Variation in natural infection outcomes and cancer cell release from soft-shell clams (Mya arenaria) with bivalve transmissible neoplasia

Bivalve transmissible neoplasias (BTNs) are leukemia-like cancers found in at least 10 bivalve species, in which the cancer cells themselves transfer from one individual to another, spreading as an unusual form of infectious disease. Before the infectious etiology was known, there were reports of lethality and outbreaks of cancer in the soft-shell clam (Mya arenaria) on the east coast of North America. Using sensitive and specific qPCR assays, we followed the outcomes of BTN in naturally-infected soft-shell clams from Maine, USA. We observed variable outcomes, with about half of clams (9/21) progressing to high levels of cancer and death, about half exhibiting long-term non-progression (11/21), and a single animal showing regression of cancer. We also observe a significant decrease in survival in animals that progress to >10% cancer in their hemolymph, while we see no effect on survival in clams with BTN that are long-term non-progressors. As most bivalves do not physically contact each other, and BTN cells can survive in seawater, it has been proposed that BTN is spread through release of cancer cells into the water. We used qPCR to detect BTN-specific sequences in environmental DNA (eDNA) in the tanks of animals throughout this experiment. We show that BTN-specific eDNA (likely from released cancer cells) can be detected in tank water of most clams with >24% cancer in their hemolymph, but not below this level. This detection of BTN eDNA is variable and occurs in bursts, but in clams with >24% cancer, the detection of BTN eDNA correlates with progression of the cancer in the hemolymph. This study demonstrates the lethality of BTN, but the observation that about half of clams with BTN do not progress to death provides evidence suggesting that there may be a block to the progression of BTN in a large portion of clams in a population with this enzootic disease. This study also further supports the hypothesis that BTN cells transmit through seawater and provides insights into the mechanisms of the transmission dynamics.