Super-shedding cattle and the transmission dynamics of Escherichia coli O157

Abstract

The prevalence of Escherichia coli O157 displays striking variability across the Scottish cattle population. On 78% of farms, in a cross-sectional survey of 952, no shedding of E. coli O157 was detected, but on a small proportion, approximately 2%, very high prevalences of infection were found (with 90-100% of pats sampled being positive). We ask whether this variation arises from the inherent stochasticity in transmission dynamics or whether it is a signature of underlying heterogeneities in the cattle population. A novel approach is taken whereby the cross-sectional data are viewed as providing independent snapshots of a dynamic process. Using maximum-likelihood methods to fit time-dependent epidemiological models to the data we obtain estimates for the rates of immigration and transmission of E. coli O157 infection - parameters which have not been previously quantified in the literature. A comparison of alternative model fits reveals that the variation in the prevalence data is best explained when a proportion of the cattle are assumed to transmit infection at much higher levels than the rest - the so-called super-shedders. Analysis of a second dataset, comprising samples taken from 32 farms at monthly intervals over a period of 1 year, additionally yields an estimate for the rate of recovery from infection. The pattern of prevalence displayed in the second dataset also strongly supports the super-shedder hypothesis.

Fig. 1

The distribution of prevalences of E. coli O157 in faecal pats sampled from finishing groups of beef cattle on 952 Scottish cattle farms.

Fig. 2

Illustrative (six out of 32 shown) time series of prevalences of E. coli O157 in faecal pats sampled monthly during a longitudinal study of beef suckler cows on 32 Scottish farms.

Fig. 3

(a) A comparison of model fits to the cross-sectional data (□) for the (null) model containing no heterogeneities (▪) and the model containing animal level variation in transmission rates () – the super-shedder model. (b) As panel (a) but with a restricted vertical axis to expose the tail of the distributions.