FARLEY ET AL.: EPIZOOTIC SARCOMA IN SOFT SHELL CLAMS 



removed, while slides were wet-fixed in an aldehyde 

 fixative (1% glutaraldehyde/4% formaldehyde) 

 (McDowell and Trump 1976) in half ambient sea- 

 water and stained with Feulgen picromethyl blue 

 (Farley 1969), dehydrated, and mounted with a 

 coverslip using a synthetic mounting medium. We 

 are designating the term "histocytology" to describe 

 this technique. The significance of this method is 

 that the monolayer preparations, which result from 

 treating living cells with histologic procedures, are 

 permanent. Cytologic artifacts are minimal and 

 cases can be accurately staged using cell counting 

 procedures. Since histocytologic preparations con- 

 tain between 100,000 and 500,000 cells in a mono- 

 layer, very early stages of the proliferative process 

 can be diagnosed. Staging is arbitrarily determined 

 by estimating the number and determining the ratio 

 of both normal and neoplastic cells (Table 2). A 

 similar diagnostic and staging method using cyto- 

 logic techniques was reported by Cooper et al. 

 (1982b); however, our method appears to have bet- 

 ter accuracy and increased sensitivity to light cases. 

 Diagnosis of histologic sections is reliable for stages 

 3-5 (Fig. 1A). As an example, comparison of late 

 spring samples shows that histocytology is the more 

 sensitive method while histology alone clearly 

 demonstrates a massive increase in prevalence from 

 zero in 1969-78 to 29.5% in 1984 (Table 1). 



Monoclonal antibody was developed against neo- 

 plastic clam cells from Massachusetts clams by tech- 

 niques described elsewhere (Reinisch et al. 1983). 

 Periodic histocytologic diagnosis and mortality ob- 

 servations were made on clams held in 55 L aquaria 

 with 15°/oo, 10 °C artificial seawater, circulated 

 through floss and charcoal filtering systems. 



RESULTS 



Sarcomas in clams were diagnosed histologically 

 in 1/25 in November 1979 from Eastern Bay; 1/25 

 in May 1981 from West River; 1/50 in November 

 1981 from Little Choptank River; and 1/75 in Janu- 

 ary 1983 from Chester River. In December 1983, 

 histocytologic diagnoses of clams obtained from a 

 local seafood restaurant showed 8/18 with sarcomas. 



An intensive survey and study of local populations 

 was initiated in December 1983 to evaluate the ex- 

 tent of this apparently new epizootic in Chesapeake 

 Bay soft shell clams. Table 1 presents epizootiology 

 of field collections while Table 2 shows comparable 

 information on laboratory-held clams. Field preva- 

 lences were found to be high in most samples from 

 December 1983 through April 1985. At the same 

 time, disease intensities which were light in Decem- 



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