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Chapter: Health Management in Aquaculture: Bacterial diseases

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Mycobacteriosis or Piscine Tuberculosis - Bacterial Diseases of Fish

Piscine mycobacteriosis is a systemic, chronic, progressive disease presenting various clinical features depending upon species and ecological conditions.

Mycobacteriosis or Piscine Tuberculosis

CAUSATIVE AGENT:

Mycobacterium marinum, M. fortuitum and M. chelonae

SPECIES AFFECTED:

Siamese fighting fish (Betta splendens)

GROSS SIGNS:

Piscine mycobacteriosis is a systemic, chronic, progressive disease presenting various clinical features depending upon species and ecological conditions. Listlessness, anorexia, emaciation, exopthalmia, skin discoloration and external lesions ranging from scale loss to nodules, ulcers and fin necrosis are signs of advancing infection. Gross internal pathology of mycobacteriosis show graywhite lesions of various sizes in most organs and tissues, but the kidney and liver are most often involved.

EFFECTS ON HOST:

Mycobacteriosis is a chronic progressive disease. The disease may take several years to progress from the asymptomatic state to clinical illness. Initially the pigment will fade, and the fish appear sluggish with loss of appetite. Skin ulcers

will develop. Fin and tail rot and loss of scale may also be seen. Nodules may form in the muscle and internal organs, which may lead to emaciation or edema or peritonitis. Infection may spread to the skeleton, in which deformities

become apparent. Mortalities will then be observed.

DIAGNOSIS:

Primary isolation of fish mycobacteria is best achieved using Ogawa and Lowenstein-Jensen media. Subcultures develop at 28°C within 3-5 days on these media. On Ogawa medium, the cultures appear creamy in the dark but brilliant yellow color when exposed to light. Cultures may not always be obtained even from fish showing unequivocal evidence of infection. Mycobacteria may also be isolated occasionally on general-purpose bacteriological media such as tryptic soy agar, or brain heart infusion agar, provided that a large inoculum is used. All fish mycobacterium have been cultured at 20-30°C for 2 to 30 days. The isolates are strongly acid-fast, rod-shaped, weakly gram-positive, cord forming, non-motile and non-spore forming. Optimum temperature for bacterial growth is between 15°C to 37°C, but the isolates grow best at 28°C.

PREVENTION AND CONTROL:

·              Sanitation, disinfection, and destruction of carrier fishes are the primary methods of controlling mycobacteriosis.

 

·              Avoid feeding fish with contaminated fish products. Pasteurize food before use.

 

·              Apply chloramine B or T at 10 mg/l for 24 h.

 

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