A leukemia-like disease affecting soft-shell clams (Mya arenaria) is a contagious cancer that spreads from clam to clam, according to research published April 9 in the journal Cell. The disease has spread along the eastern North American coastline from Prince Edward Island in Canada to Chesapeake Bay.
Researchers discovered that the cancer cells themselves migrate between clams. While the exact transmission mechanism remains unconfirmed, scientists speculate that cancer cells may float through seawater and enter the mollusks as they filter water to feed. This differs from other known transmissible cancers, such as those in dogs or Tasmanian devils, which spread through physical contact or mating.
Origins and Genetic Findings
The disease was first observed in the 1970s when large numbers of soft-shell clams began dying in Maine and Chesapeake Bay. While researchers initially suspected pollution, testing revealed the cancer could spread between clams even in clean water. Further investigation into the clams’ genetic makeup showed that while the cancer cells were nearly identical to one another, they were genetically distinct from the host clams they inhabited.
For more on this story, see Contagious Leukemia Spreading Among Atlantic Soft-Shell Clams for Decades.
Genetic analysis also identified a retroelement called Steamer within the clams. While healthy clams typically carry between two and 10 copies of Steamer, sick clams contain between 150 and 300 copies. Researchers believe the disease likely originated in a single soft-shell clam at least 40 years ago.
No Risk to Humans
Experts emphasize that the disease does not pose a threat to people. Bruce Barber, an invertebrate pathologist at Eckerd College, stated, It’s only a shellfish health issue. It’s not a human health issue.
Future research is needed to understand how the cancer cells exit and enter the clams and how the Steamer retroelement contributes to the disease. S. Anne Boettger, a physiological ecologist at West Chester University of Pennsylvania, suggested that rather than using chemical treatments in the water, a potential solution may involve identifying and breeding clams that are naturally resistant to the cancer.
