Mycobacteriosis - Fish

Also known as: Focal tuberculosis in fish, — Piscine mycobacteriosis — Piscine tubercle— chronic inflammatory foci (CIF) — tubercle granuloma.

Introduction

Mycobacteria infect many species of freshwater, saltwater and aquarium fish. The latter species are more likely to show a higher incidence of disease due to the chronicity of the disease and increased ages of aquarium kept fish compared to commercial fish.

Some mycobacteria found in fish species are zoonotic and M. marinum and M. Fortuitum, can cause skin disease in humans. Outbreaks have been reported from cutaneous abrasions and exposure to contaminated swimming pool water. M. fortuitum has been cultured from patients with pulmonary disease and local abscesses and M. chelonei has been isolated from heterograph heart-valve transplants, synovial fluid and muscle , .

Signalment

The disease has been reported in many species of fish as well as Penaeidae (prawn spp) and sea horses. Outbreaks have been reported in salmon, cod, halibut, bass, mackerel, turbot , trout, morays, silversides, tilapia and perch. Pond dwelling species such as pejerrey and snakehead fish and freshwater aquarium fish such as the Siamese fighting fish, Goldfish, Eight-banded cichlid, Zebrafish, Neon tetra, and Labyrinth fish. The severity of the disease has been linked to age and disease outbreaks can be linked to nutrient and water quality and quantity, stress and stocking density.

Clinical Signs

Clinical signs consist of haemorrhagic lesions, erosion and scale loss on skin and fins, enlarged organs, lordosis, cachexia, exophthalmia, loss of pigmentation, and pale and cystic gills. Behavioural signs include cessation of feeding, lethargy, and swimming near the surface.

Epidemiology

Fish may be infected by ingesting feed and water contaminated with faecal material, urine or exudates from diseased animals that contain mycobacteria . The identification of Mycobacteria and acid-fast bacteria in piscine ova and tubercle granulomas in ovarys of different species suggests that transovarian transmission may be a possibility. Mycobacteriosis can be sub divided into subacute and chronic forms and once fish are infected, mycobacteria spread via the circulatory or lymphatic system to other organs such as the spleen, kidney and liver. Both hard and soft granulomas are formed in the chronic proliferative form and in severe cases larger granulomas form within visceral organs and loose connective tissues. Mycobacterium has many vectors including crustaceans and molluscs.

Distribution

Worldwide - mycobacterium is ubiquitous in the water and sediment. It has been documented in fish from the Atlantic and Pacific Sea, Asia, Africa, North America, Australia, South America and Europe.

Pathology

Pathology includes necrotised and enlarged spleen, kidney and liver with diffuse greyish-white nodules, cysts (white spheres) within the body cavity and muscle, fusion of visceral membranes and increased peritoneal fluid.

Histopathology of granulomas varies for subacute and chronic forms. Sub acute forms have a large caseous necrotic areas with surrounding diffuse reticuloendothelial cells and macrophages. Acid-fast bacilli can be found within the reticuloendothelial cells and within the cytoplasm of phagocytic macrophages. Whereas the chronic proliferative form is characterized by soft granulomas with four distinguishable layers; a central caseous necrosis, (with or without nuclear debris), spindle-shaped epithelioid cells, eosinophilic, flattened, epithelioid cells and then a fine fibrous connective tissue encircling to form a thin capsule. Hard granulomas can also been seen with chronic forms and are composed of epithelioid cells encapsulated by fibrous connective tissue with or without calcification. Infected crustaceans show melanised lesions in muscle, cuticle, ovary, gill and heart.

Diagnosis

Mycobacteriosis can be confirmed by PCR, DNA probes- in situ hybridisation and antibody-based methods and presumptive diagnosis can be made from clinical signs, bacteriology and histopathology.

Treatment

Tetracycline and kanamycin sulphate can be used to treat mycobactriosis and experimentally cotrimoxazole, cycoserine, ethambutol, isoniazid, rifampacin and streptomycin have been used.

References

Mycobacteriosis - Fish Learning Resources
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