Non-myxosporidan blacktail of salmonids

Yearling Atlantic salmon, Salmo salar , rainbow trout, S. gairdneri , and brown trout, S. trutta , at several hatcheries in Portugal showed clinical signs that mimicked some features of whirling disease caused by infection with the myxosporidan Myxosoma cerebralis . Populations of small fish had a noticeable prevalence of blacktail, involving abnormally dark pigmentation of the entire caudal peduncle. On the basis of some of the above signs, a tentative diagnosis of whirling disease was made; however, histologic examination in Portugal, as well as examination of scrapings of gill arches and-surfaces of sagittally cut heads, failed to reveal spores of M. cerebralis . Considering that no myxosporidans were found, that there was marked evidence of skeletal abnormalities - particularly in the older and larger fish - and that there were suggestions of less blacktail in older fish, the authors hypothesize that the basic cause was a deficiency of tryptophan or ascorbic acid or both.