Marine transmissible cancer navigates urbanised waters, threatening to spillover

Transmissible cancer, a unique form of microparasites that spreads through direct transmission of living cancer cells, is increasingly reported in marine bivalves. In this study, we sought to understand the ecology of the propagation of Mytilus trossulus Bivalve Transmissible Neoplasia 2 (MtrBTN2), a transmissible cancer affecting four Mytilus mussel species worldwide. We investigated the prevalence of MtrBTN2 in the mosaic hybrid zone of M. edulis and M. galloprovincialis along the French Atlantic coast, sampling contrasting natural and anthropogenic habitats. We observed a similar prevalence in both species, likely due to the proximity of the two species in this region. Our results showed that ports had higher prevalence of MtrBTN2, with a hotspot observed at a shuttle landing dock. No cancer was found in natural beds except for two sites around the hotspot, suggesting spillover. Ports may provide favourable conditions for the transmission of MtrBTN2, such as high mussel density, confined sheltered shores, or buffered temperatures. Ships may also spread the disease through biofouling, with maritime traffic being the best predictor of MtrBTN2 prevalence. Our results suggest ports may serve as epidemiological hubs, with maritime routes providing artificial gateways for MtrBTN2 propagation. This highlights the importance of preventing biofouling on docks and ships’ hulls to limit the spread of marine pathogens