The spirochaete Brachyspira hyodysenteriae causes swine dysentery, a potentially severe mucohaemorrhagic colitis of pigs that is endemic in many countries. Swine dysentery depresses growth, and can cause disruptions to pig trade and potential litigation if herds supplying replacement animals are infected. Diagnosis is based on clinical signs, typical colon pathology and the detection of B hyodysenteriae . This letter highlights the increasingly common situation where B hyodysenteriae is detected in apparently healthy herds. In one likely scenario pathogenic …