Recent evidence suggests that the biodiversity of host communities mediates the transmission of parasites in humans and other species; however, careful field studies and controlled experiments are necessary to understand the nature and consequences of the interaction between host biodiversity and parasite transmission. This project examines the influence of biodiversity in the two-host system of Myxobolus cerebralis, the parasite that causes salmonid "whirling disease," using field studies and manipulative experiments. Whirling disease is transmitted to fish by stream sediment-dwelling worms. Worm communities are relatively simple and comprised of taxa that interact with each other, differ in the way in which they interact with the parasite, and differ in abilities to transmit the parasite. Thus, there is great potential for worm community structure to influence parasite transmission to fish. This project has the following goals: (1) determine statistical correlations between the relative and absolute abundances of the worm taxa and fish disease using field assays, (2) quantify the amount of genetic variation for parasite transmission within and among the worm taxa that can transmit the parasite, (3) examine how interactions among worm taxa affect the number of parasite spores produced, and thus fish disease, in laboratory experiments, and (4) develop statistical models to predict fish disease risk using data from goals (1)-(3). Broader impacts of this project include training high school, undergraduate and graduate level students from diverse groups in the climate, conduct and culture of scientific research. The research approach highlights hypothesis testing, one-on-one mentoring, diversity, and inclusion. Benefits to society emerge through the complementary field assays, manipulative experiments and synthetic statistical models that can be used not only to develop ecological theory, but also in research-based management.
There has been much recent interest in how biodiversity influences parasite transmission and disease risk in many systems. We completed field studies and manupulative experiments examining the effects of invertebrate host community structure on parasite transmission using whirling disese of trout in the Madison River, MT as a model system. Whirling disease is caused by the parasite, Myxobolus cerebralis, and transmitted by the aquatic worm, Tubifex tubifex. Both the worm and trout are required to complete the life cycle of the parasite--the worm produces a spore that is infective to the trout and the trout produces a different spore that is infective to the worm. The parasite, introduced to North American in the mid-1950's, has devastated wild trout populations in the intermountain west of North America. As research accumulates, worm community structure emerges as a key factor controlling trout disease risk. In addition, our project has had broader impacts through the training and mentoring of a diverse group of high school, undergraduate and graduate students in the conduct and culture of scientific research, through broadly disseminating our results though publication at national and international conference presentations, and through the development of molecular and genetic and natural resource management techinques that will benefit society. The goal of the study was to elucidate the role of biodiversity in an invertebrate host in the transmission of a parasite to a vertebrate host In support of this goal we (1) related worm biodiversity and density and environmental characteristics to whirling disease risk in trout using field assays, (2) quantified within worm taxa variation in parasite transimission in laboratory experiments, (3) used laboratory community experiments manipulating worm taxa to examine their effects on parasite transmission, and (4) used statistics and mathematical models to relate laboratory and field data. Our results showed that disease risk to trout varied across the landscape in the Madison River. It was difficult to distinguish among worm taxa, so in order to relate worm communities to disease risk in trout we developed genetic markers to distinguish among the worm taxa. In laboratory experiments, we showed that these worm taxa varied in their abilities to produce spores that are infective to trout. Thus, which worm taxa are present at a site will have a direct effect on disease risk to trout--sites that are dominated by high-spore-producing worm taxa should have higher whirling disease risk to trout than sites dominated by worm taxa that produce fewer spores. However in other experiments, we showed that the worm community may also indirectly influence disease risk to trout. For example, worm taxa that are not hosts to the parasite can consume and deactivate spores that are infective to the host, thus reducing infection in the host and potentially reducing infection risk to the trout. We also showed that the worm host interacts with the other non-host worm taxa in complex ways. For example, the worm host competes with one non-host worm species and when the host is infected it is a much worse competitor than when it is not infected. We have developed a mathematical model that will help us understand how these complex direct and indirect effects of the worm community integrate to influence disk risk in trout. The model will also help us to statisitcally relate our field data to our labooratory experimental data. Results from the whirling disease system will help us understand other disease systems, particularly those that are not amenable to experimentation.
