Proportion of Willow Ptarmigan (Lagopus lagopus lagopus) seropositive for louping-ill virus in the Norway-wide sampling relative to latitude in geographic degrees. The size of the circles is proportional to the number of samples in each location. The proportion of seropositive birds generally is higher in the southern than in the northern locations. Results from local sampling in Njardarheim in 2017 are not included.