Mathematical models of a tick borne disease in a British game bird with potential management strategies

Abstract

Louping ill virus (LIV) is a tick borne disease that causes mortality in red grouse, an economically important game bird of British uplands. The aim of this thesis is to extend previously published models of LIV , to consider the potential impact of different management strategies. In addition a new route of infection and the seasonal biology of both grouse and ticks will be explored. Grouse chicks are known to eat ticks as part of their diet in the first three weeks of life which may contribute to virus persistence if chicks consume infected ticks. This novel route of infection is incorporated in to the model which predicts that ingestion increases the range of host densities for which the virus is able to persist. The ingestion of ticks by grouse also reduces the tick population so that for low host densities the ingestion of ticks by grouse reduces the tick population so virus cannot persist. The model is adapted to take account of the seasonal biology of grouse and ticks. Although the temporal predictions of the seasonal models show some differences the addition of seasonality does not alter the model predictions of when LIV is likely to persist at different grouse and deer densities. Consequently seasonality is felt to be unimportant when considering management strategies. The treatment of sheep with acaricide in an attempt to reduce the tick population on a grouse moor is currently being trialled in Scotland. We use a model to predict the likely effect of this strategy at different deer densities. The number of ticks found attached to sheep varies so we consider the effect of tick attachment rates as well as acaricide efficacy. Although we predict that acaricide treated sheep can reduce the tick population and therefore LIV in grouse in some circumstances the treatment is less effective in the presence of deer. Consequently we use a model to make theoretical predictions of the effectiveness of acaricide treated deer as a control strategy for reducing LIV in red grouse. The effect of culling deer on LIV in grouse is also modelled and contrasted with the effect of acaricide use. It is predicted that acaricide treatment of deer could be highly effective, particularly if the deer density is first reduced by culling. Finally we considered the direct treatment of red grouse with acaricide. Female grouse can be given an acaricidal leg band which protects her directly and indirectly protects her chicks as they acquire some acaricide whilst brooding. Trials have suggested this can reduce tick burdens for individuals. We use the model to determine the potential effect that treating individual broods may have on the whole grouse population. The model predictions suggest that unless acaricide efficacy on chicks is high and long lasting treating individual broods is unlikely to reduce LIV in the whole population but will still provide some benefit for the individuals. The effectiveness of treatment is reduced by higher deer densities. The success of the management strategies considered in this thesis appear to be restricted by the presence of deer. It may therefore be that a combination of treatments including the treatment of deer may be of the greatest benefit to the grouse population.