Selective initiation and transmission of disseminated neoplasia in the soft shell clam Mya arenaria dependent on natural disease prevalence and animal size

Abstract

Disseminated neoplasia, a diffuse tumor of the hemolymph system, is one of the six most destructive diseases among bivalve mollusk populations, characterized by the development of abnormal, rounded blood cells that actively proliferate. Though the specific etiology of disseminated neoplasia in Mya arenaria remains undetermined, the involvement of viral pathogens and/or environmental pollutants has been suggested and considered. The current study used 5-bromodeoxyuridine (BrDU) known to induce the murine leukemia virus and filtered neoplastic hemolymph to initiate disseminated neoplasia in clams from different populations and size classes respectively. M. arenaria from three locations of different natural neoplasia occurrences were divided into a control and three experimental treatments and injected with 200μl of sterile filtered seawater or 50–200μg/ml BrDU respectively. In a concurrent experiment, animals from different size classes were injected with 2.5% total blood volume of 0.2μm filtered blood from a fully neoplastic animal. Animals were biopsied weekly and cell neoplasia development was counted and scored as 0–25, 26–50, 51–75 and 76–100% neoplastic hemocytes (stages 1–4) in 50μl samples. BrDU injection demonstrated that neoplasia development in M. arenaria was dose dependent on BrDU concentration. In addition, natural disease prevalence at the source location determined initiation of neoplasia induction, with animals from the area of the highest natural disease occurrence displaying fastest neoplasia development (p=0.0037). This could imply that depending on the natural disease occurrence, a potential infectious agent may remain dormant in normal (stage 1) individuals in higher concentrations until activated, i.e. through chemical injection or potentially stress. The size experiment demonstrated that only M. arenaria between 40 and 80mm developed 26–100% neoplastic hemocytes when injected with filtered neoplastic hemolymph, indicating that individuals smaller than 20mm or larger than 80mm were not or no longer susceptible to disease development. So far neoplasia studies have not considered natural disease prevalence or size involvement in neoplasia development and our results indicate that these should be future considerations in neoplasia examinations.