Campylobacter Attribution Extension

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Aims: Campylobacter continues to be the most common cause of foodborne illness in Scotland (a situation which is similar to the UK and most of the developed world). This study is the latest in a series of research projects that FSS has been funding since 2005 to improve our understanding of the key sources which contribute to the burden of campylobacter infection in the Scottish population.

Methods: Campylobacter isolates were collected from clinical cases in Grampian, Scotland from April 2015 to December 2016, along with isolates from abattoir sampled whole birds (representing strains found on retail ready chicken) and caeca (representing strains found in the chicken gut and faeces). The isolates were analysed using whole genome sequencing and Multi-Locus Sequence Typing (MLST). Molecular attribution models were used to attribute clinical isolates to specific host reservoirs: chicken, cattle, sheep, pigs and wild birds. Information obtained from the cases via questionnaire allowed greater understanding of routes of infection, seasonal trends and the risk factors associated with human campylobacteriosis in Grampian (where the epidemiological picture is considered to be representative of Scotland as a whole). Molecular phylogeny methods (family trees) were used to detect case clustering of subtypes of campylobacter in order to establish whether a direct link between cases and sources might be discernible.

Results: In Scotland, there was a decrease of 20.2% in the number of campylobacter cases reported between 2014 and 2016.  The findings of this study suggested that there was a downward trend in the number of chicken-attributed cases between 2015 and 2016 although the sensitivity of the molecular analyses used for determination of source attribution was not adequate to show this as a significant reduction. However chicken-related campylobacter strains are still the most common cause of illness in humans, followed by strains from ruminant (cattle and sheep) sources. The STRUCTURE attribution model allocated clinical isolates to the following reservoirs: chicken 52%, cattle 11%, sheep 26%, pigs 2% and wild birds 8%. The Asymmetric Island attribution model allocated clinical isolates to: chicken 68%, cattle 9%, sheep 14%, pigs 1% and wild birds 7%.

Analysis of retail campylobacter isolates from Scottish abattoirs showed that overall, prevalence and strain types were similar to that of abattoirs from the rest of the UK. Abattoir carcasses had heavier campylobacter loads than retail birds which was likely to be due to the birds being sampled shortly after slaughter and there being less time for the organism to become less viable or die off. At the beginning of the study, a secondary scalding tank was introduced to the Scottish abattoir but unfortunately due to intermittent use and subsequent absence of comparable samples, it was not possible to show a reduction in campylobacter loads as a result of this intervention.

Conclusions: This study has provided an insight into the dynamic nature of campylobacter and provides key data on prevalence and strain types in the main food vehicles and animal reservoirs. It provides evidence that chicken continues to be the most important source of human infection, most likely through the consumption of undercooked chicken or through cross-contamination in the kitchen.

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