With the summer lake season just around the corner, fish and game officials are on high alert for additional cases of the deadly whirling disease in fish. The disease is caused by a tiny parasite called Myxobolus cerebralis, a microscopic pathogen that attacks the cartilage of growing fish, causing damage and inflammation in the skull and spine and the spinal cord and brainstem within these structures.
The backbone of affected fish often twists and contorts, leading to the abnormal swimming behaviour that gives the disease its name. Other signs of the disease include sudden death, slow growth, darkening of the tail portion of the body and abnormal skull shape.
Fish in the salmonid family are susceptible to the disease including trout, various salmon species and graylings. There are also age differences when it comes to disease susceptibility. Up to 100 percent of newly hatched fish may die of the disease.
The rate of fatal infections reduces as the fish age. After about six months old, fish are nearly resistant to infection. This is because the cartilage skeleton is gradually replaced by bone as fish age, which leaves less cartilage for the parasite to attack and hence, less severe disease.
The lifecycle of this parasite is complex and requires the presence of a worm. When fish die and decompose or are eaten by a predator or scavenger, the parasite is released into the water. Next, a Tubifex worm eats it. Inside the worm, the parasite proliferates in the worm’s guts and develops into a stage than can live freely in the water. This free-living stage is released into the water when the worm poops. The parasite then seeks out and adheres to the gills, mouth and skin of fish. From there, it reproduces and spreads to the cartilage. The lifecycle is complete when the newly infected fish dies or is eaten.
What makes this disease particularly problematic and difficult to control is that the stage of the parasite released from dead fish is highly adapted to survive in the environment until a worm ingests it. It can remain infective for more than a year. Furthermore, the worms remain infected and can be an ongoing source of the parasite.
Fish with the typical whirling swim pattern and spinal abnormalities may be suspected of the disease but there are other conditions in fish that may look similar.
Confirmatory testing includes autopsy of the fish, microscopic examination of the tissues and molecular laboratory tests. Under the microscope, the parasite is particularly sinister, with a pair of alien-like spheres that resemble eyes.
Whirling disease first appeared in the northeastern region of the United States in the 1950s and has since spread to western states including Montana. Then in 2016, the disease was confirmed in fish from Johnson Lake in Banff National Park. Despite efforts to contain the disease, it has spread to several Alberta river systems including the Oldman, Red Deer and North Saskatchewan.
There is no direct risk to human or other animal health beyond fish. However, this disease can have devastating impacts of farmed fish, hatcheries and also wild fish populations.
It isn’t known exactly how the parasite reached western Canada, although like many emerging disease issues, it was likely moved through human actions. Preventing the spread of this disease should be a high priority. People who fish should seek out information about how to avoid moving this disease. Basic recommendations include avoiding the movement of fish between waterbodies and cleaning, draining and drying all equipment after each use and especially if switching from one lake or river to another.
Dr. Jamie Rothenburger is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine.