Much like a dog chasing its tail, some fish are swimming in tight circles, but they are unable to stop. Since its introduction to the United States some 60 years ago, “Whirling Disease” has ridden relatively clean waters with disease. Fish in the slamonid family such as salmon, trout, and whitefish are susceptible to the disease via a microscopic parasite, known as Myxobolus cerebralis. The parasite is found in the rivers and streams of 23 states, several European countries, South Africa and New Zealand.
This Myxobolus cerebralis parasite comes into contact with a common aquatic worm and uses the worm as its intermediate host. As time progresses, the parasite enters into its free-floating life phase. When the parasite contacts a slamonid during this stage in its life, the parasite forces its way through the fish’s head and spinal cartilage. The parasite begins to multiply rapidly after finding its new host. The organ pressure of the infected fish is disrupted because of the parasite’s growth and the fish begins exhibiting strange swimming behavior—swimming in circles. The abnormal swimming pattern that the parasite inflicts upon its host makes the fish unable to avoid predators or nourish itself. When an infected fish dies, the parasite thrives. Thousands of parasitic spores are released and the cycle begins again. These spores can withstand drastic temperature fluctuation and have a lifespan of an average 25 years.
The parasite was introduced to the United States in the 1950s, originating from Europe. Since 1997, extensive research initiatives have been put in action to combat the spread of and learn about Whirling Disease. This research has confirmed which fish are high risk and which are better at fending off the parasite. In 2000, low stream flows in Montana resulted in high levels of infectivity. Thus, the conclusion has been that drought, causing low stream flows, results in higher infection rates of whirling disease. For those who fear that they too will contract Whirling Disease and begin running in circles, fear not, for only fish in the slamonid family are prone; however, the disease is causing a rapid depletion of trout and salmon across the United States. Currently, there is no cure or effective remedy to eliminate the parasite’s presence in the waters, nor is there a way to eradicate the parasite from individual fish. Juvenile fish infected with Whirling Disease have a high mortality rate. Other than the characteristic “tail chasing” behavior, infected fish may exhibit a blackened tail or head or spinal deformities.
Felt-bottomed boots have been largely blamed for the widespread of the disease. When fisherman wear felt-bottomed boots, the parasite gets trapped in the fabric, so when the fisherman travels to another river, the parasite is also transported and the disease is introduced into previously Whirling Disease-free waters. Wildlife is also attributed to the rapid spread of the disease, but it’s practically impossible to restrict their movement between different streams. Thus, the focus has been on human activity.
Whirling Disease is posing a great threat to the survival of slamonid fish. The rapid spread of the disease is especially concerning because no efficient method of containment have been developed, despite great efforts. As of now, the focus is on fisherman and urging them to clean their gear after leaving a river stream.
Photo credit: nas.er.usgs.gov/queries/SpeciesList.aspx?Size=50&Group=&Sortby=1&status=0&FMB=0&pathway=0&HUCNumber=02070003