Contagious Cancer Among West Coast Clams Likely Crossed An Ocean

A Transmissible Cancer Is Devastating Washington State Clams. The Source Points to New England.

A form of cancer is spreading among crabs in Puget Sound, Washington. As if that wasn’t disconcerting enough, new research indicates this contagious form of cancer crossed an ocean to arrive on the west coast. Scientists have found genetic evidence suggesting the Puget Sound outbreak was likely seeded by infectious cancer cells from a New England lineage, a rare documented case of this clam cancer apparently crossing from the Atlantic to the Pacific.

A team of researchers publishing in the journal Proceedings of the National Academy of Sciences reported a severe outbreak of contagious cancer in soft-shell clams at two Puget Sound locations, where more than 75% of clams were infected by 2024. East Coast populations, where this cancer has existed for decades, currently show stable infection levels of about 1% to 5% in studied populations, though severe outbreaks have been documented in the past, reaching 90% prevalence with major population losses. Puget Sound is alarming because the cancer appears to have arrived recently and escalated quickly.

Cancers that jump from one animal to another are exceedingly rare. Well-known examples include facial tumors in Tasmanian devils and a sexually transmitted tumor in dogs. In clams, these cancers travel not through touch but through the water itself. Cancer cells shed by one clam can survive in seawater and infect its neighbors.

Contagious Clam Cancer Tracked to East Coast Source

MarBTN, the cancer at the center of this research, has been documented in Eastern soft-shell clams on the Atlantic coast since the 1970s. Working like a leukemia, it floods the clam’s blood-like fluid with abnormal cells until the animal dies. MarBTN had not previously been reported in soft-shell clams on the West Coast.

When researchers first collected clams from Triangle Cove in Puget Sound in 2022, they weren’t expecting to find anything unusual. Soft-shell clams there aren’t native; they were likely introduced from Atlantic populations in the 1870s. So when 27 of 60 clams tested positive, it caught scientists off guard. Nearby Stanwood also turned up cases. Three other sites showed no cancer across all three years of sampling.

Using DNA markers unique to each branch of the cancer lineage, the team found that the Puget Sound cancer closely matches the East Coast USA branch. Infectious cancer cells were likely recently transported from New England, though by what route remains unknown.

Infection Rates at Two Puget Sound Sites Surged to Over 75% by 2024

After the 2022 discovery, the research team returned each year to track the spread. At Triangle Cove, prevalence climbed from 45% in 2022 to 32% in 2023, a dip the researchers attribute to possible sampling variation, before surging to 81% in 2024, with 30% of clams showing severe infections. At Stanwood, numbers rose from 13% in 2022 to 37% in 2023 to 77% in 2024, with 39% severely infected. Three other locations remained completely clear across all three sampling years.

While tracking the outbreak, researchers made an unexpected finding. Genetic testing showed that Japanese soft-shell clams are present throughout Puget Sound alongside Eastern soft-shell clams, and the two species appear to be interbreeding, something not previously reported.

That matters because the data hint that Japanese soft-shell clams may be less susceptible to this cancer. Among clams with the highest cancer burdens, the vast majority carried genetic markers of the Eastern species. Only a single clam with high cancer levels was identified as fully Japanese soft-shell at the markers tested. The researchers noted this trend is not statistically significant and that further sampling is needed, but the pattern raises the possibility that some clams carry natural resistance.

Scientists Used Water Samples to Track the Cancer Across Puget Sound

Perhaps the most useful tool to emerge from this research isn’t about the clams. Collecting and testing individual animals is labor-intensive and limits how broadly researchers can monitor a large waterway. So the team developed a method to detect the cancer’s DNA directly from seawater, called environmental DNA testing, or eDNA.

Each cancer cell carries thousands of copies of its genetic material, giving researchers far more DNA to work with than most pathogens leave behind. By targeting mutations unique to the USA sublineage of MarBTN, the team found a reliable signal: water near Triangle Cove’s entrance contained up to 406 copies of cancerous DNA per milliliter, still detectable at 51 copies per milliliter as far as 2.8 kilometers away. A broader 2024 survey of 51 Puget Sound sites found cancer DNA at the two outbreak locations and at sites stretching north into Skagit Bay, south into Port Susan, and along parts of Whidbey Island. Those detections don’t confirm infected clams at every location but flag where follow-up sampling may be warranted.

Areas with cancer DNA in the water largely overlapped with areas where Eastern soft-shell clams dominated. Regions where Japanese soft-shell clams were more prevalent showed no detectable cancer DNA.

East Coast clam populations have coexisted with this cancer for generations, and there are hints of early resistance in some, though that remains unconfirmed. West Coast clams have had no such history. Surpassing 75% infection at two sites in just three years shows how hard this disease can hit a population encountering it for the first time.

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