This condition is found throughout fish in Saskatchewan and world wide. In young wild or hatchery raised fish, abnormal curvature of the spine is frequently seen. Many of these abnormalities are so severe that the fish does not live past the fingerling stage. Other anomalies of the spine are less lethal. Many skeletal deformities are a result of previous severe muscle or nervous system damage. Toxins such as pesticides or heavy metals as well as some bacteria, protozoan parasites and vitamin deficiencies can severely affect the muscle of mature fish, or the developing skeleton of maturing fish. Lightening strike can also cause such a severe muscle contraction that the spine is broken.

Human Safety: Because the specific cause of a skeletal abnormality in any particular fish is unknown, it is safest if the affected fish is not consumed by humans.

The disease is caused by the protozoan Myxosoma cerebralis. The spores of the organism can live for years in the environment. The spores infect mud-dwelling invertebrates and mature to the stage infective to fish. Infection is by ingestion of, or skin contact with, salmonids. These mature spores infect and damage the cranial and vertabral cartilage of young fish over a two to eight month time period. The infection causes death of the cartilage, and calcium deposits in the cartilage. This results in skeletal deformities and damaged nerves.
Damage to the nerves results in the darkening of the tail area. The disease causes death of young fish and can go undetected for years in the wild. In young fish, the disease may cause death with no visible lesions nor microscopic evidence of spores. Infected hatcheries can lead to the spread of this disease to stocked waterways and the infection of native fish. Infected fish usually die in their first year as they are more susceptible to predation and malnutrition.

Human Safety: The parasite is not known to infect humans. Fish are affected within the first year of life. All fish suspected of this disease should be examined immediately to allow early detection.