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

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.