Figure 1: — Photograph of black -tail 

 rainbow trout caused by t^^yxosoma 

 cerebralis - about 3 months post- 

 infection. Photograph by S. F. 

 Snieszko. 



Figure 2: — Photograph of eastern brook 

 trout with spinal curvature and 

 black tail due to >^osoTna cerebralis 

 infection. Photograph by S. F. 

 Snieszko. 



6 months of age; an estimated 100 percent of 

 these developed whirling symptoms, but no 

 black tail. 



Survivors' symptoms (figs. 2 and 3). Those 

 infected fish which were not killedby the parasite 

 during the early stages of the disease tend to 

 recover althoug^i they may be misshapen, part- 

 icularly in the head. The black tail and whirling 

 usually disappear but tlie spinal curvature and 

 misshapen head may reflect permanent damage. 

 The two most common and obvious head symptoms 

 are the sunken areas behind the eyes and the 

 permanently open or twisted lower jaw. All of 

 these symptoms are caused by the loss of carti- 

 lage during bone formation when the skeleton is 

 weakened and support is lost. EXjring the latter 

 part of the first year much of the damaged area 

 is filled in with an epitheloid granuloma type of 

 tissue which tends to proliferate in many instances 

 and cause secondary damage. Plehn (1904) p. 163 

 has an excellent illustration of such a tissue pro- 

 liferating from a vertebra and causing pressure 

 on a sympathetic nerve. Presumably these 

 "recovered" fish can live for a long period of 

 time so long as they are not crippled too badly 



Figure 3' — Photograph of yearling rainbow 

 trout with spinal curvatvire due to 

 J^ ^QSom a cerebral is infection. 



to feed. In spite of apparent recovery, however, 



the spores remain in the tissues of the fish for 

 at least 3 years. 



