Escherichia coli O157 in Finnish Slaughter Cattle

Introduction: The Finnish EHEC Control Programme (FECP) for cattle was launched in Finland in 2004. According to it, faecal samples of individual, randomly chosen animals are to be collected at slaughterhouses evenly year-round in a way that reveals a 1% prevalence of E. coli O157 with an accuracy of 0.5% and a confidence level of 95%. Positive animals must be traced back to the farms and restrictive measures activated.

During the years, the apparent prevalence of slaughter animals has varied from 0.20% (2008) to 2.1% (2013) with sample sizes 1495 and 1570, respectively. An evaluation of the effectiveness of the FECP was conducted in 2010 based on 2006-2010 data, revealing both incapability of the system to detect infected animals or herds and unsteadiness in its implementation.

Purpose: After several years with a relatively low O157 prevalence, the results of the years’ 2012-2013 FECP showed about 3-fold increase. A new study was therefore started to find out whether the outcome resulted from actual increase of the prevalence or weaknesses of the programme.

Methods: True prevalences on annual data sets of the FECP were assessed with WinBUGS 1.4.3. Test sensitivity 0.988 was included.

Results: The true prevalence among slaughter animals was lowest in 2008 (95% CI: 0.074%-0.58%) and highest in 2013 (95% CI: 1.53%-2.99%). The number of O157 infections among slaughter cattle in 2012 and 2013 showed a significant increase compared to the previous years. However, the O157 level of low-prevalence years differed significantly only from the high-prevalence years 2012-2013.

Significance: According to the statistical analysis, the increase of O157 within the FECP during 2012-2013 was true. Because previous evaluation of the FECP already revealed many deficiencies, e.g., in detection probability and sampling implementation, the conclusions about the trend cannot be based on annual FECP results without detailed examination of the data.