Covariation of parasite intensities in willow ptarmigan, Lagopus lagopus L.

The distribution of parasites in individual willow ptarmigan (Lagopus lagop us) hosts and associations between parasite numbers were used to investigat e whether host susceptibility, in addition to factors associated with expos ure, may have influenced the patterns of infection. Eight species of parasi tes were found, Trichostrongylus tenuis, Capillaria caudinflata, Splendidof ilaria papillocerca, Hymenolepis microps, Paroniella urogalli, Leucocytozoo n lovati, Eimeria sp., and Trypanosoma sp., in addition to microfilariae. F requency distributions of all but one of these parasite species were overdi spersed. Host age was more closely related to parasite abundance than was h ost gender. Intensities of different parasites in individual hosts were fou nd to be more similar than expected by chance. Parasite intensities therefo re tended to covary in individual hosts, and this covariation became more p ronounced when only microparasites and the most abundant macroparasites wer e analysed. Pairwise correlations between intensities gave significantly mo re positive than negative values. Common transmission routes cannot account for this pattern, and we conclude that differences in host susceptibility must have been an important factor.