Interactions between Paramoeba perurans, the causative agent of amoebic gill disease, and the blue mussel, Mytilus edulis

Amoebic gill disease (AGD) is caused by the ectoparasite Paramoeba perurans found free-living in seawater. In recent years outbreaks of AGD have occurred in most salmon farming countries causing significant economic losses. Mussels co-cultured with salmon in integrated multi-trophic aquaculture (IMTA) systems may change pathogen dynamics on sites by acting as reservoirs or biological controls. Through the use of an 18S rRNA gene quantitative real-time PCR we tested the interactions between P. perurans and blue mussels ( Mytilus edulis ) under experimental conditions by means of water-borne transmission. Quantification of DNA from water samples revealed a rapid decrease in P. perurans DNA over two weeks in the presence of mussels under experimental conditions. P. perurans was detected on swabs from mussel shells up to 48 h post-exposure. Additionally, no P. perurans were detected in mussels collected from natural mussel beds and fish farms. These results indicate that mussels are not a likely reservoir host for P. perurans but may in fact actively remove water-borne P. perurans. Statement of relevance The blue mussel does not appear to pose a biosecurity risk as a vector for the pathogen responsible for amoebic gill disease in salmon ( Paramoeba perurans ), instead the presence of blue mussels in experimental challenges led to a rapid removal of the parasite. The findings provide valuable information for how mussels may modulate pathogen densities on finfish-mussel farms.