Kocuria rhizophila As the Etiological Agent Infecting Rainbow Trout (Oncorhynchus mykiss Walbaum, 1792) in Iran

Document Type : Research Article

Authors

Nazila Yeganeh ¹

Amir Hossein Smiley ²

Mohammad Reza Kalbassi ²

Margaret Crumlish ³

¹ Department of Fisheries Engineering and sciences, Faculty of Natural Resources and Marine Sciences, University of Tarbiat Modares,Tehran, Iran.

² Department of Fisheries Engineering and sciences, Faculty of Natural Resources and Marine Sciences, University of Tarbiat Modares, Tehran, Iran.

³ Institute of Aquaculture, Faculty of Natural Sciences, University of Stirling, Stirling, UK.

10.22067/ijvst.2026.89008.1400

Abstract

An natural acute disease outbreak occurred in two rainbow trout farms located in northern Iran. Affected fish exhibited clinical symptoms, including ocular opacity, scale desquamation, skin necrosis, superficial to deep lesions on the dorsal and lateral body surfaces, loss of caudal peduncle tissue, mild exophthalmia, skin melanization, hepatic hemorrhage and paleness, excessive mucus accumulation in the abdominal cavity, renal paleness, and intestinal infection. Following biochemical analysis of the bacterium, gene sequencing was conducted, leading to the identification and registration of a strain known as K. rhizophila TMU97910 in the NCBI gene bank. After the elucidation of the bacterium’s identity, salinity tolerance, and antibiogram tests were conducted. The results demonstrated that the strain exhibited complete resistance to salinity, allowing it to thrive in a wide range of salinity conditions. In antimicrobial assays, the isolate showed resistance to Fosfomycin, while remaining sensitive to Florfenicol, Doxycycline, Oxytetracycline, Enrofloxacin, Sulfamethoxazole/Trimethoprim, Erythromycin, Gentamycin, and Kanamycin. To investigate the behavior of this bacterium in rainbow trout, pathogenicity, LD50, and histopathology analyses were conducted in three replicates. Pathogenic symptoms in trout remained consistent. Histopathological examination of the liver, kidney, spleen, and gills revealed extensive tissue damage caused by this strain, including necrosis, hemorrhage, degeneration, and tissue wounds. Subsequently, bacterial recovery was achieved, and negative results for the detection of other pathogens confirmed that Kocuria rhizophila was responsible for the disease outbreak.

Keywords

Kocuria rhizophila

Rainbow trout

Pathogenicity

histopathology

antibiogram test

Susceptibility

Subjects

• Aquatic animal diseases

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