Super-shedding and the link between human infection and livestock carriage of Escherichia coli O157

Abstract

Cattle that excrete more Escherichia coli O157 than others are known as super-shedders. Super-shedding has important consequences for the epidemiology of E. coli O157 in cattle--its main reservoir--and for the risk of human infection, particularly owing to environmental exposure. Ultimately, control measures targeted at super-shedders may prove to be highly effective. We currently have only a limited understanding of both the nature and the determinants of super-shedding. However, super-shedding has been observed to be associated with colonization at the terminal rectum and might also occur more often with certain pathogen phage types. More generally, epidemiological evidence suggests that super-shedding might be important in other bacterial and viral infections.

Figure 1

Map of the worldwide relative burden of E. coli O157 in humans in 2005 per 100,000 population. Crude rates are presented for countries where there are surveillance programs,,, noting that surveillance and detection methods differ and therefore direct comparisons of burden are problematic. Cross hatching represents the detection of E. coli O157 in that country but no estimate of incidence rate was available. White represents no data available, noting that infections may have occurred in (some of) these countries, especially those without developed E. coli O157 surveillance systems. This figure reflects information in published reports as of August, 2008.

Figure 2

Diagrammatic representation of the chain of events linking super-shedding cattle and the risk of human infection with E. coli O157.

Figure 3

Results of mixture distribution analysis on a histogram of E. coli O157 counts (log transformed) gathered from faecal pats collected during a cross-sectional survey of Scottish store and finishing cattle (2002-2004). This data was analyzed using MIX (Ichthus Data Systems, Hamilton, Ontario, Canada), a program that analyzes histograms as mixtures of statistical distributions. Parameters of the mixture distribution (mean, standard deviation, proportion) were calculated as maximum likelihood estimates (MLE) using a combination of the Expectation-Maximization (EM) algorithm and Newton-type methods. Final models were fitted assuming two normal components. This figure shows the original histogram of E. coli O157 counts (purple line) fitted with 2 component normal distributions (red lines). The green line is their sum, the mixture distribution, which matches the histogram as closely as possible. The red triangles mark the mean E. coli O157 count within each component distribution. The point estimate of threshold for a super- shedder was chosen as the point of overlap of the two unscaled distributions, approximately 3135 C.F.U. g^-1; 95% CI for threshold estimate, 1658 and 10395. Note that this does not match the value which might be inferred from this figure, where the components are scaled relative to the sizes of the observed populations. Reproduced with permission from REF 13 © 2007American society of Microbiology.

Figure 4

Colonization of cattle terminal rectum by E. coli O157. The top panel shows immuno-peroxidase staining of a microcolony of E. coli O157 at the rectal epithelium of an experimentally-colonized calf. The brown staining is of the surface O157 LPS and shows that the E. coli O157 micro-colony has eroded the epithelium (x2000). The bottom panel shows a scanning electron micrograph of a bacterial micro-colony from the terminal rectum of a calf colonized with E. coli O157 (scale indicated). Images provided courtesy of Pablo Nart, University of Edinburgh.