Localization of the pathogen Vibrio P1 in clams affected by Brown Ring Disease

Since 1987, Brown Ring Disease, caused by a bacterial pathogen named Vibrio P1 (VP1), has spread throughout aquacultural sites for the manila clam Ruditapes philippinarum along the French Atlantic coast. Experimental pathology revealed that the heterotrophic bacterial microflora increased during the development of the disease and VP1 levels remained relatively stable. The greatest proportion of VP1 bacteria was associated with the surface of the clam flesh. Surface bacteria represented about 90% of the VP1 amount in experimentally diseased clams. Similarly, the mantle did not seem to be deeply colonized by VP1, since an alcohol rinse precluded detection of the pathogen by bacteriological methods. Microscopic observations of heavily diseased clams also revealed that VP1 is particularly associated with the periostracal lamina. The possibility of any other major site of infection can be eliminated.