EPIDEMIOLOGY AND CONTROL OF SCRAPIE WITHIN A SHEEP FLOCK

Mathematical models of the transmission dynamics of scrapie are used t o explore the expected course of an outbreak in a sheep flock, and the potential impacts of different control measures. All models incorpora te sheep demography, a long and variable scrapie incubation period, ho rizontal and vertical routes of transmission and genetic variation in susceptibility. Outputs are compared for models which do and do not in corporate an environmental reservoir of infectivity, and which do and do not incorporate carrier genotypes. Numerical analyses using paramet er values consistent with available data indicate that, in a closed fl ock, scrapie outbreaks may have a duration of several decades, reduce the frequency of susceptible genotypes, and may become endemic if carr ier genotypes are present. In an open flock, endemic scrapie is possib le even in the absence of carriers. Control measures currently or like ly to become available may reduce the incidence of cases but may be fu lly effective only over a period of several years.