a some studies of preconceptions held by parents demonstrate how such an expectation may affect how caregivers can be affected by treatments. This report discusses how the common perception that sugar or other foods cause hyperactivity in young children is unfounded, and a recent review shows how parents, teachers and medical personnel can be effected by knowledge and expectation of treatment. I have seen several clients give their pets rescue remedy, a bach flower remedy which is a type of homeopathic remedy to their pets to help with anxiety just before a visit. I have always thought that the remedy seemed to calm the owner down more than the pet, and these reports help to explain why that may be.
A reasonable understanding of these effects on caregivers should instill some humility in any caregiver, and a critical analysis of how effective any treatment is. Unfortunately, such humility and critical analysis seems to be nearly entirely absent in many practitioners of alternative treatments and often in conventional practitioners as well. The difference between the effective treatments that have been developed over the last century or so and ineffective alternatives is testing and analysis by the scientific method. If someone claims that a treatment is not testable by science, or that science cannot detect the effect, that should be a red flag that the claim that the treatment works is bogus.