FARLEY ET AL.: MICROCELL DISEASE IN OYSTERS 



Episode 2 



Microcell disease in 0. edulis transferred from 

 Milford, CT to Elkhorn Slough, CA (Code WAC). 



Epizootiological gross and histopathologic data 

 are presented in Table 3. Clear association is ap- 

 parent between high mortality, emaciation (watery 

 condition), mantle recession, hemocytic infiltration, 

 and heavy microcell infection. 



Infections were particularly intense in gill and GI 

 tract epithelia which contained dense infiltrations 

 of infected hemocytes (Fig. 11). Infections were 

 characterized by the intracellular (in hemocytes) and 

 extracellular presence of "microcell" organisms 

 (Fig. 12) similar to those seen in the Chincoteague 

 Bay (FK) samples. 



Episode 3 



Microcell disease in 0. edulis transferred from 

 Milford, CT to aquaria in Oxford, MD (Code FMT). 



The experiment ran from 20 February 1968 until 

 9 October 1968. No transmission was noted. How- 

 ever, tank C drained accidentally on 19 March and 

 the filtered seawater was replaced with unfiltered 

 seawater from Chincoteague Bay, VA. A combined 

 total of 9 of 11 animals died between 22 March and 

 27 March and the remaining 2 live oysters were 

 fixed on 28 March. 



The first six that died were from tank C and had 

 moderate to heavy cases of microcell disease with 

 associated hemocytic infiltration. Histologic exam- 

 ination of both species, either after mortalities, or 

 when they were sacrificed in October, failed to 

 reveal any more infections. The parasites and 



disease characteristics were identical to the pre- 

 viously described cases (Fig. 14). 



Microcell Disease Study in Ostrea lurida 



Microcell disease in 0. lurida from Yaquina Bay, 

 OR (Codes WA0-7-A and WA0-12-A). 



Monthly to bimonthly samples of 50 specimens 

 were examined grossly and for histopathology for 

 a 2-yr period. A 24% prevalence of microcell disease 

 was seen in WA0-7-A samples collected in February 

 1969, and 12% prevalence was seen in the WAO- 

 12-A samples collected in February 1970. No micro- 

 cell infections were seen in any of the other samples. 

 However, neoplasms and Mytilicola orientalis in- 

 fections were seen commonly throughout the study. 



Microcell infections tended to be less intense than 

 in 0. edulis, but organisms were similar in size and 

 appearance to the organisms seen in flat oysters 

 (Fig. 15). Intracellular infections were in VCT cells 

 and not hemocytes. 



Ultrastructural Studies 



Electron microscopy revealed 0. edulis parasites 

 that were usually intracellular with up to three 

 organisms per cell. Parasites were 1-3 f^m in diam- 

 eter, and contained an eukaryotic nucleus about 1 

 /im in diameter. A crescent-shaped, peripheral 

 nucleolus was evident in most organisms (Fig. 16). 

 The cytoplasm contained numerous ribosome-like 

 organelles and spherical dense bodies, presumably 

 membrane bound, which were 90-130 nm in diam- 

 eter, and generally resembled the "haplosporo- 

 somes" (Fig. 17) described in other haplosporidan 



Table 3. — Epizootiology of microcell disease in O. edulis from California. 



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