Neoplasia (Tumors)

bivalve mollusc miscellaneous Neoplasia (Tumors)

I. Causative Agent and Disease mesenchymal (connective tissue origin) Cancers or neoplasms are growths of tumor in a blue mussel, disseminated abnormal cells that proliferate uncontrol- neoplasia (hemocyte origin) in a blue lably. In bivalve molluscs, neoplasms mussel and littleneck clam and a secre- are rare in comparison to vertebrates but tory cell adenoma of the gill arch in a more common than in other invertebrate geoduck clam. phyla. Neoplastic lesions have been reported most commonly in oysters but III. Clinical Signs also in other marine clams and mussels Clinical signs of neoplasia may including mesenchymal tumors, epithe- include a grossly visible abnormal tissue lioma, myofibroma, germinoma, neu- mass but more commonly neoplasia in rofibroma, sarcomas and disseminated bivalve molluscs is an incidental finding neoplasia resembling leukemia. Bivalve during routine histological examination. molluscs develop cancer in much the High prevalences of disseminated or same way as do higher animals. Known hemic neoplasia in blue mussels and soft and suspected factors contributing to shell clams have been associated with neoplasia in molluscs include viruses, significant mortality of 30-80%. environmental chemicals (carcinogens), repeated physical trauma, hormones, IV. Transmission age, sex and genetic predisposition. Most neoplasms described in bivalve molluscs are considered spontaneous II. Host Species resulting from environmental contami- Neoplasia has been reported in: nation, congenital malformation, age or Pacific and Olympic (native) oysters on genetic predisposition and are not trans- the Pacific coast of North America as missible in nature. A major exception is well as the eastern oyster on the Gulf disseminated neoplasia in clams, mussels and Atlantic coasts; ocean quahog, hard and cockles which has been transmit- and soft-shell clams and bay scallop ted in mussels by cohabitation and in on the Atlantic coast; Pacific oyster mussels and cockles by injection of and mussel in Japan; pearl oyster in cancerous cell-free tissue homogenates Australia; Chilean oyster in Chile and suggesting an infectious agent. In soft- New Zealand; the European flat oyster shell clams mixed results using cell-free in Spain and Yugoslavia; the common homogenates are reported but a retro- cockle in Ireland and Spain; the Mediter- virus is suspected to cause the disease ranean mussel in Spain; the blue mussel in all three bivalve species. The highest in Great Britain, Scandinavia, Tasmania prevalences of disseminated neoplasia and North America including British Co- occur during autumn and winter months lumbia, Canada. Disseminated neoplasia with highest bivalve mortality occurring is reported to occur in at least 15 marine in late winter and early spring. bivalve species from four continents and every ocean except the Antarctic. In V. Diagnosis Alaska, neoplasms observed in bivalve Neoplasms are diagnosed and clas- molluscs have included germinoma (go- sified using histological methods to nadal origin) in an adult Pacific oyster, determine the cell or tissue of origin and

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are further grouped based on benign or VI. Prognosis for Host malignant characteristics. Benign tumors Neoplasia usually results in death of are often well-differentiated, grow the affected bivalve mollusc, although slowly, are well circumscribed without remission has been known to occur with invading surrounding normal tissue disseminated neoplasia. High preva- and do not metastasize. Most benign lences of disseminated neoplasia are neoplasms usually end in the suffix associated with high mortality of bivalve “oma”. Exceptions are benign neoplasms mollusc populations, especially those of the brain and some endocrine organs that have been previously unexposed. in higher vertebrates that can be life Due to the transmissible nature of this threatening due to their location and cancer, known infected stocks of bivalve deleterious physiological effects on the molluscs should not be introduced to new host. Malignant tumors are often not areas. well differentiated, may grow rapidly, infiltrate normal tissues and tend to me- VII. Human Health Significance tastasize. The names of these neoplasms Although aesthetically disturbing, are often preceded by the word “malig- there are no zoonotic human health nant” or with the suffixes “sarcoma” or concerns associated with neoplasia in “carcinoma”. Disseminated neoplasia bivalve molluscs. Except for dissemi- in bivalve molluscs is characterized by nated neoplasia that has a suspected viral intense infiltration of tissues by ab- etiology, cancer is generally a rare event normal hemocytes that have very little in bivalve molluscs affecting one animal cytoplasm and enlarged pleomorphic in several thousand. Should these other nuclei with frequent mitotic figures. The tumors occur more frequently in a popu- neoplastic cells can be examined from lation, an indirect human health concern wet and stained preparations of collected would be whether the cause is linked to

hemolymph and during histological environmental contamination. examination of cut tissue sections. 

Histological section of mesenchymal neo- Higher magnification of fibroblastic plasm (arrow) in blue mussel type cells composing the mesenchymal neoplasm 62 63 bivalve mollusc miscellaneous

Neoplasia (Tumors) 

Histological section of disseminated neoplasia (arrow) in blue mussel

Higher magnification of typical leukemic-like cells with large nuclei and scant cytoplasm comprising disseminated neoplasia

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bivalve mollusc Miscellaneous 

Two white nodular foci (arrow) of a secretory cell adenoma on the gill arch of a

geoduck clam 

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Histological section of geoduck adenoma Higher magnification of collagen (arrowhead) composed of concentric whorled layers and secretory-type cells (arrow) containing of basophilic mucinous material (arrow) eosinophilic droplets also found free in the containing secretory type cells and areas of tumor mass collagen

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