Key words: Myxobolus cerebralis, lesion location/severity, Yellowstone cutthroat trout

Myxobolus cerebralis infection patterns in Yellowstone cutthroat trout

Silvia Murcia (1)*, Billie L. Kerans (1), Beth MacConnell (2), Todd M. Koel (3)

1  Department of Ecology, 310 Lewis Hall, Montana State University, Bozeman, Montana 59717, USA  smurcia@montana.edu; bkerans@montana.edu

2  Fish and Wildlife Service, Bozeman Fish Health Center, 920 Technology Blvd S,
Bozeman, Montana 59718, USA  Beth_MacConnell@fws.gov

3  Center for Resources, Fisheries and Aquatic Sciences Section, P.O.Box 168
Yellowstone National Park, Wyoming 82190, USA  Todd_Koel@nps.gov

Susceptibility to Myxobolus cerebralis infection varies among salmonid species and sub-species. Little information exists regarding lesion severity and location, or time required for parasite myxospores to develop in subspecies of cutthroat trout. The Yellowstone cutthroat trout Oncorhynchus clarki bouvieri of Yellowstone Lake face significant threats posed by whirling disease (first detected in the lake in 1998). Loss of this native fishery would have devastating ecological and economic consequences. In 2002-2003 we performed six 10-day exposures of Yellowstone cutthroat fry in Pelican Creek, an infected tributary to Yellowstone Lake. Ninety and 150 days post-exposure, we examined the fish for clinical signs, for infection prevalence by PCR, and by histology for M. cerebralis infection location and severity of lesions. Unlike previously reported for rainbow trout Oncorhynchus mykiss or brown trout Salmo trutta, the most prevalent clinical sign in Yellowstone cutthroat was whirling behavior. Prevalence of infection and severity of cartilage lesions were not statistically different between fish held for 90 or 150 days post-exposure.  Histopathology was most severe in cartilage of the cranium and jaws, whereas cartilage of the vertebrae and gill arches was seldom damaged; peak infection severity was observed 90 days post-exposure. We suggest that Yellowstone cutthroat trout are highly vulnerable to M. cerebralis infection.  Current population declines in the Yellowstone Lake basin may in part result from whirling disease.  Our results answer important questions in fish health and will aid in developing possible diagnostic and management efforts against this introduced pathogen in native cutthroat trout and other vulnerable salmonids.