A review of the risk posed to Scottish mollusc aquaculture from Bonamia, Marteilia and oyster herpesvirus

Shellfish aquaculture forms a small but expanding section of Scotland's seafood production. This production benefits from a good health status with the absence of notifiable diseases, except for localised cases of Bonamia ostreae. In this paper we review factors that could lead to the introduction and/or spread of key diseases in order that associated risk can be minimised. We selected B. ostreae and Bonamia exitiosa, Marteilia refringens and oyster herpesvirus as key pathogens; all of which have been found in and are notifiable within the United Kingdom. Vulnerabilities of molluscs farmed in Scotland to these pathogens are reviewed, as are sources of infection in neighbouring areas from which the pathogens might be introduced. We then examine: key pathways of introduction, whether conditions are suitable for the pathogens to persist in Scotland, potential for spread within Scotland and likelihood, and consequence of disease outbreaks. The major pathway for potential introduction is with the import of Pacific oysters from hatcheries. B. ostreae is the pathogen that has the highest probability of causing disease, however B. ostreae is restricted to native oysters whose production in Scotland is very small. Oyster herpesvirus could cause far larger economic consequences due to the size of the Pacific oyster industry but is less likely to invade. Therefore risks for B. ostreae and oyster herpesvirus are similar. Risks could change if imports are sourced from higher risk areas or pathogens spread in areas where currently used hatcheries are located. Water temperature in Scotland is marginal for oyster herpesvirus and M. refringens so continuation of warming trends could increase risks from these pathogens.

► Scottish shellfish production is small, but growing, and has good health status.
► Key disease risks to the industry are assessed.
► Bonamia and oyster herpesvirus are rated as the highest risk diseases.
► Greatest exposure risk comes with imported Pacific oysters.
► The Strathclyde region is the most exposed to potential disease invasions.