Does the Presence of the Parasite Always Result in Outbreaks of the Disease?
No! Although some research demonstrates that whirling disease can have major effects on wild populations, other studies also show that healthy trout populations can co-exist where the M. cerebralis parasite is present. The severity of the impacts are influenced by a number of factors including: * Infective dose – as fish are exposed to increasing levels of the infective stage of the parasite, the severity of the disease increases. * Fish age – severity of infection is related to age of fish. This is because after hatching the skeletal structure of newly hatched sac-fry is built from cartilage which is attacked by the parasites. As fish age, the cartilage is replaced by bone which increases resistence to continued infection. * Fish species – different species, and perhaps strains, vary in their susceptibility to the disease. For example, rainbow trout are typically much more susceptible than brown trout * Environmental stress – stressors such as pollution, crowding, and abnormal tempera
