Myxobolus cerebralis in Norway
Myxobolus cerebralis is a myxosporean parasite known to cause whirling disease in rainbow trout (Oncorhynchus mykiss) and other salmonids. The parasite has a two-host life cycle involving the aquatic oligochaete worm Tubifex tubifex as the definitive host and salmonid fish as the intermediate host. The first documented occurrence of M. cerebralis in Norway was in farmed rainbow trout held in earthen ponds near Oslo in 1970. Toward the late 1980s, additional cases of whirling disease were reported in salmonids reared under similar conditions. Despite these early reports, and sporadic monitoring for M. cerebralis in connection with declarations related to roe export, only three subsequent cases are recorded in the Norwegian Veterinary Institute, suggesting that the parasite has remained rare. This sequence demonstrate the rediscovery of M. cerebralis in Atlantic salmon and brown trout in Norway.