The epidemiology of tick-borne diseases in desert bighorn sheep

Ticks and tick-borne diseases are a global concern, posing health risks to domestic animals, humans, and wildlife. Predicting how pathogen distributions will shift with global change is a key challenge in disease ecology. This thesis investigates the diversity, distribution, and factors influencing tick-borne pathogens in desert bighorn sheep (Ovis canadensis nelsoni) and their primary tick vector, Dermacentor hunteri. The first data chapter uses a 40-year dataset to show that environmental factors, particularly complex interactions between minimum elevation, minimum temperature, and median NDVI, are the most significant drivers of Anaplasma spp. seroprevalence. A subsequent molecular chapter provides a finer-detail characterisation of key tick-borne pathogens, identifying a community of three co-circulating pathogens with distinct epidemiological and immunological profiles. Anaplasma ovis was confirmed as a widespread, endemic pathogen associated with an immunoregulatory host response. In contrast, A. marginale was identified for the first time in wild desert bighorn and D. hunteri; its spatially restricted distribution suggests a recent introduction, and co-infection elicited a strong pro-inflammatory response. The canid-associated Babesia vogeli was also newly discovered and highly prevalent in D. hunteri ticks but absent in bighorn hosts, revealing a cryptic transmission cycle. No Ehrlichia or Theileria spp. were detected. Despite these different immunological signatures, neither single nor co-infection status was associated with a detectable difference in host body condition. The findings of this thesis identify key locations where tick-borne pathogen risk is highest and identify the drivers of these pathogens, providing insights to inform future surveillance and targeted conservation strategies.