Myxobolus cerebralis in native Cutthroat trout of three spawning tributaries to Yellowstone Lake : a qualitative ecological risk assessment
Most environments impose periodic or stochastic stress on natural populations, which increase susceptibility to diseases. Infection by Myxobolus cerebralis (exotic parasite causing salmonid whirling disease) is strongly influenced by a stream's physicochemical attributes and stressors, which may also affect host pathology. Susceptibility to M. cerebralis varies greatly among different species and subspecies of the salmonid host, but little is known about lesion severity or location of infection among the native Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri). In 2002 and 2003 we performed a series of 10-day sentinel cutthroat fry exposures and habitat assessments in various sites of three M. cerebralis-positive tributaries to Yellowstone Lake: the Yellowstone River, Pelican Creek, and Clear Creek. At 90 and 150 days post-exposure, fry were examined by polymerase chain reaction and histology to determine prevalence, severity, and location of infection. The goal was to identify spatiotemporal patterns of infection, and physicochemical features of the streams influencing it, and potentially facilitating parasite invasion and establishment. Results on fish (young and adult) host infection data, environmental attributes, and tubificid host presence/absence data in the study streams were used to develop an ecological risk assessment for parasite establishment and whirling disease in this ecosystem. Results from our qualitative risk ranking systems suggest that the cutthroat trout of the Yellowstone Lake basin are highly susceptible to M. cerebralis infection, with the most severe lesions in cartilage of the cranium and jaws, especially in systems with high water temperatures and ionic content. Our results also suggest that such environmental features are most conducive to parasite establishment, especially in tributaries of the lake basin used by cutthroat trout as spawning and rearing habitats. Thus, this study has implications for both ecology and parasitology as it reveals that environmental components can affect when and where a pathogen resides within the host, and thereby affect manifestation of disease. Recognition of the specific environmental attributes most conducive to parasite establishment, and disease, can increase future diagnostics, detection, and management efforts, strengthening the likelihood of correctly predicting M. cerebralis' and similar pathogenic invasions and establishment in unsampled sites.