ABSTRACT
The original report of M. cerebralis infection in 1903 (Höfer, 1903) described spores in the brain of the affected fish, hence the designation ‘cerebralis’. However, subsequent studies of the infection revealed that the parasite localizes in the cartilage, primarily of the cranium and spine. Destruction of this tissue causes constriction of the spinal cord and brain stem, and severely infected fish are unable to maintain equilibrium. The resulting erratic whirling or tail chasing behaviour is the hallmark of the disease. Another common disease sign in young fish is darkening of the tail (blacktail), a condition that fades as the fish age. In older fish that survive infection, the disease is usually manifest as skeletal deformities such as a misshapen cranium, shortened opercula, misaligned jaws and spinal curvature (Hedrick and El-Matbouli, 2002).
