DAO 49:171-178 (2002)  -  doi:10.3354/dao049171

Response of rainbow trout Oncorhynchus mykiss to exposure to Myxobolus cerebralis above and below a point source of infectivity in the upper Colorado River

Kevin G. Thompson1,*, R. Barry Nehring2, David C. Bowden3, Terry Wygant4

1Colorado Cooperative Fish and Wildlife Research Unit, Colorado State University, Room 201 Wagar Building, Fort Collins, Colorado 80523, USA
2Colorado Division of Wildlife, Colorado Division of Wildlife, 2300 South Townsend, Montrose, Colorado 81401, USA
3Department of Statistics, Colorado State University, Room 221, Statistics, Colorado State University, Fort Collins, Colorado 80523, USA
4Colorado Division of Wildlife, Meeker Service Center, PO Box 1181, Meeker, Colorado 81641, USA
*Present address: Colorado Division of Wildlife, 2300 South Townsend, Montrose, Colorado 81401, USA. E-mail:

ABSTRACT: We exposed 9 wk old rainbow trout Oncorhynchus mykiss to ambient levels of Myxobolus cerebralis infectious stages at 4 sites of suspected differing infectivity in the Colorado River. Exposure was estimated by periodic filtration of river water at each exposure location. After a 32 d exposure, the fish were held in the Colorado River at a common site for over a year. Resulting infection was evaluated by the presence of clinical signs (whirling behavior, cranial deformity/exophthalmia, and black tail), severity of microscopic lesions, and myxospore counts (8, 10, 12, and 14 mo post-exposure). Two exposure sites that were immediately downstream of Windy Gap Reservoir were much higher in infectivity than the site above the reservoir or the site 26 km downstream of the reservoir. Rainbow trout exposed at those locations showed higher prevalence of clinical signs of whirling disease, more severe histological evidence of infection and higher average myxospore concentrations than those exposed above the reservoir or 26 km below the reservoir. Many more M. cerebralis actinospores were observed from water filtration at the 2 sites immediately below the reservoir compared to the other sites.

KEY WORDS: Myxobolus cerebralis · Whirling disease · Rainbow trout

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