FARLEY ET AL.: MICROCELL DISEASE IN OYSTERS 



monthly from February 1969 to January 1971. 

 Samples were coded from WA0-7A to WAO-22A. 

 Tissues were fixed in Zenker's acetic, Davidson's 

 fluid (Shaw and Battle 1957), or McDowell's fixative 

 (McDowell and Trump 1976) as modified by Farley 

 et al. (1986) (1% glutaraldehyde/4% formaldehyde; 

 pH 7.2-7.4 in one-half ambient seawater). Six /im 

 sections were stained with Harris' hematoxylin- 

 eosin, Ziehl's fuchsin, periodic acid Schiff reagent 

 (PAS) with malt diastase digestion, Feulgen picro- 

 methyl blue (Farley 1969), or Giemsa (Howard and 

 Smith 1983). 



Electron Microscopy Procedures 



Lesions from C. gigas from Denman Island were 

 fixed in 2% glutaraldehyde in pH 7.2 seawater, post- 

 fixed in 1% osmium tetroxide in phosphate buffer, 

 pH 7.2, and embedded in Epon-Araldite (Feng et al. 

 1971). Fifty to 100 nm sections, selected on the basis 

 of interference color (silver), were cut and stained 

 with lead citrate and uranyl acetate. Ostrea edulis 

 sections from the WAG material (WAC-2-19) were 

 deparaffinized, postfixed in 2% glutaraldehyde and 

 1% osmium tetroxide, plastic embedded, ultrasec- 

 tioned, and stained in the same fashion as the C. 

 gigas materials. Sections were examined in a Zeiss^ 

 EM 9 electron microscope. 



RESULTS 



condition index remained high (most oysters in 

 medium to fat condition). Mantle recession occurred 

 most commonly from April through June and was 

 most prevalent in June. Pale digestive gland was 

 present in up to 24% of the oysters in the spring 

 and was also seen in up to 16% of the oysters in fall 

 samples. Shell pustules (Fig. 1), abscesses, and 

 ulcers as described by Quayle (1961) (Fig. 2) were 

 present from April through June. 



Microscopical examination (Table 2) revealed 

 granular hemocyte infiltration of vesicular connec- 

 tive tissue (VCT) in most samples with high prev- 

 alences of microcell infections occurring sporadically 

 throughout the years. Abscesses (Fig. 3) in the VCT 

 of the mantle and gonad consisted of apparently 

 viable granular hemocytes at the periphery with 

 phagocytosis of moribund cells deeper in the lesion 

 and coagulative necrosis in the center. Microcell 

 organisms were associated with these abscesses. 

 The parasite (Fig. 4) was 1-3 p<m in diameter, con- 

 tained a small (1 f^m) Feulgen-positive nucleus, and 

 occurred cytozoically in VCT cells and extracellu- 

 larly adjacent to and within abscesses. Microcells oc- 

 curred in 60% of the abscesses found in histologic 

 sections. However, microcells were never found out- 

 side of abscesses. 



A similar disease was discovered by F. Kern^ in 

 C. gigas from Hawaii. Microcell parasites were 

 similar in morphology and size (Figs. 5, 6) to the 

 Denman Island organism, but infections were more 



"Denman Island Disease" Studies 



Table 1 presents the seasonal prevalence of gross 

 features of samples collected in this study. Visual 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



^F. G. Kern, Northeast Fisheries Center Oxford Laboratory, 

 National Marine Fisheries Service, NOAA, Oxford, MD 21654, 

 pers. commun. 



Table 1.— Gross pathology in oysters from Denman Island, B.C. 



583 



