Dynamics of the waterborne stage of Myxobolus cerebralis estimated directly by packed-bed filtration

Abstract

Past assessments of whirling disease infection risk on wild trout populations have relied on disease severity experienced by cultured fish exposed in sentinel cages or flatscreen filtration of Myxobolus cerebralis triactinomyxons. The former technique is indirect and inexpedient whereas the latter is imprecise and inaccurate. I examined spatial and temporal dynamics of Myxobolus cerebralis triactinomyxon concentrations directly using packed-bed filtration at five mainstem sites on the upper Madison River and at Willow Creek, a tributary to the Jefferson River. Packed-bed filtration provides accurate and precise estimates of ambient triactinomyxon concentrations. I also compared findings inferred from indirect sentinel-cage trials and my concurrent direct measurements. Triactinomyxon concentrations were consistently higher at Willow Creek than at the five sites on the Madison River, at which concentrations did not differ spatially. Concentrations at all sites peaked in spring and autumn. Variation in concentrations among five consecutive days was absent, but strong diel variation was evident; at Willow Creek, concentrations were elevated during periods of low light intensity. Indirect sentinel-cage sampling and simultaneous, direct packed-bed filtration largely corresponded, except in one low-temperature sample when a low percentage of fish became infected despite high concentrations of triactinomyxons. Packed-bed filtration proved to be a rapid, efficient, and effective method for assessing whirling disease infection risk among wild trout.