Whirling Disease: Reviews and Current Topics

Review: The Ecology of Myxobolus cerebralis

Billie L. Kerans and Alexander V. Zale

doi: https://doi.org/10.47886/9781888569377.ch14

ABSTRACT. The myxosporean parasite Myxobolus cerebralis is the causative agent of salmonid whirling disease. Containing its spread and limiting its effects in the Intermountain West will require judicious management programs, but such actions await a comprehensive understanding of the biology and ecology of this parasite and its hosts and how these elements interact; we do not yet know the weaknesses of this organism. To better guide efforts aimed at such an understanding, we assembled available information on the ecology of the parasite, organizing it into a conceptual model of its life cycle, to help foster understanding, focus future research, and lead eventually to a mathematical model for evaluating control measures. Myxobolus cerebralis has a complex life cycle with two obligate hosts, a salmonid fish and the oligochaete Tubifex tubifex, parasitized by the myxosporean and the actinosporean, respectively, and two infective “spore” stages, the myxospore and the triactinomyxon. This complexity is enhanced by the variable suitability of multiple salmonid species to serve as hosts, varying host suitability of genetic variants of T. tubifex, relatively recent introduction of M. cerebralis to North America, and unique traits of the parasite that preclude easy classification into conventional modeling categories. Much is known about the anatomy and function of myxospores and triactinomyxons from laboratory studies, but information on their distribution, abundance, and dispersal in natural systems is limited and based on indirect observations. Similarly, we understand development of the parasite within its hosts and resulting pathologies well but know little about host immune reactions and other mechanisms controlling proliferation within hosts or how environmental factors affect these defenses. Population-level effects on fish in natural systems have been quantified only rarely, where good prewhirling disease data exist, and effects on T. tubifex populations are unknown. Most rates and frequencies needed to infer relationships and model system dynamics have not been directly quantified in natural systems, but rapid progress is being made. Larger issues, including effects of M. cerebralis on community dynamics and ecosystem structure and function, have yet to be explored.