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Ecology of disease in bighorn sheep in Hells Canyon, USA Cassirer, Elizabeth Frances


I investigated the dynamics of eight populations of a bighorn sheep (Ovis canadensis) metapopulation in Hells Canyon, USA from 1997 - 2003. Pneumonia was the most common cause (46%) of adult mortality and the primary factor limiting population growth. Cougar (Puma concolor) predation was the second most frequent source (30%) of adult mortality but did not reduce the rate of population growth significantly. Average annual survival of adult males (0.84) was lower than females (0.91). Pneumonia was the most common known cause of lamb mortality (86%) and pneumonia-related mortality was detected whenever summer lamb survival was less than 50%. Summer pneumonia epizootics in lambs were independent of epizootics in adults. Survival of transplanted sheep was initially high, but was lower than resident sheep during two of six years and at two of three release locations, due to pneumonia-related mortality. Survival and pneumonia outbreaks were not related to population size, growth rates, climate, or nutrition. Sex and source (resident or transplant) were the best predictors of adult survival and disease-related mortality. Individuals that died from pneumonia were not closer to domestic sheep (O. aries) or goats (Capra hircus) than those that did not, but the potential for transfer of pathogens between wild sheep and domestic and goats existed for all populations. Over 70 biovariants of Pasteurella spp. and Mannheimia spp. bacteria were cultured from live sheep during health sampling at capture and from dead sheep at necropsy. Only two biovariants of P. trehalosi were isolated from live sheep in all populations at capture. Both biovariants were also prevalent in sheep that died from pneumonia Pasteurella multocida was detected more commonly in the lungs of dead sheep than in resident sheep at capture and was not found in transplanted sheep. Lungworms (Protostrongylus spp.) did not appear to play a role in pneumonia-related mortality. Micro- and macroparasites differed among populations, but prevalence of potentially pathogenic organisms in pneumonic sheep was lower or did not differ from that sheep at capture or sheep that died from causes other than pneumonia. Clinical results suggested multiple causes of pneumonia outbreaks within this population, presence of undetermined factors affecting immunocompetence, or an inability to detect or correctly classify virulent organisms.

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