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Salmonid whirling disease-sometimes referred to as "blacktail" -is a chronic, noncontagious parasitic infection of trout and, to a lesser extent, other members of the family Salmonidae. The parasite has specific tropism for cartilage. If the infection is heavy and the fish is young, mortality sometimes results. More commonly, the parasitosis evokes abnormal swimming behavior and the transient melanism of the caudal fin and peduncle that is the blacktail sign (Fig. 1).
When the parasite is given sufficient time for development and the infection is sufficiently intense, the head and axial skeleton become disfigured in some vIictims (Fig. 2). However, the behavior and appearance of lightly infected fish are commonly normal, or nearly so (Fig. 3).
The parasite belongs to the group of protozoans known as myxosporeans. Although it was first encountered in Europe at the turn of the century, its life cycle was not discovered until the early 1980's. During the intervening years, methods of diagnosis, detection, and identification were developed and improved.