Within-host parasite dynamics may scale infection dynamics in natural population

Background/Question/Methods The rapid clearance or long term persistence of parasites within hosts is determined by the interaction of both parasite life-history characteristics and the immune response of the host to infection. The duration of parasite infection and shedding in turn affects patterns of transmission at the host-population level. Using a rabbit-helminth system we integrate data from experimental manipulations and mathematical modeling to examine the density and immune mediated mechanisms that determine the dynamics of a worm population within the rabbit host and potential transmission via egg shedding. In phase one of the experiment we infect rabbits with the gastrointestinal parasite Trichostrogylus retortaeformis (TR) continuously over a 60 day period.  In the second phase we clear the rabbits of parasites and repeat the continual infection over another 60 days. We measure the worm population indirectly through counts of eggs shed in feces through and obtain a single direct measure of the worm numbers in rabbits sacrificed every 15 days. We use state-space models to fit population dynamic models to both the indirect time series and direct point measures of worm population. We compare dynamic models that combine the effects of immune and density regulation of worm establishment and mortality.

Results/Conclusions We show that the dynamics are consistent with the combined effects of density dependence and immune regulation in driving worm burden. In secondary infections, following clearance, we find that worm populations increase to similar levels, but do so at a slower rate, suggesting an interaction between lower, immune mediated worm establishment and density dependence.  The consequence of this interaction leads to both a decline in total egg shedding over time and a delay in the timing of worm clearance and maximal egg shedding due to the combined interactions of density and immunity. Variation in the rate of parasite shedding with time has implications for the accumulation of transmissible stages in the environment and the transmission rate in the naïve hosts.