In 2016, Banff National Park found itself hosting an unwelcome guest: Myxobolus cerebralis, a parasite that kills trout, salmon, and whitefish. One year later, the disease has spread beyond Banff and has now infected the entire Oldman River and Bow River watersheds.
Whirling disease, the disease caused by the parasite, is far less whimsical than it sounds. Infected fish often develop bent or shrunken tails, which can cause them to swim in an unusual “whirling” pattern. In some infected fish communities, the mortality rate of whirling disease is up to 90%.
The disease is spread via spores, which are released when an infected fish dies. The spores infect a particular type of worm and mature into a new form, which is released into the water until it is ingested by a fish, which then becomes infected. Once the parasite is inside a host, it can cause cartilage damage and skeletal deformity.
Birds that eat fish can cause the disease to spread to new bodies of water, and so can humans, by transferring boats or items of clothing between bodies of water without first cleaning them.
Plans have been proposed to control the spread of the disease, including using nets to remove all the fish from Johnson Lake or potentially even draining the entire lake.
Whirling disease does not infect humans, and it is safe for people to use infected bodies of water and even consume infected fish. However, to prevent the disease from spreading, people who spend time in infected lakes and rivers should clean and dry boats, clothes, and gear before moving them from one body of water to another.