The study, published in the Proceedings of the National Academy of Sciences (PNAS), reports that the cancer’s prevalence exceeded 75% at two Puget Sound locations by 2024, making it one of the most significant outbreaks of bivalve transmissible cancer documented in the region.
Unlike most cancers, which remain confined to a single individual, bivalve transmissible neoplasia (BTN) spreads when living cancer cells pass from one animal to another through seawater. The disease has previously been found in soft-shell clams along the Atlantic Coast but had never been documented in West Coast populations.
Researchers first detected the disease in 2022 in clams collected from Triangle Cove and the Stanwood area of Puget Sound. Follow-up surveys revealed that prevalence increased dramatically over the next two years. By 2024, more than three-quarters of the clams sampled at both locations tested positive for the disease.
“We were surprised to find a transmissible cancer spreading at this level in Pacific Northwest clams,” said Michael Metzger, Ph.D., senior author of the study and Associate Investigator at Pacific Northwest Research Institute (PNRI). “The scale of the outbreak makes it an important system for understanding how these rare cancers emerge and move through wild populations.”
Genetic analyses showed the cancer belongs to the same lineage previously identified on the East Coast of the US, indicating a recent transcontinental transfer. The outbreak appears to be in its early stages, offering researchers a rare chance to observe how wild clam populations respond to the disease.
The study also found that soft-shell clam populations in Puget Sound include hybrids of two closely related species, Mya arenaria and Mya japonica. Preliminary evidence suggests the Japanese species may be less susceptible to the disease, raising new questions about how host genetics influence cancer transmission and disease resistance.
To better understand the outbreak’s extent, the researchers developed a highly sensitive environmental DNA (eDNA) assay capable of detecting cancer-specific genetic markers directly from seawater samples. Using the method, they surveyed 51 locations across Puget Sound and detected cancer DNA at sites with infected clams and throughout surrounding waters, but did not find it spread throughout all of Puget Sound.
“This approach gives researchers a new way to monitor the spread of transmissible cancers in marine ecosystems and may help identify emerging outbreaks before they become widespread,” said Metzger.
While genetic evidence indicates the cancer originated from Atlantic Coast populations, researchers have not yet determined how it reached the Pacific Northwest.
“The most likely explanation is accidental human-assisted transport of an infected clam or seawater containing cancer cells, but we don’t yet have evidence that allows us to determine the precise route,” said Metzger. “Understanding how transmissible cancers move between regions will be critical for monitoring and managing future outbreaks.”
The findings demonstrate how contagious cancers can spread across vast geographic distances and highlight the potential of environmental DNA to track emerging marine diseases in the wild. Because the outbreak was identified at an early stage in a hybridizing clam population, it also provides a rare opportunity to study how resistance to a transmissible cancer may evolve in a natural population.
