RESUMO
The epidemiology of louping-ill in red grouse was studied in northern Britain concentrating on the possible role of other species and mechanisms of disease persistence. This tick borne viral disease caused heavy mortality in red grouse, particularly chicks. Louping-ill induced mortality reduced the strength of the density dependence that generates the tendency of grouse populations to cycle and in some populations may cause population sinks. Four routes of transmission were examined and non-viraemic transmission of virus between ticks cofeeding on hares was considered significant. Field data supported the hypothesis that disease dynamics is influenced greatly by mountain hares, both as passive amplifiers and as hosts for the tick vector. Genetic variation in louping-ill within Britain was small.