The objective of this study was to determine whether the low prevalence of Tubifex tubifex infected with Myxobolus cerebralis in a Montana stream was due to a large percentage of worms being resistant to the parasite or whether other factors were involved. For this, specimens of T. tubifex were collected from various localities throughout the Rock Creek drainage of west central Montana, U.S.A., a stream known to be heavily contaminated with M. cerebralis, the causative agent of salmonid whirling disease. The natural occurring prevalence of infection was determined, and then noninfected worms were exposed to myxospores of M. cerebralis under controlled, laboratory conditions. Results showed that the percentage of noninfected T. tubifex that became infected with M. cerebralis was always greater than the percentage of naturally infected worms found at the same locality. Further, T. tubifex susceptible to infection were found at localities where no naturally infected worms were recovered. However, the prevalence of infection was low in both groups and did not exceed 4.1% for naturally infected T. tubifex or 19.0% for experimentally infected worms. In comparison, a control group of highly susceptible T. tubifex from California, U.S.A., had a 72–88% prevalence of infection after laboratory exposures to myxospores of M. cerebralis. Therefore, it appears that a large percentage of T. tubifex in Rock Creek are resistant to infection, although the mechanism of resistance is not known.
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