Factors affecting the resistance of juvenile rainbow trout to whirling disease

Whirling disease has been linked to significant losses of rainbow trout in wild trout populations. I examined the relationships among the whirling disease pathogen (Myxobolus cerebralis), its hosts, and the environment, with emphasis on factors affecting the resistance of rainbow trout to whirling disease. I first determined the effects of age on the development of whirling disease in rainbow trout. Whirling disease was substantially reduced when the fish were exposed to the parasite for the first time at 9 weeks posthatch (756 degree-days) or older. Second, I determined if the relationship of increasing age at exposure causing a reduction in disease severity was a factor of age or size of fish at exposure. I demonstrated that both the age of fish at first exposure and the size of fish at first exposure were important for the development of whirling disease. Third, I determined if rainbow trout can develop an acquired immune response to M. cerebralis and whether it provides the fish with protection against subsequent exposures. Rainbow trout did develop an acquired immune response to the parasite and an initial immunization exposure provided the fish with protection against subsequent exposures. However, the immunization, which provided the fish with protection, also induced disease in the fish, which outweighed the benefits of the protection. To reduce the effects of whirling disease on rainbow trout, they should be reared in M. cerebralis-free waters for 756 degree-days of development or until they are 40 mm in length, whether in the wild or in a hatchery situation.