Host as ecosystem: Networks of internal host-parasite interactions
Traditional disease ecology examines host-parasite interactions primarily by simplifying host states (e.g. into susceptible, infected, or recovered) and examining these host states within a larger ecosystem framework. Such “black boxing” of the host, however, ignores the complex interactions between hosts and parasites, and between parasites and parasites in the complex internal host environment. We thus aimed to examine the relationships between host physiological and immunological parameters and parasite coinfections, by quantifying these relationships as part of a complex internal host “food web” with trophic interactions. We used data from a longitudinal study of wild plains zebras (Equus quagga) in Etosha National Park Namibia, animals that had been sampled up to five times over five seasons (three wet and two dry) over three years. These data represented 154 animal sampling events and 69 individual animals, with 45% of animals sampled two to three times, and 20% of animals sampled a total of four to five times. From these animals we collected ten morphometric measures (including age from tooth measurements), nine measures of immune function, three measures of parasite exposure and prevalence, five measures of reproductive status, and one measure of stress. We also controlled for environmental factors such as season, rainfall, and sampling area. We used multiple imputation methods to replace missing data and, with our full data set, used cluster analysis and path analysis methods to quantify the relationships between host and parasite metrics. We also repeated our analyses by a priori compartmentalizing our metrics into “basal resources” (i.e. within-host resources essential for parasite growth and maintenance), “intermediate level actors” (i.e. the parasites using host resources, and host physiological processes, such as reproduction and stress, that use host resources and affect host immunity), and “top trophic predators” (i.e. the host immune system).
TBA – analyses are currently being conducted, and we should be able to update our abstract by April.