Rare contagious cancer is tearing through Washington's Puget Sound clams after East Coast jump

Scientists are tracking a serious outbreak of a contagious cancer (bivalve transmissible neoplasia) in soft-shell clams in Washington's Puget Sound.

Genetic evidence suggests the cancer lineage likely came from Atlantic Coast populations.

By 2024, more than 75% of the clams tested at two sites were infected, alarming researchers because the species helps sustain coastal habitats and communities.

What happened?

The findings were detailed in a study published in PNAS and highlighted by Eco Magazine. Known as bivalve transmissible neoplasia, the disease is unusual because it is a cancer that can pass from one animal to another through seawater.

The first infected clams were identified in 2022 from samples collected at Triangle Cove and near Stanwood. Subsequent surveys showed the outbreak worsening rapidly, with positive test rates exceeding 75% at both locations by 2024.

Genetic analysis showed that the Puget Sound cancer belongs to the same lineage previously found in Atlantic Coast soft-shell clams, indicating a relatively recent arrival in the Pacific Northwest. Researchers consider it one of the region's most significant documented outbreaks of this kind of cancer.

To check for wider spread, the team used a highly sensitive environmental DNA test on seawater collected from 51 sites around Puget Sound. They found cancer-related DNA at the infected sites and in nearby waters, though not across the entire sound.

Why does it matter?

A fast-spreading disease in clam populations could have effects beyond the clams themselves, since these animals help filter water, recycle nutrients, and support marine food webs and coastal livelihoods. That kind of outbreak puts pressure on an ecosystem already under strain.

The health of Puget Sound affects biodiversity, recreation, and shellfish resources that many Washington residents rely on and value. If contagious diseases can spread more easily between regions, protecting marine life and restoring coastal habitats may become even harder.

The outbreak could also help scientists study why some animals are less susceptible. Soft-shell clams in Puget Sound include hybrids of Mya arenaria and Mya japonica, and early evidence suggests the Japanese species may be less vulnerable to infection. That may offer clues about how wild populations adjust to new disease threats.

What are people saying?

Michael Metzger, Ph.D., a senior author and associate investigator at Pacific Northwest Research Institute, said, "We were surprised to find a transmissible cancer spreading at this level in Pacific Northwest clams."

He added, "The scale of the outbreak makes it an important system for understanding how these rare cancers emerge and move through wild populations."

Metzger also said of the new monitoring method: "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."

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