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 these interactions. 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.
The field of community ecology seeks to link biological processes operating on lower ecological scales with community patterns. Of particular interest are the dynamics between biodiversity of host communities and host-parasite, particularly how biodiversity influences parasite transmission and disease risk. Our study focuses on how the biodiversity in stream sediment dwelling worm communities influences infection prevalence from Myxobolus cerebralis a parasite that causes Whirling Disease. The parasite is native to Eurasia and since being introduced to North America in the 1950’s has devastated wild trout populations in the intermountain west. As research accumulates, worm community structure emerges as a key factor in salmonid disease risk. Researchers from Montana State University and the University of Vermont combined ecological measurements from natural communities, manipulative laboratory experiments and Bayesian modeling to elucidate the role of worm-host biodiversity in the transmission of the parasite to the fish host. A parasite genetic-structure study across the landscape indicated patterns of parasite dispersal. We found the parasites collected in Montana to be similar to parasites in Arkansas, Oregon, California, Virginia and Germany, but parasites from West Virginia to be different. Overall, the parasite was more variable than previously thought and there were likely multiple introductions into NA from Europe. Our work on the worm host is important not only with respect to host-parasite dynamics, but also understanding of aquatic habitat quality in general. Sediment-dwelling, filter-feeding, aquatic worms are highly sensitive to their habitat quality and thus, often used for habitat quality assessments. Interactions between worm species and abiotic habitat features result in a characteristic assemblage of organisms; thus, the worm community composition is thought to be more informative than individual species. The mixture of species in aquatic worm communities can range from non-hosts to highly competent. The worm community genetic structure across the landscape found the most competent worm host to be most abundant at sites previously identified as having high fish disease and also most likely to be infected. Thus, differences in relative abundance and infection for this highly competent species may explain the range of infection in natural streams. At a watershed scale, worm community studies across four Montana watersheds found relationships between whirling disease variability and worm genetic population structure and biodiversity for three worm genera. Almost 200 worms were assayed from locations classified as positive or negative for whirling disease over several years of monitoring. We found genetic variation was non-randomly distributed, with more similarity among locations with different fish disease history, yet within the same watershed, than locations from different watersheds that had similar fish disease history. This shows parasite dispersal among watersheds by the worm host is likely limited; and studies at multiple spatial scales are needed to better understand disease ecology. The role of worm host community on fish disease dynamics is of increased interest because host community structure also appears to influence some major human and animal diseases. The aquatic worm/salmonid disease system is a tractable model system to initiate understanding of the host community effects. We also used experiments to further explore interactions identified by the field surveys. One experiment showed that both host and non-host filter-feeding worm species were able to remove infective spores from the environment, thus reducing the infection in the highly competent worm host. Lowered infection should translate to reduced infection risk to fish in the field. A second experiment, using clones from asexually reproducing field-collected worms, found significant genetic variation in the highly competent worm host for spore production both within and among sites. The mechanisms maintaining this genetic variation warrant further study because field sites dominated by the more productive clones may have higher fish disease risk. In addition, we developed a qPRC assay to distinguish among host and non-host worm taxa. This is a significant contribution because only adults can be distinguished morphologically and most of the worm community consists of juveniles. The new assay was able to identify immature worms that are potential hosts for whirling disease making the assay a powerful tool for assessing worm community composition. We also developed a method using Bayesian inference to characterize biological communities relating worm host abundance to fish disease risk. Ongoing analyses of additional field data will relate worm biodiversity, fish disease risk and environmental characteristics among sites across years. The highly interdisciplinary nature of this project provided a unique opportunity for the five graduate students funded on this project that crossed an unusually diverse set of disciplinary boundaries spanning biology, engineering and remote sensing. Such transdisciplinary training prepares them for leadership roles in the integrated, technology-intensive field of environmental research and disease control ranging from highly local outbreaks to global pandemics. In addition, 11 undergraduate students were trained during the course of this project, as well as multiple high school students and two high school teachers.
