Whirling Disease (Myxobolus cerebralis): A Disease Guide

Whirling disease, caused by the myxozoan Myxobolus cerebralis, damages the cartilage of young salmonids, causing skeletal deformity and whirling swimming. There is no cure.

Overview

Whirling disease is a parasitic disease of salmonid fish caused by Myxobolus cerebralis, a myxozoan parasite (now placed in the phylum Cnidaria). It is most damaging in young fish, where the parasite attacks developing cartilage and the nervous system, producing skeletal deformity and the characteristic tail-chasing 'whirling' swimming behavior. The disease has caused serious losses in trout hatcheries and severe declines in some wild trout populations.

Hosts and life cycle

Myxobolus cerebralis has a complex two-host life cycle. It alternates between a salmonid fish, especially rainbow trout (Oncorhynchus mykiss), which is highly susceptible, and the aquatic oligochaete worm Tubifex tubifex. Durable myxospores released from infected fish are ingested by Tubifex tubifex, where they develop over roughly 60 to 90 days into triactinomyxon (TAM) actinospores. These waterborne spores then infect fish through the skin, completing the cycle.

Clinical signs

• Tail-chasing, whirling swimming when startled
• Blackening (darkening) of the tail or caudal region
• Skeletal and spinal deformities
• Shortened or deformed gill operculum and head
• Impaired feeding and reduced growth
• High mortality in heavily infected fingerlings

Age susceptibility and transmission

Young fish, generally less than about five months old, are most vulnerable because their skeletons are not yet fully ossified, leaving more accessible cartilage for the parasite. Transmission requires the presence of Tubifex tubifex; the parasite is spread by waterborne spore stages and by the movement of infected fish, contaminated water, mud, or equipment. The parasite is not transmissible to humans.

Diagnosis

Moderate to heavy infections can be presumptively diagnosed from behavior and appearance, typically about 35 to 80 days after infection. Confirmatory diagnosis uses microscopic detection of spores in cartilage, the pepsin-trypsin digest method, histopathology, and PCR targeting the parasite's ribosomal genes.

Treatment, control and prevention

There is no known cure and no vaccine. Because the myxospores are extremely durable in the environment, control depends on preventing exposure of young fish and breaking the two-host cycle.

• Source and stock uninfected, certified breeding fish
• Avoid earthen ponds for fry; use smooth concrete or plastic-lined raceways
• Hatch eggs and rear fry in spore-free water until the skeleton ossifies
• Eliminate Tubifex tubifex habitat and avoid mud that may harbor spores
• Disinfect equipment and water, and remove infected fish from the population