Heterogeneous shedding ofEscherichia coliO157 in cattle and its implications for control

Abstract

Identification of the relative importance of within-and betweenhost variability in infectiousness and the impact of these heterogeneities on the transmission dynamics of infectious agents can enable efficient targeting of control measures. Cattle, a major reservoir host for the zoonotic pathogen Escherichia coli O157, are known to exhibit a high degree of heterogeneity in bacterial shedding densities. By relating bacterial count to infectiousness and fitting dynamic epidemiological models to prevalence data from a cross-sectional survey of cattle farms in Scotland, we identify a robust pattern: Ϸ80% of the transmission arises from the 20% most infectious individuals. We examine potential control options under a range of assumptions about within-and betweenhost variability in infection dynamics. Our results show that the within-herd basic reproduction ratio, R 0, could be reduced to <1 with targeted measures aimed at preventing infection in the 5% of individuals with the highest overall infectiousness. Alternatively, interventions such as vaccination or the use of probiotics that aim to reduce bacterial carriage could produce dramatic reductions in R 0 by preventing carriage at concentrations corresponding to the top few percent of the observed range of counts. We conclude that a greater understanding of the cause of the heterogeneity in bacterial carriage could lead to highly efficient control measures to reduce the prevalence of E. coli O157.bacterial count ͉ core groups ͉ super shedder ͉ superspreading ͉ targeted control T he role of heterogeneous infectiousness on the course of disease outbreaks was highlighted during the recent severe acute respiratory syndrome outbreak (1), in which a few individuals were responsible for a disproportionate number of transmission events. Awareness of heterogeneities in transmission dynamics can be important for the effective implementation of disease control measures and can lead to efficient targeting of interventions at a subset of the population (2-5). Factors that might lead to such heterogeneities include variability in infectiousness, exposure, genetic susceptibility, contact rates, and behavior (6-10). Quantifying their impact on the transmission dynamics can be achieved through direct methods, such as contact tracing and outbreak reconstruction (1, 11), or indirectly through their effect on the distribution of infected cases (12).Escherichia coli O157 is an important zoonosis with a known reservoir in cattle (13,14). Prevalences of infection are generally low, usually reported to be Ͻ10% of animals carrying the pathogen (14). Typically, however, the distribution of prevalences is highly skewed (15); at any one time, shedding is not detected in the majority of cattle groups, but a small proportion of groups contains high numbers of individuals shedding bacteria in their feces.The range of prevalences of an infectious agent in a small population is expected to be influenced both by stochasticity and underlying heterogeneities in the transmission dynamics. In a recent a...

Transportation and Lairage Environment Effects on Prevalence, Numbers, and Diversity of Escherichia coli O157:H7 on Hides and Carcasses of Beef Cattle at Processing

“…Because no preharvest intervention could be 100% effective (22,29), a few animals may be excreting high numbers of E. coli O157:H7 during transport and lairage. In the high-density environments of transport and lairage, a few animals shedding high numbers of E. coli O157: H7 could contaminate the hides of several animals in that lot (17,18). In the current study, a small number of the cattle shedding E. coli O157:H7 (8.6% in group 1) can lead to dramatic increases in hide prevalence and counts (from 28 to 89% and from 6 to 16 animals with high hide numbers) in a very short period (Table 1).…”

Crossing the scale from within-host infection dynamics to between-host transmission fitness: a discussion of current assumptions and knowledge

“…Several studies of transmission in animal hosts have also shown a scaling of transmission fitness with pathogen load, e.g. Salmonella and Clostridium difficile transmission in mice [61,62] and Escherichia coli in cattle [63]. All these examples suggest that for some diseases and under some scenarios, infectiousness might be directly determined by pathogen load.…”

Investigation into the Effectiveness of Pooled Fecal Samples for Detection of Verocytotoxin-Producing Escherichia Coli O157 in Cattle

“…Verocytotoxinproducing E. coli O157 is usually widespread in fecally contaminated environments of infected animals and tends to circulate between individual animals within close proximity of each other or indirectly via other epidemiological routes. 4 Currently, during outbreak investigations, a large number (often 100 to 200 or more) of fecal samples are examined for VTEC O157 based on a 95% confidence of detecting at least 1 positive at a minimum estimated within-group prevalence of 10% within each distinct epidemiological group. Adopting a pooled approach for sample collection from epidemiological groups is potentially much more practical, cost-effective, and safer for the sampler as it does not require handling of individual cattle, provided it offers adequate statistical confidence and test sensitivity.…”