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Mycobacteriosis

Posted By: Dr. Momfish <alexp@idirect.com>
Date: Saturday, 23 October 1999, at 10:14 p.m.

I don't have the book by Michael Shoskopf... but I've got Noga. So if it helps, I'll transcribe the information from this book.

Mycobacteriosis (Tuberculosis)

Method of Diagnosis 1. Culture of Mycobacterium 2. Histology of lesions (spleen, liver, kidney, skin)

History Chronic morbidity/mortality

Physical Examination Nonhealing, shallow to deep skin ulcers; corneal ulcers; pale coloration; emaciation; white nodules on viscera.

Treatment Disinfect and quarantine

Comments Epidemiology HOST RANGE Mycobacteriosis is probably the most common chronic disease that affects aquarium fish. Virtually all freshwater and marine aquarium fish are probably susceptible, especially members of the freshwater families Anabantidae, Characidae and Cyprinidae. Aquarium fish mycobacteriosis is caused by the mycobacteria, Mycobacterium marinum and M. fortuitum.

Although disease is presently uncommon in salmonids, asymptomatic Mycobacterium infections are common is some populations; over 25% of some hatchery salmonids are infected with M. chelonae along the northeastern Pacific coast. Mycobacterium was historically a serious problem in salmonids, when they were fed raw fish offal. Mycobacterium neoaurum was recently isolated from a mixed culture from Atlantic salmon with ocular lesions. Mycobacteriosis has also been a serious problem in other cultured food fish, such as European sea bass, tilapia and striped bass, especially in intensive culture systems.

TRANSMISSION

Shedding of bacteria from infected skin ulcers, as well as the intestine, is probably a major source of inoculum. Ingestion is probably the major source of infection, including fish that have recently eaten dead tankmates. The bacteria can survive for 2 years in the environment. Transovarian transmission has been demonstrated in platyfish but does not occur in salmonids.

ZOONOTIC CONSIDERATIONS

Fish-pathogenic mycobacteria can infect humans, usually causing localized, nonhealing ulcers (fish tank granuloma, swimming pool granuloma) that may be difficult to treat because of the resistance of some isolates to most antituberculosis drugs. Owners should be cautioned about contacting potentially infected fish or fomites. The ubiquity of fish mycobacteriosis coupled with the apparently low numbers of human cases suggest that it fortunately appears to be a low risk for healthy humans. However, a small number of M. marinum infections have been reported from HIV-infected persons. All acquired the infection from contact with pet fish, usually when cleaning the aquarium. Gloves should be worn by persons at rish when cleaning an aquarium or when handling fish.

CLINICAL SIGNS/PATHOGENESIS

Emaciation, poor growth, or retarded sexual maturation may be the only clinical signs of mycobacteriosis. Other lesions include skeletal defomities; chronic, nonhealing shallow to deep ulcers; or fin erosion. Internally, 1 to 4 mm white nodules may be present on the viscera, especially hypertrophc kidney or spleen.

HISTOPATHOLOGY

There is a chronic inflammatory response with epithelioid macrophages surrounding the bacteria. Lesions often have necrotic centers and may have melanomacrophages or melanocytes. Bacteria are typically located in the center of the inflammatory focus.

Diagnosis Mycobacteriosis is strongly suggested by the typical clinical signs in combination with the presence of large numbers of granulomas in wet mounts., especially spleen and kidney. Granulomas can be caused by many other pathogens, but if large numbers are present, histological material should be stained for acid-fast bacteria. Fite-Faraco is often better than Ziehl-Neelsen for demonstrating piscine mycobacteria. Tissue smears can also be stained, but this is less advisable, since fresh, infective lesion material must then be handled, risking infection of the clinician. Note that an occasional granuloma is a common incidental finding on necropsy. Granulomas also look similar to melanomacrophage centers. When in doubt about the significance of wet mount lesions, samples should be processed for histology.

Mycobacteria are ~0.4 x 1.0 to 4.0 microns long, are acid-fast, and often stain unevenly. Mycobacteria are also gram-positive but often do not stain well. Other acid-fast rods (i.e. Nocardia) are longer and branching. Nocardia is also much less common in aquarium fish and salmonids than mycobacteriosis.

Isolation on Lowenstein-Jensen or Middlebrook LH10 agar allow definitive diagnosis by biochemical identification, as well as determination of the species involved. Isolation may take up to 30 days; sometimes organisims cannot be cultured even when large number are seen in lesions. Some isolates may grow on blood agar or tryticase soy agar, if the inoculum is heavy. Cutlure is usually not necessary unless treatment is anticipated.

Treatment As with so many fish diseases, mycobacteriosis u