You may not understand it, but spend some time at the library researching Vet journals and you'll see that it exists.
And that doesn't even begin to deal with the so-called nocebo effect.
http://www.salon.com/health/feature/1999/07/15/nocebo/print.htmlWhich, uh, also appears to affect animals:
"To Moerman, the most intriguing part of the PBB episode may come from the cows. He recently charted milk production in Michigan cows in pounds of milk per cow per year ("I had never really realized how wonderful agricultural statistics are") and found a sharp drop in milk production in 1974. "It drops by 10 to 12 percent. It just goes 'kerchunk' and it drops. Ninety-nine percent of the cows never got any of this PBB, but their production of milk dropped anyway. Well, you know cows don't read the newspaper. But what do cows do?" Moerman asks. "Hang out with farmers?" I guess. "That's right. They hang out with farmers. I think we have a nocebo effect in domestic animals."
Moerman intends to research this intriguing area further before publishing, but the working hypothesis is that the farmers' concern about their cows, who for all they knew had been gobbling pure poison, was somehow communicated to the cows, who responded by slacking off on the milk production. He also points out a difference between nocebo and placebo.
Rats are apparently as vulnerable as cows. In one experiment, which didn't set out to be about nocebo, rats were given sweetened water to drink and then given shots of cyclophosphamide, which causes nausea and also induces sometimes fatal immunosuppression. To the experimenters' surprise, rats who only got one shot of cyclophosphamide but kept on being fed the sweetened water continued dying at a high rate. The rats associated feeling dreadful with the water, and so as long as they got the water they felt dreadful -- even unto death."