Pain is contagious, at least for mice. After encountering bedding where mice in pain had slept, other mice became more sensitive to pain themselves. The experiment, described online October 19 in Science Advances, shows that pain can move from one animal to another — no injury or illness required.
The results "add to a growing body of research showing that animals communicate distress and are affected by the distress of others," says neuroscientist Inbal Ben-Ami Bartal of the University of California, Berkeley.
Neuroscientist Andrey Ryabinin and colleagues didn't set out to study pain transfer. But the researchers noticed something curious during their experiments on mice who were undergoing alcohol withdrawal. Mice in the throes of withdrawal have a higher sensitivity to pokes on the foot. And surprisingly, so did these mice's perfectly healthy cagemates. "We realized that there was some transfer of information about pain" from injured mouse to bystander, says Ryabinin, of Oregon Health & Sciences University in Portland.
When mice suffered from alcohol withdrawal, morphine withdrawal or an inflaming injection, they become more sensitive to a poke in the paw with a thin fiber — a touchy reaction that signals a decreased pain tolerance. Mice that had been housed in the same cage with the mice in pain also grew more sensitive to the poke, Ryabinin and colleagues found. These bystander mice showed other signs of heightened pain sensitivity, such as quickly pulling their tails out of hot water and licking a paw after an irritating shot.
The results are compelling evidence for the social transmission of pain, says neuroscientist Christian Keysers of the Netherlands Institute for Neuroscience in Amsterdam.