Research report

​Quantifying the seasonality of E. coli O157 shedding (concentration and prevalence) in cattle and estimating its effect on the number of cases of food poisoning

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University of Aberdeen
Ogden, I ; Strachan, N

E. coli O157 infection exhibits strong seasonality in Scotland with approximately four times as many cases in the warmer summer months compared with the cooler winter months. The concentration and prevalence of E. coli O157 in cattle faeces at a local abattoir was studied from January to March 2003. The methods employed were identical to a previous study that had been carried out during May to July 2002. The aim was to compare the results between the warmer months when cattle are mainly at pasture and the cooler months when they are generally housed to try and find an explanation for the seasonality of infection in humans.

The prevalence of E. coli O157 in cattle faeces during the warmer months was found to be 7.5% which was significantly less than the 11.2% found during the cooler months (P=0.05). This is opposite to the seasonality of human infection. Both the warmer month study and the cooler month study identified virtually the same numbers of animals shedding high loads (>104 CFU/g)of E. coli O157, 0.7% and 0.6% respectively. However, during the warmer months the high shedders appear to shed higher concentrations resulting in an 8-fold increase in numbers excreted at this time of year. This may partly explain the seasonality of infection in humans.

The vast majority of strains isolated during the studies were potentially pathogenic to humans. Most (82%) were vt1negative, vt2 positive, which is comparable to the ratios of clinical E. coli O157 isolates in Scotland, where in 2002, 81% were vt1negative, vt2 positive. This is further evidence that cattle are a potential source of human E. coli O157 infection. Previous and ongoing studies determining only the prevalence of E. coli O157 in a group of animals may be a poor indicator of the total number of organisms shed by the group and may be a poor indicator of the risk of human infection. We propose that a far better indicator is knowledge of both the prevalence and the numbers of organisms shed – in particular from high shedding individuals.

Some method of removing these high shedding animals from the food chain or causing them to shed less E. coli O157 may reduce the risk of food poisoning. However, it is important to determine the duration of the high shedding (e.g. is it a day or is it a couple of weeks) as this would determine the feasibility of testing cattle at the farm prior to being moved to the abattoir.

Despite significant numbers of cattle shedding E. coli O157 in their faeces and there being high shedders the actual number of humans being infected by E. coli O157 in Scotland is low (approximately 200-300 cases per year). This indicates that hygiene measures at abattoir and along the food chain must be operating well. However, this view must be treated with caution because a breakdown of hygiene/processing standards in the food chain could potentially lead to another large food outbreak of E.coli O157.

Project Code: S01018