Time trends in the prevalence of Escherichia coli and enterococci in bivalves harvested in Norway during 2007-2012

- A weak positive correlation between enterococci and Escherichia coli were seen.
- There was a weak positive correlation between high counts of E. coli/enterococci and increased rainfall.
- When using enterococci as indicator organisms, a more sensitive method than conventional plate assays should be considered.
- The number of faecal indicator organisms wary greatly between localities for cultivation of blue mussels.
- Selection of cultivation localities should consider longer time trends for indicator organism.

The objectives of this study were to describe time trends in the prevalence of Escherichia coli and enterococci in cultured blue mussels (Mytilus edulis) harvested from 152 localities along the coast of Norway during the six-year period from 2007 to 2012. Based on the available data, possible co-occurrence of these two indicator organisms of faecal contamination was assessed. Several localities for bivalve cultivation in Norway showed single high counts of E. coli, without any previous history of E. coli detection. For other localities, the pattern of E. coli detection was recurring, however low values were found, with some sporadic findings of higher values. There was a weak positive correlation between the detection of enterococci and E. coli, and a weak positive correlation between counts of E. coli/enterococci and rainfall. Sampling intervals should take into account knowledge of the occurring variation for bacterial faecal indicators, local knowledge on possible exposure to faecal material from livestock or humans, rainfall seasons, topography of the location, as well as tidal and water current patterns.