A startling discovery in marine biology has emerged from Washington state’s Puget Sound, where researchers have uncovered a catastrophic outbreak of a rare and contagious cancer among soft-shell clams. This transmissible cancer, known scientifically as bivalve transmissible neoplasia (BTN), has manifested at an unprecedented scale in a region previously untouched by such disease. What makes this outbreak particularly remarkable is the genetic tracing that pinpoints its origin to clam populations on the distant Atlantic Coast, indicating a recent and alarming transcontinental spread of this malignancy.
The new research, published in the prestigious Proceedings of the National Academy of Sciences (PNAS) on June 23, 2026, reveals that the cancer’s prevalence in two affected Puget Sound locations has surged to over 75%, signifying one of the most severe outbreaks ever documented in marine bivalve populations. This finding illuminates not only the aggressive nature of this disease but also its capacity for rapid geographical expansion, with profound implications for marine ecology and the sustainability of clam populations.
Unlike typical cancers that develop within an individual organism, bivalve transmissible neoplasia operates under a unique paradigm: cancer cells themselves serve as infectious agents, capable of moving through seawater and invading new hosts. This transmissible characteristic sets BTN apart from conventional cancers and underscores its potential to disrupt marine ecosystems profoundly. Until now, BTN had been primarily associated with soft-shell clams along the Atlantic Coast, making its emergence on the West Coast entirely novel and unpredictable.
The initial detection came in 2022 when soft-shell clams collected from Triangle Cove and the Stanwood region of Puget Sound showed signs of infection. Subsequent monitoring over the following two years documented a dramatic increase in disease prevalence. By 2024, analysis demonstrated that more than three-quarters of the sampled clams in these areas had contracted BTN, confirming a rapid and widespread colonization within the clam population.
Dr. Michael Metzger, Ph.D., a leading scientist at the Pacific Northwest Research Institute and senior author of the study, expressed astonishment at the scale of the outbreak. He emphasized the value of this natural system as a living laboratory to understand the mechanisms by which such transmissible cancers arise, spread, and impact wild populations. Given the rarity of this cancer type, studying its dynamics in the early stages of an outbreak provides an unprecedented opportunity to gather insights into disease ecology and evolutionary biology.
Genetic investigations have revealed that the cancer lineage afflicting Puget Sound clams is identical to that previously characterized in Atlantic populations, confirming a recent and direct transcontinental transfer. This discovery raises pressing questions about the vectors and pathways that could facilitate the inter-oceanic movement of living cancer cells, which typically should be confined within a localized ecosystem.
Intriguingly, the affected clam populations in Puget Sound comprise hybrids between two sibling species, Mya arenaria and Mya japonica. Preliminary evidence suggests that the Japanese species, Mya japonica, may exhibit a degree of resistance or lower susceptibility to BTN infection. This observation points to the potential role of host genetics in mediating disease transmission and resistance, a complex interplay that warrants further molecular and ecological investigation.
To quantify the geographical extent of the outbreak, researchers developed an innovative environmental DNA (eDNA) assay. This method allows for the detection of cancer-specific genetic markers shed into seawater, providing a non-invasive and highly sensitive means of monitoring disease presence in marine habitats. Through this approach, cancer DNA was identified not only at known infection sites but also in adjacent waters, though it had not yet spread ubiquitously throughout Puget Sound.
The eDNA surveillance highlighted a vital tool for early detection and monitoring of transmissible cancers in marine environments. By enabling scientists to track cancer cells in seawater, this technique could revolutionize efforts to manage marine diseases before they become intractable epidemics. Early detection is particularly crucial given the potential ecological and economic impacts associated with widespread clam mortality.
Despite clear genetic evidence confirming Atlantic origins, the precise mechanisms underlying the cancer’s introduction to the Pacific Northwest remain unresolved. Researchers hypothesize that accidental human activity might have facilitated the transfer, such as the transport of infected clams or seawater containing viable cancer cells via shipping or aquaculture practices. Nonetheless, the absence of direct evidence emphasizes the necessity for heightened biosecurity measures and rigorous monitoring of marine introductions.
This unprecedented outbreak exemplifies the remarkable ability of transmissible cancers to breach enormous geographical barriers, challenging traditional notions of cancer ecology. The study further underscores the broader implications of such diseases in wild populations, where their dynamics can shape host evolution, community structure, and ecosystem health in unexpected ways.
Because the Puget Sound outbreak is in an early phase within a hybridizing clam population, researchers are uniquely positioned to observe natural evolutionary responses to the disease. Understanding how genetic diversity influences resistance evolution to transmissible cancers could guide conservation strategies not only for affected bivalves but for other species vulnerable to similar emergent pathogens.
Pacific Northwest Research Institute continues to lead efforts in this groundbreaking work, collaborating with local and global partners to unravel the ecological and genetic complexities of BTN and similar marine diseases. By advancing innovative diagnostic techniques and deepening our understanding of disease dynamics, these researchers aim to inform sustainable management policies and preserve marine biodiversity in the face of escalating environmental challenges.
Subject of Research: Animals
Article Title: Atlantic to Pacific: Outbreak of bivalve transmissible neoplasia detected in hybridizing soft-shell clams and eDNA in Puget Sound
News Publication Date: 23-Jun-2026
Web References: http://dx.doi.org/10.1073/pnas.2611852123
References: Proceedings of the National Academy of Sciences, 2026
Image Credits: Credit: PNRI
Keywords: Aquatic animals, Marine ecology, Genetics, Population genetics
