Whirling disease behavior and its relation to pathology of brain stem and spinal cord in rainbow trout

A long-standing assumption about whirling disease is that the whirling patt ern of swimming is caused by disequilibrium due to parasite-induced damage of the auditory-vestibular apparatus or dysfunction of the nervous system r elated to Vertebral column damage. The behavior of rainbow trout infected w ith Myxobolus cerebralis as 18-d swim-up fry was studied at 90 or 240 d aft er parasite exposure. These fish displayed a behavioral syndrome consisting of (1) sequences of tight turns (whirling), (2) periods at rest in which t he tail was elevated higher than the head, and (3) episodes of postural col lapse and immobility. In spite of these behavioral disturbances, the infect ed fish showed Virtually no behavioral indications of impaired vestibular s ystem function, such as difficulty maintaining an upright posture. Histolog ical examination of these infected fish revealed that parasite invasion of the skull and vertebral column was associated with granulomatous inflammati on that extended into the perineural cerebrospinal-fluid-containing space, producing ring-like constrictions of the upper spinal cord and sometimes co mpressing and deforming the lower brain stem. In addition, some fish showed limited degeneration of pathways that connect the medulla with the spinal cord. A neurophysiological interpretation of the whirling pattern of swimmi ng is presented, in which this abnormal behavior is proposed to result from spinal cord constriction, whereas episodes of collapse are more probably d ue to brain stem compression. The spinal cord and brain stem abnormalities that seem to be responsible for the abnormal behaviors in whirling disease appear to be caused largely by the invasion of granulomatous inflammation o f the vertebrae and skull.