Abstract

Whirling disease is caused by the myxosporean parasite Myxobolus cerebralis and affects salmonid fishes worldwide. In the United States, M. cerebralis had been recognized as a pathogen of cultured salmonids since the 1950's. In the mid-1990's, however, population declines among wild rainbow trout in regions of the intermountain western U.S. were associated with the occurrence of whirling disease. This discovery brought about a resurgence of scientific interest and widened the scope of related research to address the disease in the wild. Over the past decade, whirling disease investigations have moved in a number of new directions while continuing to develop upon the foundation built by decades of previous research. Relatively novel avenues of whirling disease research include ecological and epidemiological approaches such as those that incorporate risk assessment and dynamic modeling tools to aid in our understanding of the occurrence of whirling disease in the natural environment. Environmental factors influencing infectivity of the pathogen and population distribution of the oligochaete host are receiving increased attention as key issues explaining differences in disease prevalence among wild fish. Molecular, genetic, and immunological techniques are also being increasingly incorporated to advance many segments of whirling disease research such as evaluation of disease pathogenesis and host immune responses as well as recognition of taxonomically based differences in salmonid and oligochaete host susceptibility.