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Escherichia coli O157 infection on Scottish cattle farms: dynamics and control.

Zhang XS, Woolhouse ME - J R Soc Interface (2010)

Bottom Line: We first generate distributions of model parameter estimates using Markov chain Monte Carlo methods.Despite considerable uncertainty in parameter values, each set of parameter values within the 95th percentile range implies a fairly similar impact of interventions.Targeted interventions based on farm-level risk factors are more efficient than non-targeted interventions.

View Article: PubMed Central - PubMed

Affiliation: Centre for Infectious Diseases, University of Edinburgh, , Kings Buildings, West Mains Road, Edinburgh EH9 3JT, UK. xu-sheng.zhang@hpa.org.uk

ABSTRACT
In this study, we parametrize a stochastic individual-based model of the transmission dynamics of Escherichia coli O157 infection among Scottish cattle farms and use the model to predict the impacts of both targeted and non-targeted interventions. We first generate distributions of model parameter estimates using Markov chain Monte Carlo methods. Despite considerable uncertainty in parameter values, each set of parameter values within the 95th percentile range implies a fairly similar impact of interventions. Interventions that reduce the transmission coefficient and/or increase the recovery rate of infected farms (e.g. via vaccination and biosecurity) are much more effective in reducing the level of infection than reducing cattle movement rates, which improves effectiveness only when the overall control effort is small. Targeted interventions based on farm-level risk factors are more efficient than non-targeted interventions. Herd size is a major determinant of risk of infection, and our simulations confirmed that targeting interventions at farms with the largest herds is almost as effective as targeting based on overall risk. However, because of the striking characteristic that the infection force depends weakly on the number of infected farms, no interventions that are less than 100 per cent effective can eradicate E. coli O157 infection from Scottish cattle farms, implying that eliminating the disease is impractical.

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Related in: MedlinePlus

Effectiveness of targeted interventions where the selected farms assume a transmission coefficient β = 0. (a) Targeting farms based on herd size, estimated risk of infection and known presence of infection (positive farms). For comparison, the targeted control based on randomly chosen farms is also shown. The random targeted intervention is very similar to the non-targeted intervention that reduces the transmission coefficient by the control effort (cf. figure 4). (b) Effect of combining targeted interventions (based on herd size) with no movement-related transmission for either the targeted farms or all farms. Error bars denote the standard deviation.
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RSIF20100470F5: Effectiveness of targeted interventions where the selected farms assume a transmission coefficient β = 0. (a) Targeting farms based on herd size, estimated risk of infection and known presence of infection (positive farms). For comparison, the targeted control based on randomly chosen farms is also shown. The random targeted intervention is very similar to the non-targeted intervention that reduces the transmission coefficient by the control effort (cf. figure 4). (b) Effect of combining targeted interventions (based on herd size) with no movement-related transmission for either the targeted farms or all farms. Error bars denote the standard deviation.

Mentions: The effectiveness of different targeted interventions is shown in figure 5. We focus on the impact of reducing transmission rates, β, with or without eliminating movement-related transmission. Setting the infectious period (1/γ) = 0 days on targeted farms has similar effects to setting β = 0. Random targeting has similar impacts to partial, non-targeted interventions (cf. figure 4). Interventions targeted at larger herds are considerably more efficient (figure 5a), reflecting that the susceptibility of a farm is a function of herd size (equation (2.3)). Eliminating movement-related transmission can increase effectiveness if only a minority of farms are targeted (figure 5b). Eliminating movement-related transmission to and from the same targeted farms is almost as effective unless the control effort is small (figure 5b). If interventions are targeted only at farms known to be infected (roughly 16% of all farms), the prevalence of infection only decreases to about 13 per cent. Interventions targeted at farms based on the estimated probability of being infected, , are only slightly less effective than those targeted on the basis of herd size (figure 5a). This is because, in practice, herd size accounts for most of the variation in risk. In all cases, however, to eliminate the infection from Scottish cattle farms, 100 per cent control effort is needed (figures 4 and 5).Figure 5.


Escherichia coli O157 infection on Scottish cattle farms: dynamics and control.

Zhang XS, Woolhouse ME - J R Soc Interface (2010)

Effectiveness of targeted interventions where the selected farms assume a transmission coefficient β = 0. (a) Targeting farms based on herd size, estimated risk of infection and known presence of infection (positive farms). For comparison, the targeted control based on randomly chosen farms is also shown. The random targeted intervention is very similar to the non-targeted intervention that reduces the transmission coefficient by the control effort (cf. figure 4). (b) Effect of combining targeted interventions (based on herd size) with no movement-related transmission for either the targeted farms or all farms. Error bars denote the standard deviation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3104328&req=5

RSIF20100470F5: Effectiveness of targeted interventions where the selected farms assume a transmission coefficient β = 0. (a) Targeting farms based on herd size, estimated risk of infection and known presence of infection (positive farms). For comparison, the targeted control based on randomly chosen farms is also shown. The random targeted intervention is very similar to the non-targeted intervention that reduces the transmission coefficient by the control effort (cf. figure 4). (b) Effect of combining targeted interventions (based on herd size) with no movement-related transmission for either the targeted farms or all farms. Error bars denote the standard deviation.
Mentions: The effectiveness of different targeted interventions is shown in figure 5. We focus on the impact of reducing transmission rates, β, with or without eliminating movement-related transmission. Setting the infectious period (1/γ) = 0 days on targeted farms has similar effects to setting β = 0. Random targeting has similar impacts to partial, non-targeted interventions (cf. figure 4). Interventions targeted at larger herds are considerably more efficient (figure 5a), reflecting that the susceptibility of a farm is a function of herd size (equation (2.3)). Eliminating movement-related transmission can increase effectiveness if only a minority of farms are targeted (figure 5b). Eliminating movement-related transmission to and from the same targeted farms is almost as effective unless the control effort is small (figure 5b). If interventions are targeted only at farms known to be infected (roughly 16% of all farms), the prevalence of infection only decreases to about 13 per cent. Interventions targeted at farms based on the estimated probability of being infected, , are only slightly less effective than those targeted on the basis of herd size (figure 5a). This is because, in practice, herd size accounts for most of the variation in risk. In all cases, however, to eliminate the infection from Scottish cattle farms, 100 per cent control effort is needed (figures 4 and 5).Figure 5.

Bottom Line: We first generate distributions of model parameter estimates using Markov chain Monte Carlo methods.Despite considerable uncertainty in parameter values, each set of parameter values within the 95th percentile range implies a fairly similar impact of interventions.Targeted interventions based on farm-level risk factors are more efficient than non-targeted interventions.

View Article: PubMed Central - PubMed

Affiliation: Centre for Infectious Diseases, University of Edinburgh, , Kings Buildings, West Mains Road, Edinburgh EH9 3JT, UK. xu-sheng.zhang@hpa.org.uk

ABSTRACT
In this study, we parametrize a stochastic individual-based model of the transmission dynamics of Escherichia coli O157 infection among Scottish cattle farms and use the model to predict the impacts of both targeted and non-targeted interventions. We first generate distributions of model parameter estimates using Markov chain Monte Carlo methods. Despite considerable uncertainty in parameter values, each set of parameter values within the 95th percentile range implies a fairly similar impact of interventions. Interventions that reduce the transmission coefficient and/or increase the recovery rate of infected farms (e.g. via vaccination and biosecurity) are much more effective in reducing the level of infection than reducing cattle movement rates, which improves effectiveness only when the overall control effort is small. Targeted interventions based on farm-level risk factors are more efficient than non-targeted interventions. Herd size is a major determinant of risk of infection, and our simulations confirmed that targeting interventions at farms with the largest herds is almost as effective as targeting based on overall risk. However, because of the striking characteristic that the infection force depends weakly on the number of infected farms, no interventions that are less than 100 per cent effective can eradicate E. coli O157 infection from Scottish cattle farms, implying that eliminating the disease is impractical.

Show MeSH
Related in: MedlinePlus