Without a viable rainbow trout population, it is difficult to determine what the actual level of infectivity might be. On the upper Colorado River during the 1996 field season, we observed far greater numbers of juvenile brown trout that were suffering significantly from the debilitating effects of clinical whirling disease than we had observed in 1994 and 1995. This may indicate that the ambient level of infectivity in some portions of this stream may still be increasing. Interestingly, we saw fewer significantly affected brown trout in 1997 than we did in 1996. Clearly, there are differences among and between years and rivers that affects the dynamics of transmission of this disease from T.tubifex worms to the fish that remain enigmatic.
Much work remains to be done.
The sentinel fish studies that have been conducted over the past four years yielded significant insights into many facets of the etiology and epidemiology of this disease as it applies to salmonids in the intermountain west. When exposed at a very young age, i.e., 1-2 weeks post emergence from the gravel, brook trout are also highly vulnerable to the parasite. Infected CRR trout produce the highest levels of MC myxospores of all salmonids tested over the past four years. However, CRR trout have survived longer and at somewhat higher levels than similarly exposed brook trout, greenback, Rio Grande, and Colorado River cutthroat trout. Except for brown trout, Snake River cutthroat trout appear to have greater resistance to the debilitating effects of the parasite than any of the other salmonids tested in Colorado.