Pathology and infectious agents of unionid mussels: A primer for pathologists in disease surveillance and investigation of mortality events

Abstract

Freshwater mussels are one of the most imperiled groups of organisms in the world, and more than 30 species have gone extinct in the last century. While habitat alteration and destruction have contributed to the declines, the role of disease in mortality events is unclear. In an effort to involve veterinary pathologists in disease surveillance and the investigation of freshwater mussel mortality events, we provide information on the conservation status of unionids, sample collection and processing techniques, and unique and confounding anatomical and physiological differences. We review the published accounts of pathology and infectious agents described in freshwater mussels including neoplasms, viruses, bacteria, fungi, fungal-like agents, ciliated protists, Aspidogastrea, Digenea, Nematoda, Acari, Diptera, and Odonata. Of the identified infectious agents, a single viral disease, Hyriopsis cumingii plague disease, that occurs only in cultured mussels is known to cause high mortality. Parasites including ciliates, trematodes, nematodes, mites, and insects may decrease host fitness, but are not known to cause mortality. Many of the published reports identify infectious agents at the light or ultrastructural microscopy level with no lesion or molecular characterization. Although metagenomic analyses provide sequence information for infectious agents, studies often fail to link the agents to tissue changes at the light or ultrastructural level or confirm their role in disease. Pathologists can bridge this gap between identification of infectious agents and confirmation of disease, participate in disease surveillance to ensure successful propagation programs necessary to restore decimated populations, and investigate mussel mortality events to document pathology and identify causality.

Multisystemic inflammatory disease in Pheasantshell (Unionidae,Actinonaias pectorosa) associated withYokenella regensburgeiinfection at sites experiencing seasonal mass mortality events

“…To the authors' knowledge, bacterial diseases have not yet been described in wild freshwater mussels (Knowles et al, 2023).…”

“…(non-mortality-associated) lesions in mussels (Burcham et al 2023;Knowles et al 2023). Gonadotropic microsporidia were unique to case mussels, potentially reflecting immunosuppression, or simply predilection for larger (sexually mature) mussels.…”

“…Overall, our study provides baseline information and description of normal and abnormal cell and tissue anatomy of freshwater mussels, which is generally lacking (Knowles et al 2023;McElwain and Bullard 2014). Most importantly, we provide a comprehensive pathological description of lesions associated with Y. regensburgei and address its pathogenicity (Leis et al 2023).…”

“…To the authors’ knowledge, bacterial diseases have not yet been described in wild freshwater mussels (Knowles et al, 2023). This finding is remarkable because bacterial diseases in bivalves more broadly are largely limited to aquaculture, particularly hatchery-reared juvenile stages, facilitated by various artificial conditions (Paillard and Borrego 2004; Travers et al 2015).…”

Special focus issue on invertebrate pathology: A growing discipline requiring veterinary diagnosticians

“…Its articles were selected to familiarize the readership with emerging issues and pertinent questions, including how to perform mortality investigations in invertebrates. 14,21 It includes several case studies, 1,2,3,9,11,13,17 which demonstrate how traditional investigative and descriptive pathology methods can effectively improve our understanding of invertebrate diseases, even in the face of the many challenges posed by invertebrates. Articles, including studies on arachnids and giant African millipedes, 7,20 bring together current knowledge so as to enable diagnosticians wishing to engage in important frontiers.…”