Nocardiosis in Fish: A Disease Guide

Fish nocardiosis, caused by Nocardia seriolae, is a chronic granulomatous disease producing white nodules in internal organs. It is hard to treat and resembles mycobacteriosis.

Overview

Nocardiosis is a chronic, systemic, granulomatous bacterial disease of fish. The main causative agent is Nocardia seriolae, with Nocardia asteroides also reported. These are gram-positive, partially acid-fast, aerobic bacteria that grow as branching filaments, resembling fungi but being true bacteria (actinomycetes). The disease develops slowly and is an important problem in warm-water marine and freshwater aquaculture, with significant economic losses and few effective control measures.

Affected species

Nocardia seriolae has been reported from a wide host range, with more than 40 freshwater and saltwater fish species recorded. Commonly affected hosts include yellowtail and amberjack (Seriola species), sea bass, snakehead, and various other cultured and ornamental fish. The bacterium is regarded as an opportunistic pathogen that more readily infects stressed or immunocompromised fish.

Clinical signs

• Whitish nodules (granulomas) in the gills, spleen, liver, kidney, and body wall
• Skin ulceration and external nodules
• Emaciation and loss of condition
• Lethargy and reduced feeding
• Slow, chronic mortality rather than sudden mass die-offs

Diagnosis and differentiation from mycobacteriosis

Diagnosis is based on examination of granulomatous lesions, where Gram and Ziehl-Neelsen (acid-fast) staining of tissue imprints or sections reveals gram-positive, acid-fast, branching, beaded filamentous rods. Confirmation uses bacterial culture, which is slow, and PCR such as Nocardia-specific assays. Nocardiosis closely resembles fish mycobacteriosis, since both produce acid-fast bacteria in granulomas; the branching filamentous morphology of Nocardia and molecular testing help distinguish the two.

Transmission, treatment and control

Infection is thought to occur through wounds, the gills, and contaminated feed. Nocardiosis is difficult to control because it is chronic and intracellular, antibiotic efficacy is limited, and there is no widely effective vaccine. Management therefore emphasizes prevention and reducing the stressors that predispose fish to infection.

• Reduce crowding and stress and maintain good water quality
• Source healthy stock and quarantine new fish
• Remove and properly dispose of diseased and dead fish to lower environmental load
• Disinfect equipment and maintain general biosecurity
• Use antibiotics only under veterinary direction, recognizing their limited efficacy against this chronic intracellular pathogen