Effects of ambient temperature on mechanisms of pathogen transmission in house finches (Haemorhous mexicanus)

Ambient temperature is an important abiotic factor shaping the process of pathogen transmission because of its effects on hosts, pathogens, and interactions between them. However, most experimental studies demonstrating the effects of temperature on transmission remain correlative and often exclude endothermic taxa, which modify behavior and energy allocation strategies in colder environments in ways that could increase pathogen spread. Additionally, because many endotherms serve as important reservoirs for zoonotic diseases and are facing conservation threats due to disease, understanding how temperature influences transmission in these systems has downstream relevance to human and wildlife health. In this dissertation, I use three laboratory experiments to determine how temperature affects several mechanisms of transmission in a naturally occurring songbird-pathogen system. House finches (Haemorhous mexicanus) are small songbirds that rely on bird feeders to meet thermoregulatory demands during winter. However, interactions with other birds at the feeder and contact with contaminated feeder surfaces are important sources of transmission of the bacterial pathogen Mycoplasma gallisepticum (MG). These interactions likely contribute to the fall and winter outbreaks of mycoplasmal conjunctivitis, a disease characterized by severe conjunctival swelling and changes in behavior in house finches. In my first experiment, I simulated infection in house finches to determine how temperature (warm versus cold) affected contact-relevant sickness behaviors, and in turn, the potential for transmission. I found that ambient temperature had a complex effect on some but not all contact-relevant sickness behaviors in this system, which could have key implications for downstream pathogen spread. Next, I investigated how ambient temperatures influenced another mechanism of transmission, the viability and pathogenicity of MG harbored on bird feeder surfaces. I found that MG remained viable and pathogenic to birds significantly longer when incubated on feeder surfaces at colder versus warmer temperatures. In my final chapter, I determined how temperature influenced the pairwise-transmission of MG from an experimentally-inoculated "donor" bird to its susceptible "receiver" bird cagemate. Here I examined how temperature influenced host infectiousness and estimated exposure dose, as well as the behaviors of both sick and healthy birds. I found that donor birds in colder temperatures were slower to recover from infection, and thus remained infectious for longer, compared to donor birds in warmer temperatures. I also found that receiver birds had more contacts with bird feeders and higher estimated doses of MG in colder temperatures. Despite evidence suggesting that MG transmission could be more successful in colder versus warmer temperatures, overall transmission success did not differ by temperature treatment. My work highlights the complex and non-uniform effects of temperature on aspects of the MG transmission process and suggests ways that temperature could have major implications for seasonal disease dynamics in this system. More broadly, my dissertation provides a framework for testing how different abiotic factors could influence the spread of other directly-transmitted diseases, which will be needed now more than ever in the face of global climate change.