EPIZOOTIOLOGY OF MYXOBOLUS CEREBRALIS, THE CAUSATIVE AGENT OF SALMONID WHIRLING DISEASE IN THE ROCK CREEK DRAINAGE OF WEST-CENTRAL MONTANA

Abstract Whirling disease, caused by the myxozoan parasite Myxobolus cerebralis, remains a serious health threat to salmonid fish in the western United States. A previously published study on the epizootiology of whirling disease in the Rock Creek watershed of west-central Montana, conducted from 1998 to 2003, showed that the intensity of M. cerebralis infections in sentinel trout increased significantly throughout the drainage and that the range of M. cerebralis had expanded considerably. In addition, the parasite had apparently caused a dramatic decline in rainbow trout densities, but the brown trout population numbers had increased. This earlier study was continued from 2004 to 2008 and the results are reported here. It now appears that the disease intensity may have peaked in 2006 and is on the decline in this watershed. The decline cannot be directly attributed to a change in the prevalence of M. cerebralis–infected Tubifex tubifex, as these numbers remained statistically the same from 1998 to 2008. ...