Myxobolus cerebralis, “Whirling Disease”

Cartilage necrosis with intralesional Myxobolus cerebralis spores.
Cartilage necrosis with intralesional Myxobolus cerebralis spores.

What is Myxobolus cerebralis?

M. cerebralis is the myxozoan parasite that causes Whirling Disease in salmonids. This parasite infects wild and cultured salmonid populations, and in many states it is a reportable disease due to its potential to cause both significant production loss as well as resulting in fish movement restrictions that can inhibit activities of producers who export fish. M. cerebralis is often required to be tested for prior to exporting and restocking fish.

How do fish become infected with M. cerebralis?

An infected fish releases spores into the environment which are then eaten by aquatic tubifex worms. The worms must be exposed to spores from an infected fish to be able to transmit M. cerebralis; tubifex worms cannot spread this parasite without infected fish being present in their environment. Inside the worm, the parasites replicates and the infective stage is released into the water. The infective stage invades fish by attaching to their gills and skin. M. cerebralis cannot be transmitted directly from fish-to-fish; the aquatic tubifex worm is required for parasite transmission.

This image illustrates the life cycle of the parasite Myxobolus cerebralis, which causes whirling disease in fish. The life cycle involves multiple stages and hosts, including fish and Tubifex worms.

What happens to fish infected with M. cerebralis?

M. cerebralis infects the cartilage around the skull and spinal cord. Infected fish can have head and spinal deformities, dark tails, and poor growth rates, as well as behavioral changes such as abnormal swimming (sometimes in a “whirling” pattern) and inappetence. Sometimes infected fish can display no clinical signs at all and appear normal.

Can Whirling Disease be treated?

Fish cannot be treated for Whirling Disease. Infected fish can clinically recover and be carriers of the parasite. Upon death, infected fish can release spores into the environment propagating infections in new areas.

What tests are available for M. cerebralis?

Certification testing to screen fish for M. cerebralis before export or stocking almost always requires following methodology from the American Fisheries Society-Fish Health Section Blue Book’s USFWS/AFS-FHS Standard Procedures for Aquatic Animal Health Inspections. This test involves performing a pepsin-trypsin digest on cartilage from fresh or frozen samples and then visualizing the spores under a microscope. This health topic has more information about and pictures of how WADDL performs the pepsin-trypsin digest testing. If myxospores consistent with M. cerebralis are visualized, PCR and histopathology testing can be performed as confirmatory testing. Because WADDL is an AAVDL-accredited laboratory with a robust Quality Management System, samples are kept frozen for three months following results being reported in case clients require any additional testing on submitted samples.

For fish that are suspected to have Whirling Disease, diagnostic testing also includes performing histopathology and PCR.

How can I get more information about certification and diagnostic testing for Whirling Disease?

For more information about testing aquatic species at WADDL, please call us at 509-335-9696.

This resource was created by visiting veterinary externs Karly Evans and Marisa Gazzola.