What’s killing mussels across Scandinavia?

I have collected samples from seven mass mortality events (MMEs) in Norway and Sweden over four field trips. These MMEs appear to be caused by some infectious disease. In brief, this is because the mortality appears downstream of locations in rivers where mussels have been transplanted for restoration programmes, indicating the potential transmission of some etiological agent during restoration attempts.

The general sampling design I have undertaken is to sample both sick and healthy mussels from the impacted area and healthy mussels from a non-impacted area. I also sample water from both impacted and non-impacted areas. I have been searching these samples for any signals of potential pathogens. So far, I have completed several studies, including metagenomics, histology, hemocytology, experimental infections, and bacteriology. However, I haven't found any significant results. Well, that's not entirely accurate—there may be some evidence suggesting the involvement of a gregarine parasite. I will provide an update in a few weeks!