Epidemiology: from individuals to populations
Christopher A. Gilligan, University of Cambridge
J.E. Van der Plank wrote in 1963 that "Chemical industry and plant breeders forge fine tactical weapons but only epidemiology sets the strategy". This is still true, more than forty years later. It underlines the continued quest for sustainable disease control, which, itself, rests on a paradox. Since most plants are self-evidently resistant to most pathogens, it seems perfectly reasonable to assume that advancing knowledge of the genetical, molecular, and cellular bases of host-pathogen interaction will identify the means not only to engineer or to select durable resistance but also to produce effective and environmentally neutral forms of chemical control. Yet failures still occur and the problems are exacerbated by escalating costs for release of new varieties and for the development and registration of new chemicals. These problems partly reflect differences in scales between screening and deployment underlining the need to integrate epidemiology - 'the science of disease in populations' - with molecular biology, and host-pathogen genetics and physiology. Most novel forms of disease control are screened for effectiveness at the small scale. Often this is done at scales as small as the single plant for initial screening, though more usually it involves multiple field plots and ultimately fields. Yet successful deployment - and the risk of failure - occurs at scales much larger than this, at the regional, national or even international scales. The epidemiological challenges are not all technologically driven: agriculture and natural vegetation continue to be confronted by new and recurrent epidemics. The problems in minimising the risks of failures of control and in managing emerging epidemics demand a common epidemiological approach that considers invasion, persistence, scaling and chance.
Having reviewed briefly the progress in epidemiology during the past 25 years, I shall illustrate likely future developments in constructing an epidemiological framework to model invasion, persistence and variability of epidemics that encompasses a wide range of scales and topologies through which disease spreads. By considering how to map control methods onto epidemiological parameters and variables, some new approaches towards optimising the efficiency of control at the landscape scale will be described. Some epidemiological strategies to minimise the risks of failure of chemical and genetical control will be presented and, if time permits, some consequences of heterogeneous selection pressures in time and space on the persistence and evolutionary changes of pathogen populations discussed. Finally, brief mention will be made of how we might embed epidemiological models in an economically-plausible framework for the deployment of control.