OTHER DISEASES, OTHER MOLLUSCS Hemic Neoplasia of Bivalve Molluscs The disease known as hemic, hematopoietic, or hemocytic neoplasia HCN! is also referred to as hemic proliferative disease, leukocytic neoplasia, sarcomatous neoplasia, sarcomataidproliferative disorder, disseminated sarcoma, and atypical hemocyte condition. As a neoplasia, it is considered ta be a form of cancer of shellfish similar to leukemia in higher animals and man in the way it affects the host. It should be emphasized, however, that this is a cancer of shellfish, not of humans, and that consuming shellfish with this condition poses na known health threat ta humans. Soine research has suggested that the disease is caused by a virus, but this is not yet confirmedor generally accepted. However, it has been shawn in some cases to be highly contagiousfrom one individual shellfish to another. The disease occurs throughout the warld in a variety af bivalve molluscs and appears to causesignificant mortality in certain farmed populations of shellfish. Geographic Range and Species Infected The diseaseaffects many speciesthroughout the world. Like many other shellfish diseases,it is probably more widely distributed than is now known. The following species and locations have been identified: Adula cali fornica, Pacific coast of North America; Artica i slandica mahagany quahog!, Rhode Island Saund, Atlantic caast of Narth America; Ceras- todermaedule common cackle!, Cork Harbour, Ireland; Saccostrea commerciali,s Australian rockoyster!, Australia; Crassostreagigas Japaneseor Pacific oyster!, Matsushima Bay, Japan;Crassostrea rhizophorae, Brazil; CrassostreaUirgi,mica Eastern or American oyster!, Atlantic coast of North America, discontinuously fram the Chesapeake Bay ta Long Island Soundand sites on the Gulf coast; Macoma calcarea, Baffin Island, Canada; Macoma nasuta andM. irus, Yaquina Bay, Oregon; Mya arenaria soft shell clam!, Atlantic coast of North America,discontinuously from Chesapeake Bay to England; Mya truncata, BaRin Island, Canada;Mytilus edulis bay or blue mussel!, Pacific caastof North America, discontinuously froni Yaquina Bay, Oregon, to sites in British Columbia, North Wales, Denmark, Finland, andsouthern coast of England; Ostrea chilensis Chilean oyster!, Chiloe, Chile; Ostrea luri da Olympia oyster!, Yaquina Bay, Oregon; and Ostrea edulis European flat oyster!, Mali-Stonarea of Yugoslavia near Dubrovnic, Ria de Noya in Galicia, Spain, and the Brit- tany region of France. 29 80 / MOLLUSC DISEASES / Elston Mortality Rate, Environmental Factors, and Seasonality Some cultured populations appear to be 100% infected if individual animals are monitored over severalmonths. Mortality rates due to the diseaseare reported to approach 100%over an annual period in soinespecies. In other cases,in cultured populations,annual mortality rates of 30%-50% are typical. Specific environmental factors that induce or enhance the disease are not known. Although much researchhas beenconducted to determine whether various types of pollu- tion contribute to the disease, no single factor which has these effects has been identified. Hemic neoplasia appears to be highly infectious, and dense populations of farmed shellfish maintain high disease levels because of ease of transnussion from one animal to another. In all speciesin which seasonalityhas beeninvestigated, the diseaseis reported to be most prevalent judging by percentageof infected individual shellfish! during fall and winter months, typically from Octoberthrough March. The prevalencedrops in the spring and summer, possibly because heavily infected individuals die in the winter and the disease does not start another cycle of infection until autumn. Diagnosis Diagnosisis basedon microscopicexamination of blood or histological examination of tissuesby a qualifiedpathologist. Common signs of the diseasefor all affectedspecies are not established,but the followingare knownto applyin soinecases: failure to produce maturereproductive follicles; high levelsof mortality whichspread geographically; and tissues swollen from the massive proliferation of abnormal blood cells in individuals with advanced cases of the disease. Prevention an8 Management Disease-free Areas Every effort should be made toavoid introducing infected shellfish into areas that do not have the disease, Hemic neoplasiais known to be contagiousfrom one animal to an- other within a given species. It is not known, however, if it can be transmitted from one speciesto another. It is possible,using techniquesof pathologicalexamination, to establish reasonableassurance of the presenceor absenceof the diseasein givenpopulations of shellfish.Field observationsindicate that somepopulations within a givenspecies may be more resistant than others. Areas Known to Have the Disease Eradication of the diseaseis not feasible sincethe diseasecan persist at low levels in natural populations of shellfish. General managementmethods based on available knowl- edgeconsist of keepingcultured population densities as low as practicaland scheduling harvests so that market-sizedshellfish are harvested beforethe typical period of increased diseasein the fall and winter. Spat from wild spawn should not be allowed to collect on the shells of older infected bivalves. It alsoappears that the severityof infectionin individual shellfishand the percent- ageof individualsthat are infectedmay increaseas the animalsget older. Thus,it maybe desirableto harvest all individuals at as early an ageas possibleand to removeolder shellfish from the population. Other Diseases, Other Molluscs / 81 References Alderman,D. J., P. Van Banning,and P. Colomer.1977. Two European oyster Ostrea edulis! mortalities associatedwith an abnormal hernocyticcondition. Aquaculture 10:335- 340. Brousseau,D. J. 1987. Seasonalaspects of sarcomatousneoplasia in Mya arenaria soA;- shellclam! from Long Island Sound. Journal of InvertebratePathology 50:269-276. Cooper,K R,, R. S.Brown, and P. W. Chang.1982. Accuracy of bloodcytological screening techniquesfor thediagnosis of a possiblehematopoietic neoplasm in thebivalve mollusc, Myaarenaria. Journal of InvertebratePathology 39:281-289. Cooper,K. R.,R. S. Brown, and P. W. Chang.1982. The course and mortality of a hernato- poieticneoplasm in the soft-shellclam, Mya arenaria. Journal of InvertebratePathology 39;149-157. Cosson-Mannevy,M. A., C. S.Wong, and W, J, Cretney. 1984. 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