FARLEY ET AL.: MICROCELL DISEASE IN OYSTERS 



Mikrocytos mackini sp. n., which has eccentric 

 nucleoH, and M. roughleyi sp. n., which has a cyto- 

 plasmic vacuole and a nucleus that is displaced to 

 the periphery of the cell, are morphologically distinct 

 from B. ostreae (the only closely related species). All 

 three species occur in separate host species and all 

 appear to be host specific. 



DISCUSSION 



Denman Island Disease 



Quayle's original study (1961) documented heavy 

 mortality (40%) clearly associated with surface 

 tissue pustules in C. gigas in an area of British 

 Columbia in May. Studies of C. gigas by Mackin 

 resulted in the discovery of the "microcell" organ- 

 ism and its association with tissue abscesses. Sub- 

 sequent histological examinations done by the senior 

 author, in cooperation with D. Quayle and N. 

 Bourne, confirmed the association of mortality, 

 pustules, tissue abscesses, and microcell infections 

 that have continued to occur during each May and 

 June to the present time. 



Histopathologically, this disease (caused by Mikro- 

 cytos mackini) is characterized by acute inflam- 

 matory abscesses which remain focal until the oyster 

 dies or resolution occurs. While microcell organisms 

 are not always found in abscesses, they are never 

 found in oysters that do not have abscesses, in- 

 dicating at least an associational relationship. 



Electron microscopy has demonstrated only one 

 stage of a small protistan organism that contains 

 organelles resembling haplosporosomes. No clear 

 demonstration of mitochondria has been accom- 

 plished. The haplosporosome-like organelles often 



IGURE 16.— Scale units in Figures 16-21 are in nanometers. Elec- 

 tron micrograph of deparaffinized Ostrea edulis from Califor- 

 nia. Two intracellular microcells showing prominent nuclei with 

 peripheral nucleoli. 36,000 x. 



IGURE 17.— Higher magnification of California microcells show- 

 ing haplosporosome-like bodies (A) and a probable mitochondria 

 (B). 108,000 X. 



IGURE 18.— Electron micrograph of several microcells in vesicular 

 connective tissue of Crassostrea gigas (A) from Denman Island, 

 British Columbia. Probable degenerate microcells can also be 

 seen in the cytoplasm of phagocytic hemocytes (B). 7,320 x . 

 IGURE 19.— Electron "micrograph" of microcell from Denman 

 Island Crassostrea gigas. Note the eccentric nucleolus (A), myelin 

 bodies (B), haplosporosome-like bodies (C), and virus-like entities 

 (D). 36,900 x. 



IGURE 20.— Electron micrograph of Crassostrea gigas microcell 

 with structure suggestive of division. 36,900 x. 

 IGURE 21.— Higher magnification of electron micrograph show- 

 ing haplosporosomes and virus-like dense bodies. 108,000 x. 



tend to be elongated and contain layers of mem- 

 branes. Internal structure of these organelles is not 

 nearly as dense as that seen in Bonamia or other 

 haplosporidans. 



The small 45 nm dense bodies also seen in the 

 cytoplasm have characteristics suggestive of virus 

 structure; namely, uniform size, abundant occur- 

 rence in the cytoplasm only, and a suggestion of 

 icosahedral symmetry. The cytoplasmic occurrence, 

 46 nm size, icosahedral symmetry, and the presence 

 of an envelope are characteristics of the family 

 Togaviridae. The lack of paracrystalline arrays, 

 strategy of development, empty capsids, and extra- 

 cellular occurrence prevent conclusive identification 

 of these particles as virus at this time, and these may 

 prove to be an exclusive organelle that is a char- 

 acteristic of this protistan group. The presence of 

 a lytic virus in the parasite could also explain the 

 self -limiting nature of the focal abscesses character- 

 istic of the Denman Island disease. 



Australian Winter Disease 



Since Roughley described this disease in an Aus- 

 tralian S. commercialis in 1926, little progress has 

 been made toward identification of the etiologic 

 agent. Careful examination of the tissues of affected 

 animals, collected by Peter Wolf, has revealed ap- 

 parent small cytologic and histozoic organisms 

 associated with abscesses. These organisms have 

 features such as size, morphology, and tissue loca- 

 tion remarkably similar to organisms present in 

 other oyster microcell diseases. Peter Wolf (unpubl. 

 data) has stated that this disease thrives in high 

 (30-357oo) salinity (and is unknown in lower salin- 

 ities); the incubation period is about 2V2 months and 

 mortality does not occur in animals less than 3 years 

 old. The occurrence of shell lesions and focal tissue 

 abscesses appears to be common features in Sac- 

 costrea and Crassostrea. This may indicate a degree 

 of resistance characterized by the ability of the 

 animal to isolate parasites in a focal lesion and to 

 eliminate them by either rupture of the abscesses 

 or diapedesis through the mantle epithelium to the 

 shell surface, with subsequent calcification via shell 

 pustule formation, or it may indicate a host parasite 

 relationship response. This mechanism was reported 

 in oysters that had acquired resistance to Haplo- 

 sporidium nelsoni (Farley 1968). 



Kern (fn. 6) found microcell infections in C. gigas 

 from Hawaii (Fig. 6) that were not always asso- 

 ciated with focal abscesses. General systemic 

 infection and inflammatory infiltration were noted 

 in these cases (Fig. 5), but the organisms appear 



591 



