Bits have the potential to disrupt the breathing mechanism in horses, a recently published review says. Researchers David Mellor and Ngaio Beausoleil explore what they describe as breathing, breathlessness and bridles in their wide-ranging review of equine welfare during exercise. The pair, from New Zealand’s Massey University, said horses showed clear signs of aversion to having a bit in their mouths, varying from it being a mild irritant to very painful.