Article

Optimal reactive vaccination strategies for a foot-and-mouth outbreak in the UK.

Department of Biological Sciences and Mathematics Institute, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK.
Nature (Impact Factor: 42.35). 04/2006; 440(7080):83-6. DOI: 10.1038/nature04324
Source: PubMed

ABSTRACT Foot-and-mouth disease (FMD) in the UK provides an ideal opportunity to explore optimal control measures for an infectious disease. The presence of fine-scale spatio-temporal data for the 2001 epidemic has allowed the development of epidemiological models that are more accurate than those generally created for other epidemics and provide the opportunity to explore a variety of alternative control measures. Vaccination was not used during the 2001 epidemic; however, the recent DEFRA (Department for Environment Food and Rural Affairs) contingency plan details how reactive vaccination would be considered in future. Here, using the data from the 2001 epidemic, we consider the optimal deployment of limited vaccination capacity in a complex heterogeneous environment. We use a model of FMD spread to investigate the optimal deployment of reactive ring vaccination of cattle constrained by logistical resources. The predicted optimal ring size is highly dependent upon logistical constraints but is more robust to epidemiological parameters. Other ways of targeting reactive vaccination can significantly reduce the epidemic size; in particular, ignoring the order in which infections are reported and vaccinating those farms closest to any previously reported case can substantially reduce the epidemic. This strategy has the advantage that it rapidly targets new foci of infection and that determining an optimal ring size is unnecessary.

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    • "Ref. [45] develop a probabilistic transmission model of FMD and explore an optimal deployment strategy of limited reactive ring vaccination of cattle in a single epidemic outbreak. Ref. [33] construct and numerically analyze single-outbreak, deterministic , mean-field equations (they assume homogeneous mixing of host population) based on an SEIR (susceptible, exposed, infected, recovered) natural history of FMD to illustrate impacts of constrained vaccine supply on optimal vaccination schedule. "
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    ABSTRACT: Many countries have eliminated foot and mouth disease (FMD), but outbreaks remain common in other countries. Rapid development of international trade in animals and animal products has increased the risk of disease introduction to FMD-free countries. Most mathematical models of FMD are tailored to settings that are normally disease-free, and few models have explored the impact of constrained control measures in a ‘near-endemic’ spatially distributed host population subject to frequent FMD re-introductions from nearby endemic wild populations, as characterizes many low-income, resource-limited countries. Here we construct a pair approximation model of FMD and investigate the impact of constraints on total vaccine supply for prophylactic and ring vaccination, and constraints on culling rates and cumulative culls. We incorporate natural immunity waning and vaccine waning, which are important factors for near-endemic populations. We find that, when vaccine supply is sufficiently limited, the optimal approach for minimizing cumulative infections combines rapid deployment of ring vaccination during outbreaks with a contrasting approach of careful rationing of prophylactic vaccination over the year, such that supplies last as long as possible (and with the bulk of vaccines dedicated toward prophylactic vaccination). Thus, for optimal long-term control of the disease by vaccination in near-endemic settings when vaccine supply is limited, it is best to spread out prophylactic vaccination as much as possible. Regardless of culling constraints, the optimal culling strategy is rapid identification of infected premises and their immediate contacts at the initial stages of an outbreak, and rapid culling of infected premises and farms deemed to be at high risk of infection (as opposed to culling only the infected farms). Optimal culling strategies are similar when social impact is the outcome of interest. We conclude that more FMD transmission models should be developed that are specific to the challenges of FMD control in near-endemic, low-income countries.
    Epidemics 12/2014; 9. DOI:10.1016/j.epidem.2014.09.008 · 2.38 Impact Factor
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    • "Implementation of tactics to control pathogen invasions is often costly and may induce lowered sensitivity of the pathogen, such that a balance between negative consequences of implementation of a control strategy and its effectiveness is necessary (Muller-Schafer et al. 2004, Horie et al. 2013). Optimal control strategies, which seek a balance between effective control and negative impacts, have been proposed for a number of pathogen systems, including cholera (Tuite et al. 2011), oak wilt (Horie et al. 2013), and human influenza (Ferguson et al. 2006), among others (Lipsitch et al. 2003, Riley et al. 2003, Ferguson et al. 2005, Tildesley et al. 2006). Such strategies often require detailed knowledge of the pathogen and host, however, which may not be available during an invasion (Woolhouse 2011). "
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    ABSTRACT: Pathogen invasions pose a growing threat to ecosystem stability and public health. Guidelines for the timing and spatial extent of control measures for pathogen invasions are currently limited, however. We conducted a field experiment using wheat (Triticum aestivum) stripe rust, caused by the wind-dispersed fungus Puccinia striiformis, to study the extent to which host heterogeneity in an initial outbreak focus influences subsequent disease spread. We varied the frequency of susceptible host plants in an initial outbreak focus and in the non-focus of experimental plots, and observed the progress of epidemics produced by artificial inoculation. The frequency of susceptible hosts in the initial outbreak focus increased the spread of stripe rust in the experimental plots, while frequency of susceptible hosts outside the initial outbreak focus did not. This suggests that factors influencing pathogen reproduction in the initial outbreak focus are key to the control of epidemics of stripe rust. Two mechanisms may underlie the field results. The first is the continuing, direct infection of susceptible hosts in areas outside the initial outbreak focus by disease propagules arriving from the initial outbreak focus. The second is highly local proliferation of disease caused by direct descendants of colonizing individuals originating from the initial outbreak focus. We considered these two alternatives in simulations of a generalized pathogen exhibiting fat-tailed dispersal, similar to P. striiformis. Simulations showed a dominant effect of conditions in the initial outbreak focus, in agreement with the field experiment, but indicated that, over time, this dominance may erode. Analysis of the duration of focal dominance led to the conclusion that both mechanisms contribute to the phenomenon of focal dominance, and that the frequency of susceptible hosts in the initial outbreak focus had a stronger influence when the proportion of propagules that remained local during dispersal was higher. Overall, our results suggest that targeting pathogen reproduction in the initial outbreak focus will have a disproportionately large impact on subsequent epidemic spread
    Ecological Applications 10/2014; 24(7):1854-1862. DOI:10.1890/13-1408.1 · 4.13 Impact Factor
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    • "However, a recently published study found that the number of undetected infected animals for vaccination-to-live was not higher than for pre-emptive culling strategies after adequate final screening (Backer et al., 2012a), which supports the choice of the vaccination-to-live strategy. Epidemic models have been used to quantify benefits of alternative FMD-control strategies, such as pre-emptive culling or emergency vaccination (Kitching et al., 2005; Hutber et al., 2011; Stevenson et al., 2013) both for the 2001 FMD outbreak in the UK (Ferguson et al., 2001a,b; Keeling et al., 2001, 2003; Hutber et al., 2006; Tildesley et al., 2006) as well as other venues (Bates et al., 2003a,c; Schoenbaum and Terry, 2003; LeMenach et al., 2005; Ward et al., 2009; Traulsen et al., 2011; Carpenter et al., 2011; Backer et al., 2012b). Most of these studies revealed a benefit of vaccination or pre-emptive culling in reducing either the epidemic or the economic impact or both, although non-beneficial vaccination in respect to the epidemic size has also been reported (Hagerman et al., 2012). "
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    ABSTRACT: Foot-and-mouth disease (FMD) is highly contagious and one of the most economically devastating diseases of cloven-hoofed animals. Scientific-based preparedness about how to best control the disease in a previously FMD-free country is therefore essential for veterinary services. The present study used a spatial, stochastic epidemic simulation model to compare the effectiveness of emergency vaccination with conventional (non-vaccination) control measures in Switzerland, a low-livestock density country. Model results revealed that emergency vaccination with a radius of 3km or 10km around infected premises (IP) did not significantly reduce either the cumulative herd incidence or epidemic duration if started in a small epidemic situation where the number of IPs is still low. However, in a situation where the epidemic has become extensive, both the cumulative herd incidence and epidemic duration are reduced significantly if vaccination were implemented with a radius of 10km around IPs. The effect of different levels of conventional strategy measures was also explored for the non-vaccination strategy. It was found that a lower compliance level of farmers for movement restrictions and delayed culling of IPs significantly increased both the cumulative IP incidence and epidemic duration. Contingency management should therefore focus mainly on improving conventional strategies, by increasing disease awareness and communication with stakeholders and preparedness of culling teams in countries with a livestock structure similar to Switzerland; however, emergency vaccination should be considered if there are reasons to believe that the epidemic may become extensive, such as when disease detection has been delayed and many IPs are discovered at the beginning of the epidemic.
    Preventive Veterinary Medicine 10/2013; 113(1). DOI:10.1016/j.prevetmed.2013.10.015 · 2.51 Impact Factor
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