Appearance and spread of diseases among bivalve molluscs in the northern hemisphere in relation to international trade.

Bivalve mollusc culture is a well-developed marine aquaculture activity in many countries around the world, notably in the northern hemisphere. During the development of this activity, numerous countries have been confronted with infectious diseases of varying severity and duration. Research has been conducted to determine the aetiology, epidemiology and control measures for these epizootics. Major epizootics in bivalve molluscs have been caused by viruses, bacteria, fungi and protozoan parasites. Moreover important commercial relations exist in marine mollusc culture between different geographical areas. This must be taken into account in explaining the appearance and the spread of some infectious diseases in several countries around the world. The author concentrates on some viral and protozoan diseases of bivalve molluscs reported in the northern hemisphere, in view of their economic impact and their spread related to movement of molluscs through trade.