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Identification of MSX vectors to support decisions related to the inter-provincial movements of mussels: are mussels a vector for MSX transmission?



Multinucleated sphere "X" (MSX) is an infectious disease that causes high mortalities in American oysters (Crassostrea virginica) although it does not affect human health. Transfers of all live shellfish from one area to another is regulated under the Fisheries Act and are reviewed on a case-by-case basis by the Introductions and Transfers Committee (ITC). Hence understanding how the transfer of mussels from MSX positive areas and if they might be a vector for the transmission of MSX to naïve populations would help inform ITC decisions and identify ways to mitigate the risks of MSX exposure.

The ITCs took a precautionary approach by recommending that mussels from MSX infected areas be considered as potentially infected and that mitigation measures be undertaken before and during the transfer activities. Through this study, targeted sampling of wild oysters, mussels and associated organisms including Polychaetes, Isopods, Gastropods, Amphipods, and crabs were performed in East Bay, Nova Scotia, an MSX infected area within the Bras d'Or Lakes. In the laboratory, organisms were screened for MSX through histological and/or qPCR analysis. The results of this research project provided the aquaculture industry and the ITCs with information that can be used during risk assessments related to the movement of mussels from MSX affected areas and will help to make more informed decisions.


Targeted sampling of wild mussels and epifauna in an area with MSX infected wild oysters detected MSX by a molecular test - Real Time Polymerase Chain Reaction (qPCR). However, the disease was not visually detected histologically (i.e., through the direct examination of the tissues). These results support industry concerns that although mussels are not believed to be a host of MSX, a life stage of the parasite may either be present in the intra-valvular fluid of the mussels or the intermediate host may be found within the associated epifauna. Therefore continued mitigation to reduce the risk of the inadvertent spread of MSX should be considered during risk assessments of industry activities.

Program name

Program for Aquaculture Regulatory Research (PARR)


2013 - 2014

Principal investigator

Mary Stephenson
Head Shellfish Health Unit, Gulf Fisheries Centre, Gulf Region

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