Whirling Disease: Reviews and Current Topics

<EM>ABSTRACT. </EM>Laboratory challenges to determine the susceptibility of indigenous Deschutes River, Oregon, salmonids to <em>Myxobolus cerebralis </em>were conducted as part of a study to assess the risk of reintroducing anadromous salmon above a migration barrier on that river. Replicate groups of progeny from wild rainbow trout <em>Oncorhynchus mykiss</em>, steelhead (anadromous rainbow trout), kokanee <em>O. nerka</em>, and chinook salmon <em>O. tshawytscha</em>, were exposed to doses of 0, 200, or 2,000 triactinomyxons per fish. Fish were evaluated at 5 months postchallenge for spore concentration in the cranial cartilage, severity of microscopic lesions in the cartilage, and clinical signs of disease. The wild rainbow trout (0.7 g at exposure) were most susceptible to infection, with infection prevalence and spore concentrations similar to those of a susceptible (Mt. Lassen) control rainbow trout strain (0.7 g at exposure), although clinical disease signs were less common in the wild strain. Two year classes of steelhead, exposed at different sizes (0.3 g and 1.0 g), both showed fewer clinical disease signs, a lower prevalence of infection, a lower spore concentration, and a decreased mean lesion score, compared with the control rainbow trout (0.6 g and 1.2 g). Kokanee (1.5 g at exposure) became infected but less severely than the control rainbow trout (1.8 g at exposure). Clinical signs were not evident in the kokanee or the susceptible rainbow trout, possibly because of the large size at exposure. No signs of infection were detected in the chinook salmon (1.0 g at exposure) at either dose, despite high infection prevalence in the control rainbow trout (0.6 g at exposure). These results demonstrate that the indigenous salmonids present in the Deschutes River, both above and below the barrier, are susceptible to infection, but the rainbow trout would be most at risk should introduction of the parasite occur in this system.