1. Forty days. Slight erosion of carti- 

 lage. SchSperclaus (1954) p. 379 (our fig. 5). 



2. Sixty -five days. Still trophozoite 

 only; no tissue proliferation. Schaperclaus 

 (1931) p. 542, 554. 



3. Three months (figs. 6, 7, and 8) . 

 Cartilage still being eroded by trophozoites in 

 a lesion -like cavity in cartilage; cellular and 

 cartilage debris present in lesion, bone being 

 formed at periphery of skeleton. No tissue 

 proliferation or inflammation. 



4. Four months (figs. 10, 11, and 12). 

 Some spores now present, but still no prolifer- 

 ation. 



5. Eight - 12 months (fig. 13). Spores 

 present. Trophozoites no longer present. Epi- 

 thelioid proliferation of fish tissue now surrounds 

 the mass of spores. This cyst-like structure is 

 sometimes referred to as a granuloma which may 

 cause pressure on vital organs (Plehn, 1904). 



DIAGNOSIS 



Before development of the spores (about 

 four months) the disease can be tentatively 

 diagnosed on the basis of the symptoms- -tail - 

 chasing, whirling, and black tail. Verification 

 can be made only by finding the ameboid stages 

 in the cartilage of the head in histological sec- 

 tion. Very early stages are more difficult to 

 find but at three months they are often easily 

 found and consist of trophozoites 5 to 30|ain 

 size with many nuclei about 1-1/2 to 2|Jin 

 diameter. 



From 4 months post infection to at least 

 2 years, the spores can be found easily in wet 

 mounts or histological section. Wet material 

 may be prepared by dissecting out the auditory 

 capsule and crushing it (Plehn, 1904) or split- 

 ting the head lengthwise and scraping the poste- 

 rior part of the cranium with a scalpel to free 

 the spores. A more reliable method for fish, 

 up to at least 2 years of age, is to cut up the 

 head with scissors and macerate in a Waring 

 blendor in about 50 ml of water- -the spores can 

 usually be found in a random drop. 





SPORE 





*■!?•* # • 



t% 



EPITHELIOID 

 HOST TISSUE 





c,^' 



Figure 13: — Photomicrograph of Myxosoma 

 cerebralis spores in an epithelioid 

 granuloma at 12 months post-infection. 

 Stained with Giemsa's to show spores 

 (x U30). 



Figure lU: — Photomicrograph of spores of 

 f^osoma cerebralis from rainbow trout 

 at 12-15 months post-infection. Stained 

 with Giemsa's (x 970). Photo by Dr. 

 E. M. Wood. 



II 



