Quantifying within- and between-animal variation and uncertainty associated with counts of Escherichia coli O157 occurring in naturally infected cattle faeces

Abstract

Cattle faeces are considered the most important reservoir for human infection with Escherichia coli O157. We have previously described shedding of E. coli O157 in the faeces of naturally infected cattle cohorts. However, the data require further investigation to quantify the uncertainty and variability in the estimates previously presented. This paper proposes a method for analysing both the presence and the quantity of E. coli O157 in cattle faecal samples, using two isolation procedures, one of which enumerates E. coli O157. The combination of these two measurements, which are fundamentally different in nature and yet measuring a common outcome, has necessitated the development of a novel statistical model for ascertaining the contribution of the various components of variation (both natural and observation induced) and for judging the influence of explanatory variables. Most of the variation within the sampling hierarchy was attributable to multiple samples from the same animal. The contribution of laboratory-level variation was found to be low. After adjusting for fixed and random effects, short periods of increased intensity of shedding were identified in individual animals. We conclude that within-animal variation is greater than between animals over time, and studies aiming to elucidate the dynamics of shedding should focus resources, sampling more within than between animals. These findings have implications for the identification of persistent high shedders and for assessing their role in the epidemiology of E. coli O157 in cattle populations. The development of this non-standard statistical model may have many applications to other microbial count data.

Perspectives on Super-Shedding ofEscherichia coliO157:H7 by Cattle

“…Others have evaluated the incidence of super-shedding using single samples taken from a crosssectional survey of multiple farms (Chase-Topping et al, 2008). However, Robinson et al (2009) reported that withinanimal variation of shedding E. coli O157:H7 was greater than that among animals over time. With this in mind, we hypothesize that super-shedding may vary within an individual animal at any given time and that a single sample from animals on a single day is inadequate to accurately describe super-shedding events and impacts.…”

“…These numbers would reach or exceed 10 10 cells per day if 4 or more super-shedders were within the same pen. In the past, shedding profiles of E. coli O157:H7 in cattle have been based predominantly on point estimates on the farm and typically comprise a cluster of samples with a high level of variation (Robinson et al, 2009). Others have evaluated the incidence of super-shedding using single samples taken from a crosssectional survey of multiple farms (Chase-Topping et al, 2008).…”

Molecular Epidemiology ofCampylobacter jejuniin a Geographically Isolated Country with a Uniquely Structured Poultry Industry

“…Bacterial count data have two components: (i) whether bacteria are present or absent and (ii) how many bacteria are present. In order to capture these two components in our model, we used a Bayesian zero-inflated Poisson model (37). This model includes two linear predictors, one informing the probability that the bacteria are present and the other informing the number of bacteria that are obtained assuming that they are present.…”

Genetic diversity and antimicrobial resistance among isolates of Escherichia coli O157: H7 from feces and hides of super-shedders and low-shedding pen-mates in two commercial beef feedlots

“…As most previous studies of super-shedders have evaluated a single point in time, it is also possible that many cattle are super-shedders for only a brief period. Accordingly, Robinson et al [6] reported that within-animal variation of shedding E. coli O157 was greater than that among animals over time.…”