Experimental production of gastric dilation and its association with osmoregulatory stress and biogenic amines in chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Gastric dilation and air sacculitis in Chinook salmon, Oncorhynchus tshawytscha (Walbaum); correlation of macroscopic and microscopic lesions, and relationship of the syndrome to glomerulonephritis and serum biochemistry

Macroscopic and microscopic assessment procedures were developed to evaluate the severity and enable diagnosis from histological samples, of gastric dilation and air sacculitis (GDAS) in Chinook salmon, Oncorhynchus tshawytscha. Stomachs and swim bladders were examined from young fish with experimentally induced GDAS and from larger fish with the syndrome held in commercial saltwater net-pens. Fish fed a diet previously known to induce GDAS had significantly wider stomachs with decreased prominence of longitudinal stomach folds that contained larger amounts of fluid (P < 0.001), and thinner stomach walls with greater inter-nerve distances (P < 0.001), than fish fed an alternative commercial diet not associated with the syndrome. These fish also had swim bladders that were more likely to be opaque and contain more fluid (P < 0.001). These observations correlated well with selected criteria for stomach tissue (P < 0.002) and swim bladder (P < 0.04) that could be evaluated microscopically. Four stomach measurements, primarily measurements of wall or partial wall thickness and inter-nerve distances, were suitable for discriminating between affected and non-affected fish. A stomach width ratio, that was independent of fish weight and highly correlated with macroscopic stomach measurements (P < 0.0001), was particularly useful; this ratio was derived from the distances between both the outer border of the muscularis mucosa and mesothelium of the serosal surface to the stratum compactum. Serum biochemistry parameters (osmolality, calcium and magnesium) did not differ between fish fed different diets, but serum creatinine concentration was correlated with the microscopic thickness of the muscularis externa of the stomach wall and the total stomach thickness (P < 0.001 and P < 0.003, respectively). A glomerulonephritis was also noted in these fish. The severity of the lesion was not significantly related to GDAS nor to any serum biochemistry parameter assayed however.

Development and physiology of gastric dilation air sacculitis in Chinook salmon, Oncorhynchus tshawytscha (Walbaum)

The syndrome known as gastric dilation air sacculitis (GDAS) has previously been shown to affect Chinook salmon, Oncorhynchus tshawytscha, in seawater (SW) aquaculture. Feed and osmoregulatory stress have been implicated as potential epidemiological co-factors. The development and physiology of GDAS was investigated in SW and freshwater (FW) adapted smolts. Diet A (low-cohesion pellets) and diet B (high-cohesion pellets) were fed to both FW- and SW-adapted fish. GDAS was induced only in the SW trial on feeding diet A. Stimulated gastro-intestinal (GI) smooth muscle contractility, and fluid transport by the pyloric caeca were different in GDAS-affected fish, which also showed osmoregulatory dysfunction. Cardiac stomach (CS) smooth muscle contractility in response to acetylcholine and potassium chloride (KCl) was significantly reduced in fish fed diet A relative to controls from weeks 3-5. In contrast, maximal pyloric sphincter (PS) circular smooth muscle contraction in response to KCl was significantly elevated in fish fed diet A in weeks 4 and 5. Serum osmolality was elevated in GDAS-affected fish from week 2 of the SW trial. Fluid transport from the mucosal to serosal surface of isolated pyloric caeca was significantly reduced in weeks 3, 4 and 5 in SW fish fed diet A. Gastric evacuation from the stomach of healthy fish was shown to be significantly different when diets of low- and high-cohesion were fed. The results are consistent with the intestinal brake playing a role in the development of the disease.