taminated water. They were observed for 5 

 months; except for one which developed whirl- 

 ing symptoms, growth and behavior were normal. 

 The affected fish was sectioned at 3 months of 

 age and the developing stages of M. cerebralis 

 were readily recognized in the cartilage. 



DEVELOPMENT STAGES IN TROUT 



Prior to 3 months (fig. 5) 



Presumably the sporoplasm has made its 

 way through the intestinal wall and migrated via 

 blood or lymph channels to the cartilage, mainly 

 of the head. This seldom affects fish over 12 

 months of age (Plehn, 1904, 1924; Schaperclaus, 

 1931, 1954). Our attempts to infect fish experi- 

 mentally failed, so we could not demonstrate this 

 stage. Schaperclaus (1954) p. 379 (our fig. 5) 

 has an excellent photomicrograph of the develop- 

 ing trophozoite which has "eroded" a cavity for 

 itself in the cartilage. As near as we can deter- 

 mine this is from a 40-day-old fry, so the para- 

 site must be 40 days or less of age. 



Figure ^'. — Trophozoite of ?tp:osonia 

 cerebralis in cartilage at about 

 UO days post-infection (drawn from 

 photo of Schaperclaus, 19$h) > 



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



Our only specimen of known age is this 

 one. It was from one of many sac -fry brought 

 from Benner Spring, Pa., during the epizootic, 

 to Leetown for rearing in water free of M. 

 cerebralis. Since it must have become infected 



at Benner Spring, we know the approximate age 

 of the parasite. At this stage the parasites are 

 multinucleate ameboid trophozoites and in cross 

 sections range from 5 x 5 to 30 x 8u in diameter; 

 the smaller ones are possibly cross sections of 

 elongate trophozoites. There are at least 18 

 nuclei about 1-1/2 - 2uin diameter in each tro- 

 phozoite. These exist in lesions measuring 

 about 300 X 100 u in the cartilage. Also in the 

 lesions are freed cartilage cells and remnants 

 of disintegrating cells and cartilage matrix. 

 Apparently in normal osteogenesis in the trout 

 studied, the cartilage of the skeleton in certain 

 places is eroded from within by vascular action 

 at the same time that bone is being laid down on 

 the outside of the skeletal structures. Blood 

 vessels penetrate the cartilage, and pockets of 

 it are eroded, presumably by enzymes released 

 from the capillaries. These pockets of carti- 

 lage erosion (fig. 9) resemble M_. cerebralis 

 lesions, but contain blood vessels, disintegra- 

 ting cartilage cells and cartilage matrix and 

 sometimes multinucleate host cells but no 

 parasites. The multinucleate cells resemble 

 phagocytic giant cells, but Ruth-i suggested 

 that they might be tissue cells that have failed 

 to divide. 



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



The multinucleate trophozoite has grown 

 considerably but is still in the cartilage. Some 

 of the nuclei have divided rqjeatedly to form 

 groups of nuclei (or cells?) which are now known 

 as pansporoblasts. These distinct units produce 

 the spores, usually two each. Kudo (1960) be- 

 lieves that myxosporideans are distinctly multi- 

 cellular at this stage and, therefore, much dif- 

 ferent from other protozoa. Kudo (1930) and 

 Noble (1944) have reviewed pansporoblast form- 

 ation in other Myxosporidia . At this age some 

 of the pansporoblasts have already produced 

 spores. The trophosoite (entire parasite) has 

 probably reached its maximum size at this stage; 

 those measured were up to 1 mm in greatest di- 

 ameter although we do not Know for certain 

 whether two or more trophozoites may be in such 

 close association that their boundaries are not 

 discernible. 



3/ Ruth, Delbert. Anatomy Department, John 

 Hopkins Medical School, Baltimore, Maryland, 

 pers. comm., 1961. 



