Factors related to spatial patterns in whirling disease risk

Myxobolus cerebralis, the parasite that causes whirling disease (WD), has been a major contributor to the reduction in trout populations in the Intermountain West, U.S.A.  Myxobolus cerebralis has a two host life cycle: trout produce spores that are infective to the oligochaete intermediate host, Tubifex tubifex, and T. tubifex produces spores that are infective to trout.  Although some progress has been made correlating WD risk to various biotic and abiotic factors, much variation remains unexplained.  We assessed the relationships between WD risk (based on infection severity in sentinel trout) and characteristics of the oligochaete community and the environment in eight WD-positive and eight WD-negative drainage basins in Montana using a paired-watershed design.  More oligochaetes were collected in WD-positive than -negative watersheds, although T. tubifex was found in both watershed types.  Watersheds with low WD risk had higher proportions of land use in forest and sediments with lower proportions of fine particles, whereas watersheds with high WD risk had lower proportions of land use in forest and sediments comprised of more fine particles.  The ability to characterize areas of high WD risk is essential for improving our understanding of the dynamics of M.cerebralis such that appropriate management strategies can be implemented.