Morphologic and genetic characterization of potential oligochaete alternate hosts of Myxobolus cerebralis

Salmonid whirling disease, caused by the parasite Myxobolus cerebralis , was considered to be of concern only to fish aquaculture until the early 1990's when dramatic declines in wild trout populations in the intermountain west were attributed to the presence of the parasite. The current consensus is that once M. cerebralis becomes established in a waterway it is probably impossible to eradicate the parasite and that management of the disease will focus on reducing the impact on salmonid fish populations. Effective management strategies will rely in part on understanding and assessing the interaction of the numerous variables of the disease, including the hosts, parasite and environment. Myxobolus cerebralis requires two hosts to complete its life cycle, salmonid fish and an oligochaete worm, Tubifex tubifex. Extensive research has been conducted on interactions between the parasite and salmonid fish, however, less is known about the role of the oligochaete host.; In this study homogeneous monocultures of tubificids were developed by utilizing the ability of Tubificidae to reproduce by parthenogenesis. These monocultures were developed from geographic variants of T. tubifex that vary in their ability to release triactinomyxons, the form of the parasite infective to fish. Evidence is presented showing that current oligochaete identification keys based on chaetae morphology and internal male genitalia do not differentiate between T. tubifex strains that are susceptible to M. cerebralis infection with subsequent release of triactinomyxons and a resistant strain of T. tubifex that ingests the myxospore form of M. cerebralis but the parasite does not undergo development into viable triactinomyxons. This evidence is further supported by documentation of spurred chaetae, a previously unreported chaetal form, exhibited by a T. tubifex population in Rock Creek, Montana. Further, other studies indicate this spurred chaetal form is probably environmentally induced. Generation time may be an indication that susceptible and resistant T. tubifex are different species. In addition, genetic fingerprints were generated that distinguished between strains of T. tubifex that vary in susceptibility to M. cerebralis infection. Results of interbreeding experiments between M. cerebralis-susceptible and resistant T. tubifex are presented, and the potential use of resistant T. tubifex as a biological control of whirling disease is discussed.