What is it?
Whirling disease is a chronic disease caused by Myxobolus cerebralis, a parasitic protozoan that affects mainly juvenile salmonids.
Where and when might it occur?
Susceptibility to the disease is influenced by water temperature, age and species.
Young fish are highly susceptible as the parasite attacks their soft cartilage, resulting in nerve damage, skeletal deformities and sometimes death.
When fish are 8 to 10 cm long, cartilage forms into bone and they are no longer susceptible to disease; however they remain carriers of the parasite.
The intermediate host, the worm Tubifex tubifex, is found in Australia.
The parasite spreads mainly through the stocking of infected fish and also through the alimentary tracts of fish-eating migratory birds.
Diagnosis
Clinical signs of the disease are not evident until fish are approximately 7 cm long.
Signs of the disease often include mass mortalities in fry, convulsive movements, increased rate of breathing and jerking backwards movements.
Fish also tend to swim in a whirling motion (tail chasing) and show erratic then nervous darting movements until exhausted.
Gross pathological signs are:
- darkening of the skin from the vent to the tail (blacktail)
- spinal curvature
- skull deformation and shortened gill plates.
Control and treatment
As tubifex worms live in mud, the disease can be partly controlled in trout farms by growing young fish in concrete raceways.
Source: Australian Government, Department of Agriculture, Fisheries and Forestry