A Bayesian framework to assess the potential for controlling classical scrapie in sheep flocks using a live diagnostic test

Abstract

Current strategies to control classical scrapie remove animals at risk of scrapie rather than those known to be infected with the scrapie agent. Advances in diagnostic tests, however, suggest that a more targeted approach involving the application of a rapid live test may be feasible in future. Here we consider the use of two diagnostic tests: recto-anal mucosa-associated lymphatic tissue (RAMALT) biopsies; and a blood-based assay. To assess their impact we developed a stochastic age- and prion protein (PrP) genotype-structured model for the dynamics of scrapie within a sheep flock. Parameters were estimated in a Bayesian framework to facilitate integration of a number of disparate datasets and to allow parameter uncertainty to be incorporated in model predictions. In small flocks a control strategy based on removal of clinical cases was sufficient to control disease and more stringent measures (including the use of a live diagnostic test) did not significantly reduce outbreak size or duration. In medium or large flocks strategies in which a large proportion of animals are tested with either live diagnostic test significantly reduced outbreak size, but not always duration, compared with removal of clinical cases. However, the current Compulsory Scrapie Flocks Scheme (CSFS) significantly reduced outbreak size and duration compared with both removal of clinical cases and all strategies using a live diagnostic test. Accordingly, under the assumptions made in the present study there is little benefit from implementing a control strategy which makes use of a live diagnostic test.

Genetic Engineering and Animal Infectious Diseases

“…Genetic engineering might also affect the epidemiology of infectious diseases among wild and domestic animals, with special reference to host-parasite interaction, thereby linking its effects with those of climate change and human activities. Indeed, genetic manipulation in favor of resistance to infection may be a strategic option, as clearly exemplified by the positive results obtained in the ovine population of the European Union after more than a decade of genetic selection for resistance to sheep scrapie, the animal and human prion disease "prototype" [2]. Nevertheless, when applying genetic engineering/selection for resistance to infectious diseases, special emphasis should be also placed on the possibility that one or more "undesidered" genetic traits could become phenotypically expressed and vertically transmitted within the concerned animal species and populations.…”