Persistent cross-species transmission systems dominate Shiga toxin-producing Escherichia coli O157:H7 epidemiology in a high incidence region: a genomic epidemiology study
BackgroundSeveral areas of the world suffer notably high incidence of Shiga toxin-producing Escherichia coli, among them Alberta, Canada. We assessed the impact of persistent cross-species transmission systems on the epidemiology of E. coli O157:H7 in Alberta. MethodsWe sequenced and assembled 229 E. coli O157:H7 isolates originating from collocated cattle (n=108) and human (n=121) populations from 2007-2015 in Alberta. We constructed a timed phylogeny using BEAST2 using a structured coalescent model. We then extended the tree with human isolates through 2019 (n=430) to assess the long-term disease impact of locally persistent lineages. Shiga toxin gene (stx) profile was determined for all isolates. ResultsDuring 2007 to 2015, we estimated 108 (95% HPD 104, 112) human lineages arose from cattle lineages, and 14 (95% HPD 5, 23) from other human lineages; i.e., 88.5% of human lineages arose from cattle lineages. We identified 11 persistent lineages local to Alberta, which were associated with 38.0% (95% CI 29.3%, 47.3%) of human isolates. Of 117 isolates in locally persistent lineages, 6.0% carried only the Shiga toxin gene stx2a and the rest both stx1a and stx2a. During the later period, six locally persistent lineages continued to be associated with human illness, including 74.7% (95% CI 68.3%, 80.3%) of reported cases in 2018 and 2019. The stx profile of isolates in locally persistent lineages shifted from the earlier period, with 51.2% encoding only stx2a. ConclusionsOur study identified multiple locally evolving lineages transmitted between cattle and humans persistently associated with E. coli O157:H7 illnesses for up to 13 years. Of concern, there was a dramatic shift in locally persistent lineages toward strains with the more virulent stx2a-only profile. Locally persistent lineages may be a principal cause of the high incidence of E. coli O157:H7 in locations such as Alberta and offer opportunities for understanding the disease ecology supporting E. coli O157:H7 persistence, as well as for local prevention efforts.