Dynamics of an intestinal pathogen within and between bumble bee hosts

My thesis investigates the dynamics of infectious disease within and between hosts. I use a combination of field and laboratory studies and mathematical modeling to examine the establishment of infections within hosts, the spread of infections between hosts, and the effects of infection on host behaviour. Throughout, I use the bumble bee Bombus impatiens and its intestinal pathogen Crithidia bombi as a model system.; First, I demonstrate that a variety of factors may influence within-host dynamics of infection, including a host's exposure level, body size, and immune function. Importantly, the dynamics of C. bombi infections within bees create large variation in infectiousness among bees and alter aspects of host foraging that may influence between-host transmission at flowers.; Next, I demonstrate how two aspects of infectiousness, pathogen shedding and rate of social contact, can have direct consequences for the spread of infection among hosts. I show that infectiousness (shedding) interacts with host density to produce unexpectedly low rates of transmission among bees when host density is high. Further, within colonies, bees interact through non-random contact networks and an individual's risk of infection is strongly linked to its unique rate of contact with infected nestmates.; I then use a mathematical model of Fickian diffusion to integrate various aspects of disease dynamics and to examine the spread of pathogens from a point source of infection. Specifically, I consider the 'spillover' of C. bombi from infected commercially reared bumble bees into wild populations near greenhouses. Using field and laboratory data, I show that C. bombi may spread kilometres from greenhouses that use infected commercial bees. Importantly, sensitivity analyses show that slight changes in the infectivity of a pathogen or its rate of decay in the environment would greatly affect the rate of spread into susceptible populations.; Lastly, I examine how sub-lethal infections might reduce host fitness via adverse effects on host foraging. I show that infection by C. bombi reduces the efficiency of nectar collecting by bumble bees and alters their flower visiting behaviour in complex ways.