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

•Water-borne Paramoeba perurans densities rapidly reduced in the presence of mussels.•No P. perurans detected in mussel tissues from natural mussel beds or fish farms.•P. perurans detected on the surface of mussel shells up to 48 h post-exposure.•Artificial surfaces may provide a substrate for P. perurans attachment.

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 relevanceThe 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.