Geographic Distribution of Adult Salmon Enteritis ( ASE ), Enterocytozoon schreckii , and Ceratonova shasta in Chinook Salmon Oncorhynchus tshawytscha (Walbaum, 1792) in Select Oregon and Washington State Hatcheries and Expanded Histopathologic Descr

Adult salmon enteritis (ASE) is a severe intestinal disease affecting spring Chinook Salmon (Oncorhynchus tshawytscha) in the Pacific Northwest. ASE is strongly associated with prespawn 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 259 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.