Modelling the epidemiology and economics of sheep scab in Great Britain

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


Sheep scab is an important parasitic disease of livestock, particularly sheep, found worldwide. In Great Britain, its prevalence has grown steadily since reintroduction in 1973, despite attempted national control prior to 1992 and a number of industry-led interventions. Recent reports of resistance in the causative agent, the mite Psoroptes ovis, to macrocyclic lactones, increases importance of urgent coherent action. The aim of this thesis was to develop epidemiological and economic models for sheep scab in Great Britain that could provide tools able to identify better scab management strategies.
A within-farm transmission model was developed, based on the classic SIR model used widely in epidemiology (Chapter 2). Model results show that 100 days after the introduction of one scab index case, around 80% of a flock are likely to be infected. Sensitivity analysis suggests that the transmission rate is the most important parameter to target in future interventions.
The model was expanded into a metapopulation model, with transmission across Great Britain possible via neighbour-to-neighbour contact (Chapter 3). Farm clusters with high connectivity and transmissibility are identified, which could be targeted in future interventions. Model simulation results show that scab spreads rapidly when introduced into one of these clusters, however, it is then limited to the cluster edges, suggesting that scab is unlikely to spread across the whole of Great Britain by neighbour-to neighbour contact only and that long distance movements may be important future intervention targets.
The within-farm model is revisited in Chapter 4 and an additional compartment is added for carriers of scab. The newly parameterised model produces output which is shown statistically to be from the same distribution as experimental data.
These changes are carried into Chapter 5, where an alternative metapopulation model is presented. Approximate Bayesian Computation is used to fit this model to reported data. The importance of long-distance movements is confirmed and evidence for the importance of the timings and synchrony of treatment on the seasonality of scab dynamics is provided.
An economic game theory model looking at the prophylactic treatment choices of two farmers found that it is currently not cost-effective to use prophylaxis for scab in Great Britain, except when the risk is high and treatment costs are low (Chapter 6). Lower insecticide costs or subsidies would be required to incentivise farmers to treat prophylactically.
The models provide tools that, with further scenario analyses, could help shape effective and economical interventions for scab control.
Date of Award26 Nov 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorEllen Brooks Pollock (Supervisor) & Richard Wall (Supervisor)

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