In the case of coinfections, multiple pathogens induce pathologies independently or in combination. The present study describes the pathological and microbiological characterization of Vibrio harveyi and two strains of Photobacterium damselae subsp. damselae causing coinfection in cobia reared in sea cages. All the isolated pathogens were virulent to juvenile cobia by injection challenge. The experimental challenge with the three isolated pathogens singly or in combination revealed a significant difference in the mortality profile. Lesions in natural coinfection consisted of ascites, fibrinous deposits on internal organs, and increased melanomacrophage centres (MMC) and extensive necrosis in kidneys. Lesions in experimental infection were limited to congestion of internal organs. Enzymatic activity of both extracellular products (ECP) and live cells by well diffusion showed the highest enzymatic index for gelatinase in the ECP of V. harveyi. The enzyme activity evaluated based on the API‐ZYM system revealed the highest activity for alkaline and acid phosphatase in both ECP and bacteria. The MAR index indicated that the pathogens were from a low‐risk region of antibiotic contamination.

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