A mass mortality event (MME) affecting the fan mussel Pinna nobilis was first detected in Spain in autumn 2016 and spread north- and eastward through the Mediterranean Sea. Various pathogens have been blamed for contributing to the MME, with an emphasis on Haplosporidium pinnae, Mycobacterium sp. and Vibrio spp

This study used samples of 762 fan mussels of various health conditions, taken before and during the MME, to assess the role of pathogens in the MME. 

The authors state that the most important factor playing a main role in the onset of the mass mortality of P. nobilis throughout the Mediterranean Sea was the infection by H. pinnae. It was the only non-detected pathogen before the MME while, during MME spreading, its prevalence was higher in sick and dead individuals than in asymptomatic ones, in MME-affected areas than in non-affected sites, and it was not associated with host size, infecting both juveniles and adults.

Conversely, the study shows, that infection with mycobacteria was independent of the period (before or during MME), from the affection of the area by MME and from the host health condition, and it was associated with host size.

https://www.frontiersin.org/articles/10.3389/fmars.2022.666640/full

Grau et al. (2022) Wide-Geographic and long-term analysis of the role of pathogens in the decline of Pinna nobilis to critically endangered species. Frontiers in Marine Science 9:666640.