Susceptibility of three species of anadromous salmonids to experimentally induced infections with myxobolus cerebralis, the causative agent of whirling disease

The susceptibility of three species of anadromous salmonids to whirling disease was examined after their experimental exposures to the infectious stages of Myxobolus cerebralis. Chinook salmon Oncorhynchus tshawytscha exposed as alevins were very susceptible to infection; the appearance of clinical signs, prevalence of infection, severity of microscopic lesions, and spore counts at 130 d postexposure are similar to those of age-matched rainbow trout O. mykiss exposed at the same dose. In contrast, coho salmon O. kisutch demonstrated no clinical signs of infection and had a lower prevalence of infection and spore numbers than did the exposed rainbow trout. A comparison of two strains of steelhead (anadromous rainbow trout), one from an enzootic site (San Lorenzo River) where M. cerebralis has been present for the past 35 years and a second from a site where the parasite is not found (Dry Creek), showed them both to be highly susceptible to experimental infections with M. cerebralis. These controlled experimental exposures demonstrated that although certain species of anadromous salmonids (e.g., coho salmon) may resist the ill effects of whirling disease, other species (chinook salmon and steelhead) are highly susceptible to infection and disease. We anticipate that severe and negative impacts at the population level might occur among certain anadromous salmonids when conditions favor exposure of early life stages to high levels of M. cerebralis.