Graduate Thesis Or Dissertation
 

Characterizing the Spread and Consequences of Mycoplasma ovipneumoniae on Bighorn Sheep (Ovis canadensis) in the Northern Basin and Range Ecosystem

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  • North American bighorn sheep (Ovis canadensis) have experienced significant declines and population extirpations due to novel pathogens such as Mycoplasma ovipneumoniae. This disease continues to limit the population restoration of bighorn sheep. Therefore, understanding the demographic consequences of pathogen presence and the risk of contact between bighorn populations and potential sources of pathogens is vital to managing bighorn sheep populations effectively, especially for pathogens that cause respiratory pneumonia. My dissertation focuses on characterizing the spread and consequences of respiratory disease caused by M. ovipneumoniae in southeastern Oregon and northern Nevada. I carried out four interdisciplinary studies involving extensive fieldwork, epidemiological, genetic, geospatial, and statistical methodologies to determine factors influencing bighorn sheep demography and spatial ecology. My research relied in part on data provided by two state wildlife management agencies, Oregon Department of Fish and Wildlife (ODFW) and Nevada Department of Wildlife (NDOW), that captured adult female and male bighorn sheep, fitted them with GPS collars that generated and remotely transmitted remotely location data, and sampled them to generate diagnostics and genetic testing. I collected additional observational and non-invasive data within the system. In Chapter 2, I investigated the effect of M. ovipneumoniae on juvenile survival within our study system. I used observational data of juveniles and PCR-testing of juveniles that were found dead to analyze juvenile survival relative to M. ovipneumoniae presence, population genetic diversity, and forage characteristics. That study showed that the presence of M. ovipneumoniae can cause extremely low juvenile survival but found little influence of population genetic diversity or nutritional effects on juvenile survival. In addition, the study showed that even very low prevalence of M. ovipneumoniae in adults can have harmful effects on juveniles and that targeted removals of infected adults should be considered. In Chapter 3, I investigated the effect of exposure and infection of M. ovipneumoniae and other factors on GPS-collared adults using known-fate models. M. ovipneumoniae-exposed adults had lower survival than unexposed individuals, and I found evidence, albeit weaker, that adult survival was lower for males and in populations where genetic diversity was lower. The low prevalence of M. ovipneumoniae-exposed individuals suggests that chronic shedders and birth pulses maintain the pathogen. While targeted removals have been used as an effective tool to manage juvenile survival in bighorn sheep, these results indicate adults may benefit from this action too. I also recommended that management increase genetic diversity of populations that have suffered from sequential founder effects, although such action would have to be weighed carefully against the risk of increased disease exposure. For Chapter 4, I used GPS collar data to investigate space and habitat use patterns – key features of host behavior that impact pathogen exposure and transmission, as well as gene flow and metapopulation function. I assessed utilization distributions, site fidelity, social affinity, and resource selection functions separately for male and female bighorn. Although resource selection by both sexes was quite similar within the same seasons, female bighorn sheep exhibited extremely high site fidelity and social affinity, much higher than observed in other systems. Site fidelity and social affinity of male bighorn sheep were significantly lower, with numerous interpopulation movements. Our findings suggest that male bighorn sheep are responsible for disease transmission between the populations and maintain gene flow within the system. Still, females' high site fidelity and social affinity have resulted in the low potential for colonization of unused habitat, and I identified several areas of potential habitat that are unused by females but could increase distribution or enhance connectivity if occupied. In Chapter 5, I used the methodology of O’Brien et al. (2014) to estimate the risk of contact for each study population with potential sources of M. ovipneumoniae, including domestic sheep grazing allotments, other bighorn sheep populations, or potential sources of domestic sheep and goats. I found that all study populations had some probability of contact with other possible sources of infection risk, although those sources did not include known domestic sheep grazing allotments. This study therefore provides a tool to prioritize outreach to private landowners or management of disease within affected bighorn populations that may pose risk to other populations. This study highlights the adverse effects of M. ovipneumoniae persistence on bighorn sheep population recovery and the complications of managing metapopulation connectivity in a disease-impacted system. Management options to control pathogen spread must balance connectivity's negative and positive consequences. The management agencies responsible for these bighorn populations have initiated test and remove programs to deal with asymptomatic carriers within the system. However, our findings suggest further actions may be needed to improve genetic diversity and promote habitat colonization while considering disease risks associated with these actions. Hopefully, the data presented here inform efforts that help mitigate further exposure to novel strains of M. ovipneumoniae while maintaining necessary metapopulation functions.
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