Abstract
Identifying risk factors for the presence of Escherichia coli O157 infection on cattle farms is important for understanding the epidemiology of this zoonotic infection in its main reservoir and for informing the design of interventions to reduce the public health risk. Here, we use data from a large-scale field study carried out in Scotland to fit both “SIS”-type dynamical models and statistical risk factor models. By comparing the fit (assessed using maximum likelihood) of different dynamical models we are able to identify the most parsimonious model (using the AIC statistic) and compare it with the model suggested by risk factor analysis. Both approaches identify 2 key risk factors: the movement of cattle onto the farm and the number of cattle on the farm. There was no evidence for a role of other livestock species or seasonality, or of significant risk of local spread. However, the most parsimonious dynamical model does predict that farms can infect other farms through routes other than cattle movement, and that there is a nonlinear relationship between the force of infection and the number of infected farms. An important prediction from the most parsimonious model is that although only ∼20% farms may harbour E. coli O157 infection at any given time ∼80% may harbour infection at some point during the course of a year.
Highlights
Escherichia coli O157 emerged over two decades ago and is widespread in Scotland, where the incidence rate of human infection is generally higher than in most other UK, European or North American countries (Locking et al, 2006; Chase-Topping et al, 2008)
Human infection may arise from person to person contact and from consumption of food contaminated by asymptomatic human carriers, primary human infection can be attributed to contamination of the environment or the food chain from several livestock species, especially cattle (Espie et al, 2006)
According to the Akaike Information Criterion (AIC) value the most parsimonious model is the basic model, i.e. that described by Eqs. (2)–(4)
Summary
Escherichia coli O157 emerged over two decades ago and is widespread in Scotland, where the incidence rate of human infection is generally higher than in most other UK, European or North American countries (Locking et al, 2006; Chase-Topping et al, 2008). Cattle are the main reservoir host for E. coli O157 (Armstrong et al, 1996) and play a significant role in the epidemiology of human infections (Griffin and Tauxe, 1991). Previous work has shown that direct contact with animals, their faeces and the farming environment are all important risk factors for sporadic human infections (O'Brien et al, 2001; Locking et al, 2001; Willshaw et al, 2003).
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