Whirling disease: re-emergence among wild trout

Whirling disease of rainbow trout is caused by Myxobolus cerebralis, a myxo zoan parasite possessing a life cycle well adapted to the natural environme nts where salmonid fish are found. Whirling disease was first described in Europe in 1898 among farmed rainbow trout but recent occurrences have been devastating to wild trout in North America. The disease is considered a maj or threat to survival of wild rainbow trout in the intermountain west of th e United States. Difficulties in containing the spread and potentially elim inating the pathogen are tied to features of a complex life cycle involving two hosts, the salmonid fish and an aquatic oligochaete. Derails of the mo rphologic development of the parasite have been described in each host but only now are we beginning to appreciate the breadth of interactions between these developmental forms and the sequential responses of the host. Fundam ental mechanisms of the recognition and attachment of the parasite to the h osts, how host immunity is evaded and the unknown influences of environment al factors all contribute to a rather poor understanding of the biology of the parasite. Although the biology and ecology of the salmonid host are bet ter known than for the oligochaete host, our knowledge is inadequate to int erpret their complex interactions with the parasite. This uncertainty precl udes the development of effective management activities designed to enhance the viability and productivity of wild trout populations in M. cerebralis- positive river systems. Improving our understanding of the hosts, the paras ite and the environmental factors determining their interaction should prov ide for more focused and effective control methods for containing the sprea d and devastating effects whirling disease is causing to our wild trout pop ulations.