Abstract

Several Shewanella sp. have been identified, which affect the eyes and lenses in humans and terrestrial animals,^1-5 but there is no report of bacterial lens pathology in fish. In the present study, we describe a pathogenic incidence of Shewanella sp. S-1 infection in cultured rainbow trout (Oncorhynchus mykiss, Walbaum, 1792), as evidenced by lens atrophy.

The fish were divided into two groups; diseased fish in group A and healthy fish in group B. Bacteriological and histopathological samples were taken from the kidneys, liver, spleen, eyes, and brain of the fish. The fish were weighed on a precision scale. Morphometric measurements were carried out on ten fish from each Group A and Group B. Standard length, lens diameter, and eye diameter were measured. The optic lobes were removed from the brain and evaluated macroscopically. The length, width, and height of the optic lobes were also measured. Pathological and macroscopic findings of the lens atrophy were demonstrated by H&E staining. All samples, including those taken from the brain, eyes, lens, and other body parts of the fish (gills, muscles, liver, kidneys, and spleen) in Groups A and B were examined for the presence of bacteria by Brown-Brenn staining.

Upon macroscopic examination, no clinical findings, such as bleeding or lesions, were detected in the eyes and body of the fish in Group B. Likewise, in Group A, no clinical lesions were observed, except for the absence of one eye in some fish. No morphological difference was observed between Groups A and B for the morphometric measurements of the optical lobes. Macroscopically, the lens had a spherical shape. There was a visible difference in the lens diameter of the two groups. However, when palpated and applied pressure, the lens displayed shrinkage in Group A. Lens shrinkage was not observed in Group B. When fixed in a 10% formaldehyde solution, the lens acquired a white color in Group B. No such lens discoloration was observed in Group A. The lens diameters of Groups A and B were determined to differ significantly, such that the lens diameters of Group A were approximately 33% smaller than the diameters of Group B. Pathological findings were not associated with any ocular lesion. No pathological findings were detected in the retina, optic disc, optic nerve, cornea, and optic lobes, except for nuclear margination in the optic lobes’ neurons. Pathological examination showed that bacteria had caused nuclear margination in the optic lobes’ neurons. Gram-negative bacteria were detected in only the atrophic lenses of the fish.

In the present study, it has been demonstrated, for the first time, that Shewanella sp. S-1 causes lens atrophy in rainbow trout, leading to visual impairment. This bacterium caused blindness due to lens atrophy and induced eye loss in some fish uni- or bilaterally. The pathogenicity of Shewanella sp. S-1 for the lens, eye, brain, and other internal organs was investigated, and it was determined that while significant lens atrophy occurred, there was no accompanying morphological disorder.

*Presenting author

Literature Cited

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