Retention of viable microsporidial (Loma salmonae) spores within the blue mussel (Mytilus edulis): Use of an experimental laboratory model probing pathogen transfer within a multi-trophic aquaculture setting

An in vivo rainbow trout (Oncorhynchus mykiss) branchial xenoma expression model was used to detect the presence of viable microsporidial spores (Loma salmonae) within aquarium-held blue mussels (Mytilus edulis) experimentally exposed to a suspension of semi-purified L. salmonae spores. In two separate studies, viscera from mussels exposed to spores for 4 hr were fed to trout that subsequently developed xenomas within their gills 6 weeks post-infection. The results from these studies provide proof of principle evidence that L. salmonae spores are captured from the water column by mussels, and remain viable within mussel tissue. However, mussel-fed fish yielded significantly fewer xenomas compared to reference fish exposed to an intraperitoneal injection of spores. A spore-transfer efficiency value was calculated (xenomas/spore dose) and revealed 0.03% of spores led to xenomas in IP injected fish, while 0.005% of spores exposed to mussels yielded xenomas in viscera-fed fish. Relative spore-transfer efficiency provided by mussel filtration and IP injection is therefore 16.6%. In a separate trial, we examined the influence of time, after mussels ingest spores for 4 hr, on xenoma development in trout. While 75% of fish became infected after 4 hr of exposure (similar to previous trials), only 10% became infected after 7 days, which indicates that blue mussels may act as a reservoir for infectious L. salmonae spores for at least 7 days. In a final pilot study, we aimed to determine whether the addition of mussels to L. salmonae contaminated water provides bioremediation. The percentage of trout that became infected was similar in fish exposed to contaminated water versus those exposed to contaminated water filtered by mussels. Interestingly, trout exposed to water filtered by mussels, developed higher mean xenoma counts which suggests that viable spores are expelled via mussel feces and pseudofeces, which can then be consumed by trout.

► Upon ingestion by blue mussels, Loma salmonae spores remain infective.
► Spores remain viable within blue mussels for at least 7d post-exposure.
► Spores appear to remain viable upon release from blue mussels to the environment.
► Mussels act as a reservoir for Loma salmonae, which may impact cultured salmonids.