Diseases of salmonids resembling Myxosporean whirling disease, and the absence of Myxosoma cerebralis, in South America

Previously published reports of whirling disease (WD) of salmonids (rainbow trout Oncorhynchus mykiss, chinook salmon O. tshawytscha and coho salmon O. kisutch) in 4 South American countries (Venezuela, Colombia, Ecuador, Chile) are reviewed. The disease(s) were shown not to be associated with infections with the myxosporean Myxobolus cerebralis, the cause of myxosporean WD. Our recent observations in Ecuador and Chile of salmonid diseases with clinical signs resembling those of myxosporean WD further demonstrated that the diseases in South America were not caused by M. cerebralis. In Chile, the diseases in coho salmon and rainbow trout were attributed to bacterial infections, whereas no infectious agent was associated with afflicted rainbow trout in Ecuador. We conclude that myxosporean WD of salmonids does not occur in South America. The causes of several other diseases of salmonids with clinical signs similar to those of myxosporean WD (e.g. whirling behaviour, skeletal deformities, blackened tails) include bacterial or microsporidial infections of the meninges or brain, myxobacterial infections of the vertebrae, and dietary deficiencies. A diagnosis of myxosporean WD, therefore, cannot be made on clinical signs alone. Laboratory confirmation of the diagnosis through demonstration of the presence of M. cerebralis is necessary.