FARLEY ET AL.: MICROCELL DISEASE IN OYSTERS 



Table 2.— Percentage of prevalance of lesions and parasites in Denman Island oysters. 



systemic with diffuse inflammatory infiltration of 

 connective tissue (Fig. 5) associated with the pres- 

 ence of microcell parasites (Fig. 6). Microcells were 

 cytozoic in hemocytes and VCT. Focal abscesses 

 were present but not nearly as prominent in the 

 Hawaiian oysters as the lesions in Denman Island 

 oysters. 



Australian Winter Disease Studies in 

 Saccostrea commercialis 



Australian winter disease was characterized by 

 pustules, ulcerations, and abscesses (Fig. 7). Seven 



F'iGURE 1.— Right and left valves of Crassostrea gigas from 

 Denman Island, British Columbia. The darker rounded lesions 

 are conchiolinous shell pustules characteristic of the Denman 

 Island disease. 



Figure 2.— Scale units in Figures 2-15 are in micrometers. Macro- 

 photograph showing shell pustule (A) and adjacent tissue ulcer 

 (B). 



Figure 3.— Histologic section from Denman Island oyster. A large 

 abscess-type lesion is apparent in the connective tissue-gonad 

 region of the section. 100 x . Feulgen picromethyl blue (PPM) 

 stain (specimen WWC-2-7). 



Figure 4.— Higher magnification photomicrograph taken at the 

 edge of the lesion in Figure 3. Many microcell protistan para- 

 sites (Mikrocytos mackini sp. n.) are evident in vesicular con- 

 nective tissue cells adjacent to necrotic inflammatory cells deeper 

 in the lesion. 1,000 x . Harris' hematoxylin and eosin (HHE) 

 stain (specimen WWC-2-7). 



Figure 5.— Lesion caused by microcell infection in C. gigas from 

 Hawaii. 100 x. HHE stain (specimen S-124A-45). 



Figure 6.— High magnification photomicrograph showing micro- 

 cell parasites intracellular in hemocytes in vesicular connective 

 tissue of the Hawaiian oyster. 1,000 x . HHE stain (specimen 

 S-124A-45). 



individuals, consisting of six females and one male, 

 displayed ulcerations in the gonad and mantle. 

 Ulcerations of the gills were also common and fre- 

 quently occurred near the adductor muscle. Im- 

 paired adductor muscle contraction was character- 

 istic of the disease. 



Histologically, the animals contained abscesses 

 (Fig. 8) with intense phagocytic infiltrations in the 

 connective tissue and varying degrees of necrosis. 

 The abscesses contained a small (1-2 ^m) organism 

 (Figs. 9, 10) which contained a nucleus >1 ^m that 

 was spherical with bipolar or eccentric nucleolar 

 structures. The size and cytozoic location of these 

 organisms suggest a strong similarity to other 

 microcell type parasites seen in other species of 

 oysters. Four of the six females had gonads in a 

 state of resorption and digestive diverticular epithe- 

 lium was slightly metaplastic in two of the seven 

 oysters. 



Microcell Disease Study in Ostrea edulis 



Episode 1 



Microcell disease in 0. edulis from progeny from 

 Boothbay Harbor, ME brood stock spawned at 

 Milford, CT and transferred to Chincoteague Bay, 

 MD (Code FK). 



Two of the 13 oysters fixed between August 1961 

 and July 1962 had heavy infection of microcells (Fig. 

 13) which were 1-3 ^m in diameter with a Feulgen- 

 positive nucleus (1 yim. diameter) and found intra- 

 cellularly within hemocytes. In three of the other 

 oysters, moderate infiltrations of hemocytes were 

 observed but without parasites. 



585 



