ABSTRACT
Whirling disease, caused by the myxozoan parasite Myxobolus cerebralis Family Myxobolidae), was first detected in rainbow trout (Oncorhynchus mykiss) in Germany (1893) and has since been linked to disease and mortality in multiple species of captive and wild salmonids (Family Salmonidae). Unfortunately and despite a two-host parasite lifecycle, the widespread presence of the intermediate host, Tubifex tubifex, has facilitated further dissemination of this parasite. First discovered in the USA in Pennsylvania in 1956, the parasite has been detected in at least 24 states. In 1968, M. cerebralis made its debut in Michigan (MI) in a private trout farm located on the Tobacco River (Clare County). Despite undertaking aggressive management actions, subsequent spread was documented in multiple smaller hatcheries, ponds, and private recreational ponds that had already received infected fish prior to parasite detection. Within ten years of its first detection in MI, M. cerebralis caused an estimated $10 million dollar (US) total economic loss in 1975, when the detection of the parasite resulted in the depopulation of over 2.5 million salmonids from the MI Department of Natural Resources’ Sturgeon River Hatchery (closed in 1975) and Platte River State Fish Hatchery (currently operating). Additional introductions of M. cerebralis into Michigan waters occurred in the mid-1990s, when infected fish were stocked from another private hatchery source. The parasite is now estimated to be enzootic in ~10% of Michigan’s trout streams; however, it’s prevalence and range is incompletely understood. In fact, there have been no peer reviewed publications surrounding the parasite’s range or prevalence in Michigan waterways in nearly five decades despite periodic surveillance efforts. Thus, a year-long project supported by the Janice Lee Fenske Excellence in Fisheries Management Fellowship is currently underway to summarize the knowledge of M. cerebralis in Michigan by compiling and synthesizing decades worth of detection, surveillance, and management action data, with the objective of closing the nearly fifty-year gap of whirling disease data synthesis and understanding in the state. Spanning 16 MI counties and having been detected >100 times since 1968, M. cerebralis poses a potential threat to multiple wild trout systems in the state, particularly if abiotic conditions change. Herein, we further summarize historical and recent surveillance data, previous and ongoing management actions, and touch upon recent developments in M. cerebralis research, with an eye towards supporting and guiding fish health management decisions. This compendium of information tells, for the first time, the complicated, unresolved, and multi-decade history and story of M. cerebralis in Michigan and highlights the parasite's potential as a contemporary fish health threat in the state.