Homeopaths insist that their potions cannot be just causing a placebo, because they (supposedly) work on animals, and animal’s are “immune to placebo.” This is a failure to understand the nature of placebo, and it is insulting to the intelligence of everyone concerned, including the dogs, cats and horses (who deserve real care and medicine, by the way).
There is plenty of room for bias and placebo in the human observer of the animal. (Remember, placebo includes all non-specific effects and artifacts of investigation, not just mind-over-symptom effects.1) People know almost nothing about the animal’s experience, and nothing at all that doesn’t come through the filter of (intense) human interpretation. Anecdotes about animal healing, if anything, are actually more likely to be warped by bias than personal reports, not less.
And who says animals can’t be directly influenced by human hope and expectations and feelings? Have you ever watched The Dog Whisperer? It’s absolutely amazing how dogs are like mirrors for our souls. After watching a few episodes, I have no doubt at all that a dog might very well enjoy a placebo from a hopeful human taking charge and confidently giving a remedy!