Abstract
Whirling disease, caused by the myxozoan parasite Myxobolus cerebralis, remains a serious health threat to salmonid fish in the western U.S. Although various aspects of this host-parasite system have been studied, investigations examining the overall epizootiology of whirling disease in an ecosystem are lacking. Therefore, in June 1998, such a study was initiated in the Rock Creek watershed of west central Montana. Parameters examined included assessing the intensity of infection in trout using sentinel cages stationed throughout the drainage and determining the percentage of Tubifex tubifex releasing M. cerebralis at sentinel cage and other localities. Also, habitat and water quality parameters were measured. Overall, the intensity of M. cerebralis infections in sentinel trout increased significantly throughout the drainage between June of 1998 and 2006 with the biggest jump occurring between 1998 and 1999. In addition, the range of M. cerebralis expanded considerably over this period. There was no strict correlation between habitat condition and the occurrence of the parasite as fish became heavily infected in optimal and marginal habitats. During this period, the parasite apparently caused a dramatic decline in wild rainbow trout densities but the brown trout population numbers increased. However, it now appears that disease intensity 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 T. tubifex as these numbers remained statistically the same from 1998 to 2008. Similarly, changes in water temperature and water flow do not account for the decrease in disease intensity. However, it is possible that wild rainbow trout are developing some resistance to the parasite, a phenomenon recently documented to be occurring in the Willow Creek Reservoir of southwest Montana.
Start Date
5-3-2010 2:00 PM
End Date
5-3-2010 2:30 PM
Document Type
Presentation
Epizootiology of the Causative Agent of Salmonid Whirling Disease in the Rock Creek Drainage: 1998-2008
Whirling disease, caused by the myxozoan parasite Myxobolus cerebralis, remains a serious health threat to salmonid fish in the western U.S. Although various aspects of this host-parasite system have been studied, investigations examining the overall epizootiology of whirling disease in an ecosystem are lacking. Therefore, in June 1998, such a study was initiated in the Rock Creek watershed of west central Montana. Parameters examined included assessing the intensity of infection in trout using sentinel cages stationed throughout the drainage and determining the percentage of Tubifex tubifex releasing M. cerebralis at sentinel cage and other localities. Also, habitat and water quality parameters were measured. Overall, the intensity of M. cerebralis infections in sentinel trout increased significantly throughout the drainage between June of 1998 and 2006 with the biggest jump occurring between 1998 and 1999. In addition, the range of M. cerebralis expanded considerably over this period. There was no strict correlation between habitat condition and the occurrence of the parasite as fish became heavily infected in optimal and marginal habitats. During this period, the parasite apparently caused a dramatic decline in wild rainbow trout densities but the brown trout population numbers increased. However, it now appears that disease intensity 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 T. tubifex as these numbers remained statistically the same from 1998 to 2008. Similarly, changes in water temperature and water flow do not account for the decrease in disease intensity. However, it is possible that wild rainbow trout are developing some resistance to the parasite, a phenomenon recently documented to be occurring in the Willow Creek Reservoir of southwest Montana.