Date of Award
Doctor of Philosophy
Diseases can have a range of impacts on hosts and host populations. These impacts can be minimal, to the point of being considered nearly a commensal relationship. The other end of the spectrum is when a disease regulates a population or even drives it to extinction. Diseases that are directly transmitted in a density-dependent manner typically do not cause population extinction because as the population decreases so does transmission. However, there are several factors that can lead to extinction caused by diseases. These diseases can be frequency-dependent transmission (including vector-borne diseases) or diseases that infect multiple sympatric hosts. The parasite Cytauxzoon felis is a tick-borne apicomplexan that infects bobcats and domestic cats and is enzootic in the Midwest and Southeast US. Bobcats are considered the reservoir host of C. felis and typically do not show signs of disease associated with infection. This parasite is the etiological agent of cytauxzoonosis, a highly-fatal disease impacting domestic cats and occasionally other felid species. Domestic cats also can be subclinically infected. This parasite has increased its range and density within previously described enzootic areas. There are several aspects of the biology of C. felis that have not been explored and impact the epizootiology of this important veterinary parasite. The prevalence of the parasite has been studied in some locations, yet areas where the parasite has recently invaded need to be studied as populations of naïve bobcats and domestic cats may be at risk of epizootics. My research expands the knowledge of C. felis, adding information about a relatively recently-described enzootic area. The present dissertation is divided into 6 chapters. In the first chapter I provide a thorough literature review of C. felis and general information on pathogens. In my second chapter I describe the prevalence of C. felis in bobcats and ticks in southern Illinois from 2006-2017. This is the first documentation of C. felis in bobcats in Illinois. The prevalence in ticks is also the highest prevalence in ticks reported to date. These results have been published in the Journal of Parasitology. In the third chapter I provide evidence of chronic C. felis infections in bobcats. Some bobcats maintained C. felis infection for at least 2 years. I determined individual bobcats were infected with the same strain of the parasite at each capture event. This finding indicates that bobcats may carry the same strain over time. These infected bobcats could be spreading strains that are more pathogenic to domestic cats and possibly to other bobcats. Vector-borne pathogens (specifically microparasites) can show varying levels of intensity of infection in vertebrate hosts. The intensity of infection may correlate with activity of the vectors to facilitate transmission from the vertebrate to the vector. In the fourth chapter I tested if C. felis parasitemia (percent of red blood cells infected with the parasites) increased with environmental factors associated with tick activity. Cytauxzoon felis infections are increasing in domestic cats in the US in many areas. The fifth chapter describes the first study of clinical and subclinical C. felis infections in domestic cats in southern Illinois. I collaborated with veterinary clinics to obtain 642 domestic cat blood samples for this project. I also tested whether land cover types and host characteristics were related to risk of infection, and found only that feral cats were more likely to have subclinical C. felis infection. Cytauxzoon felis is transmitted through a tick vector; so direct contact between domestic cats and bobcats is not necessary for transmission to occur. For the sixth chapter I tested if the genetic populations of C. felis in domestic cats and bobcats were different (suggesting barriers to transmission). I found that there was high genetic diversity of C. felis in my samples. The within population variance accounted for nearly all variance detected. Therefore I conclude that the population of C. felis in bobcats and domestic cats in my study area is panmictic suggesting there are no barriers to transmission between these two host species.
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