Myxosoma cerebralis, an important pathogen of young salmonids, is responsible for whirling disease, also known as “blacktail.” Typically, infected fingerlings show rapid tail-chasing behavior when startled, and the peduncle and tail may darken significantly. As infected fish age, skeletal deformity may result from damage to the cartilaginous structures, particularly the skull and vertebral column. Recovered fish remain carriers, and adults do not show signs, although skeletal deformities do not resolve.
The disease can be prevented by purchasing uninfected breeding stock and maintaining them in an environment free of the intermediate hosts.
A presumptive diagnosis of whirling disease is made by detection of spores from skulls of infected fish. Samples can be submitted to a fish disease laboratory, or procedures described by the American Fisheries Society can be followed. Diagnosis may be confirmed histologically or serologically. Whirling disease is of regulatory concern in some states.