Untangling the role of environmental and host-related determinants for on-farm transmission of verotoxin-producing <i>Escherichia coli</i> O157

<b>Background:</b> Cattle colonised by the zoonotic pathogen verotoxin-producing Escherichia coli of serotype O157 (VTEC O157) can shed high levels of the pathogen in their faeces. A suggested key for controlling VTEC O157 is preventing colonisation of individuals. <b>Aim:</b> In this study the role of individual super-shedders and factors related to susceptibility and environmental exposure in the transmission of VTEC O157 among dairy calves are explored. <b>Methods:</b> The association between sex, age, pen hygiene, pen type and stocking density and colonisation of individual calves, established by recto-anal mucosal swabs, on farms where pathogenic VTEC O157 had been confirmed was investigated. In a follow-up sampling, the consistency of previously identified risk factors and the role of shedding pen mates was assessed by studying the risk of new/re-colonisation. <b>Results:</b> The results suggest an important role of stocking density that decreases with age, possibly due to increased resistance to colonisation following exposure. However, previous colonisation did not influence the risk of being colonised in the second sampling. Super-shedders (shedding >103 colony forming units/g faeces) significantly increased the risk of colonisation in peers (OR = 10, CI 4.2-52). In addition, environmental factors associated with survival of the bacteria, affected risk. <b>Conclusion:</b> The results confirm the suggested importance of super-shedders but also emphasises the importance of considering the combined exposure from peers and the environment.