EVALUATION OF MYXOBOLUS CEREBRALIS DISTRIBUTION AND MEASURES OF INFECTION IN COLORADO RIVER CUTTHROAT TROUT IN LABARGE CREEK, WYOMING

Abstract

Salmonid whirling disease caused by the invasive parasite Myxobolus cerebralis was firstdetected in the U.S. in 1956. It spread widely, causing declines of up to 90% in some wild salmonid populations. Colorado River Cutthroat Trout (CRCT), already threatened by habitat loss, introduced invasive species, and disease, now occupy only 13% of their historic Wyoming range. Despite extensive restoration efforts in LaBarge Creek, CRCT numbers remain critically low. My study examined the potential role of whirling disease in limiting CRCT recovery by assessing the spatial distribution and infection intensity of M. cerebralis in CRCT. I also compared two diagnostic approaches: the traditional pepsin-trypsin digestion (PTD) method and a quantitative polymerase chain reaction (qPCR) assay to evaluate whether raising fish for PTD analysis was necessary for estimating infection prevalence. Finally, I assessed if eDNA sampling could be used for detecting M. cerebralis to evaluate disease distribution and prevalence. Sampling of wild fish and a sentinel-caging experiment with naïve fish showed widespread but spatially structured infection risk, with site-level prevalence ranging from 0–100%. Infection was consistently detected in the mainstem and downstream portions of tributaries, while upstream headwater sites remained largely disease-free. Pepsin-trypsin digestion and qPCR showed high agreement (>95%) in detecting infection, and parasite maturation was not required for reliable detection using qPCR. In contrast, environmental DNA sampling detected the parasite at only 5 of 24 sites, where sentinel fish became infected at 14 of 24 sites, and detection varied widely over the diel cycle. Taken together, these results indicate that whirling disease is established across much of the drainage and may be hindering CRCT recovery in the LaBarge Creek restoration area. They also demonstrate that qPCR applied to fish tissue is an efficient and reliable method for assessing infection prevalence, while eDNA may underestimate infection risk under typical sampling conditions.