Whirling disease in juvenile salmonid fishes is a serious problem in the western part of North America. It is caused by Myxobolus cerebralis, an organism once thought to be a member of the phylum Myxozoa and a protozoan but now known to be multicellular and a cnidarian (Siddall et al., Journal of Parasitology 81: 961–967). The parasite attacks cartilage, causing, among other things, severe skeletal deformities. When one considers that the trout fishing industry in Montana alone is worth approximately $300 million a year and that mortality caused by whirling disease in rainbow trout can range up to 90% in some streams, the extent of the difficulties created by M. cerebralis assumes serious economic significance.

We invited Bill Granath, one of North America's leading experts on whirling disease, to review this parasite and the problem it is causing worldwide. He has teamed with Michael...

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