This comprehensive review examines pathogens affecting adult anadromous Pacific salmon (Oncorhynchus spp.) during their terminal freshwater migration and spawning across populations from California through Alaska, including Oregon, Washington, and British Columbia. We systematically reviewed selected pathogens based on their significance to adult salmon health or role in epizootiology, categorizing them by their impact on prespawn mortality (PSM), disease severity, and maternal or ‘egg-associated’ transmission risks to progeny. Our analysis encompasses macroparasites, microparasites, bacteria, and viruses affecting anadromous Pink (O. gorbuscha), Chum (O. keta), Coho (O. kisutch), Sockeye (O. nerka), and Chinook Salmon (O. tshawytscha) and Steelhead Trout (O. mykiss), integrating extensive literature analysis with direct field observations and case studies from representative geographic regions. Understanding pathogen impacts during the spawning life stage is crucial for salmon population sustainability, as the unique semelparous nature of Pacific salmon makes this terminal phase critical for reproductive success and the continuation of these ecologically, economically, and culturally vital species.

We performed a diagnostic disease investigation on a wild smallmouth bass (Micropterus dolomieu) with skin ulcers that was collected from Lake Oahe, South Dakota, following reports from anglers of multiple fish with similar lesions. Gross and histologic lesions of ulcerative dermatitis, myositis, and lymphocytolysis within the spleen and kidneys were consistent with largemouth bass virus (LMBV) infection. LMBV was detected by conventional PCR in samples of a skin ulcer, and the complete genome sequence of the LMBV (99,184 bp) was determined from a virus isolate obtained from a homogenized skin sample. A maximum likelihood (ML) phylogenetic analysis based on the major capsid protein (MCP) gene alignment supported the LMBV isolate (LMBV-SD-2023) as a member of the species Ranavirus micropterus1, branching within the subclade of LMBV isolates recovered from North American largemouth (Micropterus salmoides) and smallmouth bass. This is the first detection of LMBV in wild smallmouth bass from South Dakota. The ultrastructure of the LMBV isolate exhibited the expected icosahedral shape of virions budding from cellular membranes. Viral nucleic acid in infected cells was visualized via in situ hybridization (ISH) within dermal granulomas, localized predominantly at the margin of epithelioid macrophages and central necrosis. Further sampling is needed to determine the geographic distribution, affected populations, and evolutionary relationship between isolates of LMBV.

Adult salmon enteritis (ASE) is a severe intestinal disease affecting spring Chinook Salmon ( Oncorhynchus tshawytscha ) in the Pacific Northwest. ASE is strongly associated with pre-spawn mortality (PSM), a critical concern for endangered populations and continued Chinook Salmon management. This study characterizes the geographic distribution of ASE across Oregon and Washington watersheds, examines its association with two common parasites ( Ceratonova shasta and Enterocytozoon schreckii ), and provides detailed histopathological characterization of the disease. We examined intestinal tissues from 256 adult spring Chinook Salmon collected from multiple hatcheries across the Columbia River Basin and Puget Sound between 2022 and 2025 using routine histopathology, specific histochemical stains, and immunohistochemistry. ASE was highly prevalent in all Columbia Basin populations, with chronic ulcerative and necrotizing enteritis, but was absent in Puget Sound populations. C. shasta prevalence was high in most Columbia Basin locations (10-100%), while E. schreckii prevalence was more variable (0-67%). Consistent histopathological findings of ASE included ≥ 10% mucosal ulceration, mixed inflammation expanding the lamina propria and extending into surrounding intestinal layers, and dissolution of the stratum granulosum. In chronic ASE lesions, we further characterized granulation tissue formation. Cold water holding of spawning adults appears to slow disease progression and reduce PSM without preventing ASE development. These findings demonstrate that ASE is a geographically restricted disease of the Columbia Basin that may have associations with diverse environmental exposures including water temperature and pathogens.

Tilapia ( Oreochromis spp.) is an increasingly popular aquacultured fish worldwide. While tilapia enjoy a ‘hardy‐fish’ reputation among fish farmers, they are susceptible to a variety of infectious diseases that can be difficult to recognize pondside and have similarities in their gross presentations. This report describes gross and microscopic dermal, renal, and splenic granulomas; isolation and identification of Francisella orientalis ; intervention; and resolution of a F. orientalis outbreak in an aquaculture farm in the North Central United States where F. orientalis has not been reported previously. The outbreak resulted in peak daily mortality rates of approximately 2.7%. This case report highlights the current benefits of aquatic species‐focused diagnostic testing for fish farmers at veterinary diagnostic labs and provides an anecdotal clinical example of a resolved francisellosis outbreak potentially influenced by increasing the water temperature.