Population dynamics of scrapie in a sheep flock

A detailed analysis of an outbreak of natural scrapie in a flock of Cheviot sheep is described. A total of 137 cases was reported over 13 years among 1307 sheep born into the flock. The epidemiology of scrapie can only be und erstood with reference to sheep demography, the population genetics of susc eptibility to scrapie, pathogenesis during a long incubation period, and th e rate of transmission (by both vertical and horizontal routes), all of whi ch interact in complex ways. A mathematical model incorporating these featu res is described, parameter values and model inputs are derived from availa ble information from the flock and from independent sources, and model outp uts are compared with the field data. The model is able to reproduce key fe atures of the outbreak, including its long duration and the ages of cases. The analysis supports earlier work suggesting that many infected sheep do n ot survive to show clinical signs, that most cases arise through horizontal transmission, and that there is strong selection against susceptible genot ypes. However, important aspects of scrapie epidemiology remain poorly unde rstood, including the possible role of carrier genotypes and of an environm ental reservoir of infectivity, and the mechanisms maintaining alleles givi ng susceptibility to scrapie in the sheep population.