Dynamics of Myxobolus cerebralis in the Lostine River, Oregon: Implications for Resident and Anadromous Salmonids

Myxobolus cerebralis , the myxozoan parasite causing whirling disease in salmonids, was first detected in northeastern Oregon in 1986. To better understand the potential impact of M. cerebralis on resident and anadromous salmonids in this region, a series of studies was conducted in the Lostine River, where infected fish were first reported. Rainbow trout Oncorhynchus mykiss fry were exposed in sentinel cages for 14 d at five sites from near the river's mouth to the headwaters during the period from July 1998 to November 1999. A subsample of exposed fry was assayed for the presence of DNA of M. cerebralis by polymerase chain reaction. Following four exposures, fish were held in the laboratory for 5 months of observation. The severity of infection in these fish was determined by histological examination, enumeration of spores isolated from the cranial cartilage, and observation of clinical signs. Sentinel fry became infected throughout the river from March to November, although the prevalence of infection increased in groups exposed in late summer. Infection prevalence was highest among fish exposed at the lower river site, ranging from 22.2% to 100%. Among fish held for 5 months, only those exposed in the lower river developed cranial lesions, whirling behavior, and black tail. The parasite's oligochaete host Tubifex tubifex was consistently identified only from the lower river site. Of the water quality measurements taken at each location, only conductivity correlated with infection prevalence ( P < 0.001). These data indicate that juvenile rainbow trout, steelhead (the anadromous form of rainbow trout), and chinook salmon O. tshawytscha , which emerge during times of peak infection, are exposed to M. cerebralis and are likely to become infected in this river.