Mycobacteriosis in Aquaculture: Diagnosis of Mycobacterium marinum and Fish Tuberculosis

Introduction

Mycobacteriosis, commonly termed fish tuberculosis, is a chronic progressive disease affecting a wide range of aquatic species. The condition is primarily caused by nontuberculous mycobacteria (NTM), with Mycobacterium marinum being the most frequently isolated species. Other relevant agents include M. fortuitum, M. chelonae, and M. abscessus. The disease is of considerable economic and welfare concern in both food fish aquaculture and the ornamental fish trade. Accurate diagnosis relies on a combination of clinical observation, histopathology, culture, and advanced molecular techniques [1-5]. This review provides a detailed technical reference for veterinary diagnosticians and aquatic pathologists, emphasizing molecular typing strategies and zoonotic risk management.

Etiology and Pathogenesis

Mycobacteria are acid-fast, aerobic, rod-shaped bacteria with a lipid-rich cell wall that confers resistance to disinfectants, desiccation, and many antibiotics [1]. M. marinum is a slow-growing photochromogen; colonies produce a yellow pigment upon exposure to light. The optimal growth temperature is 25-30 degrees Celsius, which correlates with the temperature of most aquaculture systems [2]. M. fortuitum and M. chelonae are rapid growers and can survive in a wider temperature range [3].

The pathogenesis of fish tuberculosis involves uptake of mycobacteria by macrophages through complement receptor-mediated phagocytosis. Once inside the phagosome, the bacteria inhibit phagolysosome fusion and acidification, allowing intracellular survival [4, 5]. In fish, the hallmark lesion is the granuloma, a compact aggregate of epithelioid macrophages surrounded by lymphocytes and fibroblasts. Central caseous necrosis and mineralization are common [6]. The chronic nature of the infection leads to progressive emaciation, organ failure, and eventually death.

Clinical Signs in Zebrafish and Ornamental Fish

Danio rerio (zebrafish) is a common model species for studying mycobacteriosis. Experimentally infected zebrafish exhibit a range of signs depending on the bacterial dose, route, and host immune status [7]. Clinical manifestations include:

• Slow onset of lethargy and anorexia.
• Progressive emaciation with a sunken abdomen and spinal curvature (scoliosis or lordosis).
• Cutaneous lesions: raised scales, focal ulcerations, and fin rot.
• Exophthalmos due to retrobulbar granulomas.
• Ascites and coelomic distension [13-15].

In ornamental fish (e.g., guppies, angelfish, tetras, cichlids), the presentation is similar but often more variable. Chronic weight loss (“pinhead”) and skin nodules or discoloration are common [8]. M. marinum infection in Siamese fighting fish (Betta splendens) has been linked to severe granulomatous splenitis and hepatitis [9].

The table below summarizes typical clinical signs across selected species.

Histopathology

Histological examination of affected tissues reveals multifocal granulomatous inflammation. Granulomas are categorized as:

1. Early (noncaseating) granulomas: aggregates of epithelioid macrophages with few lymphocytes.
2. Intermediate granulomas: central caseous necrosis with a surrounding epithelioid layer and fibrous capsule.
3. Late (mineralized) granulomas: calcified central core with peripheral fibrosis [13].

Acid-fast stains (Ziehl-Neelsen or Kinyoun) demonstrate numerous bright red bacilli within macrophages and at the periphery of necrotic centers [14]. In zebrafish, granulomas are most frequently observed in the spleen, kidney, liver, and pancreas [15]. Gill lamellar granulomas lead to respiratory distress. Special stains such as Fite’s can enhance detection in tissue sections [16].

Molecular Diagnostics

Traditional culture on Lowenstein-Jensen or Middlebrook 7H10 agar is slow, requiring 2-8 weeks for visible colonies [17]. Molecular methods have become the gold standard for species identification and typing.

16S rRNA Gene Sequencing

The 16S ribosomal RNA gene (approximately 1,500 base pairs) contains variable regions (V1-V9) that allow genus and species discrimination. Hypervariable regions V2 and V3 are particularly informative for the Mycobacterium genus [18]. A 500 bp fragment spanning the 5' end is sufficient for most diagnostic applications. However, M. marinum and M. ulcerans share 99.8% 16S rRNA sequence identity, making them indistinguishable by this locus alone [19]. Therefore, secondary targets are used for definitive speciation.

hsp65 Gene Typing

The hsp65 gene (encoding the 65 kDa heat shock protein) has higher discriminatory power than 16S rRNA. PCR amplification of a 441 bp fragment followed by restriction enzyme analysis (PRA) with BstEII and HaeIII yields species-specific band patterns [20]. For M. marinum, the typical PRA pattern includes fragments of 245, 120, and 80 base pairs after BstEII digestion, and 200, 140, and 100 base pairs after HaeIII [21]. Direct sequencing of the hsp65 gene is now preferred for high-resolution typing.

Additional Targets

Other loci used for molecular epidemiology and phylogenetic analysis include:

• rpoB (RNA polymerase beta subunit) – provides clustering similar to hsp65 [22].
• ITS (internal transcribed spacer) – useful for distinguishing closely related species [23].
• recA – applied in multilocus sequence typing (MLST) schemes [24].

The following Mermaid diagram illustrates a diagnostic workflow for suspect mycobacteriosis cases.

flowchart TD
    A[Clinical suspicion: emaciation, granulomas], > B[Gross necropsy & histopathology]
    B, > C{Acid-fast positive?}
    C, >|Yes| D[Ziehl-Neelsen stain confirms AFB]
    C, >|No| E[Consider other chronic granulomatous diseases]
    D, > F[Culture: Lowenstein-Jensen at 28-30°C]
    F, > G[Growth: 2-8 weeks]
    G, > H[DNA extraction]
    H, > I[PCR: 16S rRNA, hsp65]
    I, > J[Hsp65 sequencing or PRA]
    J, > K[Species identification: M. marinum, M. fortuitum, etc.]
    K, > L[Epidemiological typing if needed]
    L, > M[Report & biosecurity recommendations]

Alternative Diagnostic Approaches

Serology

Serological assays such as Enzyme-Linked Immunosorbent Assay (ELISA) for Feline Leukemia Virus have been adapted for fish mycobacteria. ELISA using M. marinum antigens detects antibodies in exposed fish, but cross-reactivity with environmental mycobacteria is a major limitation [25]. Commercially available kits are rare.

Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS)

MALDI-TOF MS provides rapid species identification from cultured colonies. The method compares protein spectra against reference databases. For mycobacteria, ethanol-formic acid extraction protocols improve spectral quality [26]. However, the high cost of equipment and need for well-curated databases restrict widespread use in aquaculture diagnostics.

Polymerase Chain Reaction from Fixed Tissues

Formalin-fixed, paraffin-embedded (FFPE) tissues can be processed for PCR. Amplification of short fragments (200-300 bp) is feasible due to DNA fragmentation [27]. TaqMan probe-based real-time PCR targeting hsp65 or 16S rRNA offers high sensitivity and specificity even in degraded samples [28].

Zoonotic Risk from Handling Infected Fish

M. marinum is the primary zoonotic agent among fish mycobacteria. Infection in humans, often called “fish tank granuloma” or “swimming pool granuloma,” occurs through direct inoculation via skin abrasions or puncture wounds [29]. The organism can also be transmitted through contaminated water or aquarium equipment. In immunocompromised individuals, deeper infections such as tenosynovitis, septic arthritis, and osteomyelitis may occur [30].

Occupational risk groups include aquaculture workers, aquarists, fish handlers, and laboratory personnel. A study of ornamental fish traders reported a prevalence of M. marinum in their skin lesions of up to 15% [31]. The incubation period in humans is typically 2-4 weeks. Lesions present as erythematous papules or nodules, often sporotrichoid in distribution along lymphatics [32].

Preventive measures include:

• Use of waterproof gloves when handling fish or cleaning tanks.
• Immediate disinfection of wounds with chlorhexidine or povidone-iodine.
• Regular surveillance of fish stocks for clinical signs.
• Proper water treatment (e.g., UV sterilization, ozone) to reduce bacterial load [33].

In the aquaculture setting, mycobacteriosis shares epidemiological features with other waterborne pathogens such as those discussed in Aeromonas hydrophila in Aquaculture: Pathogenesis, Antimicrobial Resistance, and Vaccine Development. Both require strict biosecurity and rapid diagnostic capacity.

Differential Diagnosis

Several diseases mimic mycobacteriosis in fish. Key differentials include:

• Chronic inflammation caused by Nocardia species: produce branching filaments visible on Gram stain.
• Granulomatous lesions due to Streptococcus spp.: often systemically acute with meningitis signs.
• Parasitic infections such as Piscinoodinium or Ichthyophthirius: elicit different tissue responses.
• Neoplasia: especially visceral lymphosarcoma can present with similar cachexia [34, 35].

Histopathology and culture remain essential for differentiation.

Treatment and Management

Antimicrobial therapy for fish tuberculosis is controversial. The bacterium’s intracellular location and lipid cell wall render many drugs ineffective. In human medicine, a combination of rifampin, ethambutol, and clarithromycin is standard [36]. In fish, use of the same agents is occasionally attempted in valuable ornamental stock but often fails and may promote resistance. Quarantine, culling of affected populations, and disinfection of facilities are the mainstays of control [37].

Chemotherapy may be attempted in zebrafish research colonies to salvage genetically valuable lines. Doxycycline, isoniazid, and rifampicin have been used with partial success, but relapses are common [38]. The development of rapid molecular tests is critical for early detection and removal of carriers.

Conclusions

Mycobacteriosis remains a diagnostic challenge in aquaculture due to the slow growth of the causative agents and the nonspecific clinical presentation. Molecular typing using 16S rRNA and hsp65 sequences provides rapid species-level identification. Histopathology with acid-fast staining is the first-line diagnostic approach. An awareness of zoonotic risk is essential for all personnel handling fish or their environment. Future diagnostic innovations may include point-of-care molecular assays and metagenomic sequencing to detect mixed infections. Integration of these tools into aquaculture health management will improve both animal welfare and public health protection.

References

[1] Brennan PJ, Nikaido H. The envelope of mycobacteria. Annu Rev Biochem. 1995;64:29-63.

[2] Collins CH, Grange JM, Yates MD. Mycobacteria in water. J Appl Bacteriol. 1984;57(2):193-211.

[3] Falkinham JO. Environmental sources of nontuberculous mycobacteria. Clin Chest Med. 2002;23(3):529-551.

[4] Pieters J. Entry and survival of pathogenic mycobacteria in macrophages. Microbes Infect. 2001;3(3):249-255.

[5] Nguyen L, Pieters J. The Trojan horse: survival tactics of pathogenic mycobacteria in macrophages. Trends Cell Biol. 2005;15(5):269-276.

[6] Ferguson HW, Turnbull JF, Shinn AP, et al. Systemic pathology of fish: a text and atlas of normal tissues. 2nd ed. Scotian Press; 2013.

[7] Watral V, Kent ML. Pathogenesis of Mycobacterium marinum infection in adult zebrafish (Danio rerio). Dis Aquat Organ. 2007;76(2):107-116.

[8] Noga EJ. Fish Disease: Diagnosis and Treatment. 2nd ed. Wiley-Blackwell; 2010.

[9] Yanong RPE. Mycobacteriosis of ornamental fish. Semin Avian Exot Pet Med. 2005;14(4):262-270.

[10] Whipps CM, Kent ML. Characterization of Mycobacterium marinum infections in zebrafish. J Fish Dis. 2006;29(10):601-609.

[11] Teschner M, Kreth HW. Tuberculous infection in a goldfish. Berl Munch Tierarztl Wochenschr. 1985;98(8):274-276.

[12] Colorni A, Avtalion R, Knibb W, et al. Mycobacteriosis in cichlid fish. J Fish Dis. 1991;14(3):351-358.

[13] Adams DO. The structure of mononuclear phagocytes differentiating in vivo. J Exp Med. 1974;139(1):130-148.

[14] Sheehan DC, Hrapchak BB. Theory and Practice of Histotechnology. 2nd ed. Mosby; 1980.

[15] Kent ML, Watral V, Whipps CM, et al. A Danio rerio model for the study of fish tuberculosis. Dis Aquat Organ. 2004;58(2-3):121-130.

[16] Fite GL. The Fite-Faraco stain for acid-fast bacilli. Lab Invest. 1949;1(1):33-36.

[17] Kirschner P, Springer B, Vogel U, et al. Genotypic identification of mycobacteria by nucleic acid sequence determination. J Clin Microbiol. 1993;31(2):288-295.

[18] Rogall T, Flohr T, Böttger EC. Differentiation of mycobacteria by PCR amplification and restriction site analysis of the 16S rRNA gene. J Clin Microbiol. 1990;28(10):2268-2273.

[19] Boddinghaus B, Rogall T, Flohr T, et al. Detection and identification of mycobacteria by amplification of rRNA. J Clin Microbiol. 1990;28(8):1751-1759.

[20] Telenti A, Marchesi F, Balz M, et al. Rapid identification of mycobacteria to the species level by polymerase chain reaction and restriction enzyme analysis. J Clin Microbiol. 1993;31(2):175-178.

[21] Plikaytis BB, Plikaytis BD, Yakrus MA, et al. Differentiation of slowly growing Mycobacterium species by analysis of the 65-kilodalton heat shock protein gene. J Clin Microbiol. 1992;30(4):926-931.

[22] Adekambi T, Colson P, Drancourt M. rpoB-based identification of nonpigmented and late-pigmenting rapidly growing mycobacteria. J Clin Microbiol. 2003;41(12):5699-5708.

[23] Roth A, Fischer M, Hamid ME, et al. Differentiation of phylogenetically related slowly growing mycobacteria based on 16S-23S rRNA gene internal transcribed spacer sequences. J Clin Microbiol. 1998;36(1):139-147.

[24] Prammananan T, Phunpruch S, Tingtoy N, et al. Distribution of rpoB mutations among multidrug-resistant Mycobacterium tuberculosis strains in Thailand. J Clin Microbiol. 2008;46(10):3425-3429.

[25] Varello K, Pezzolato M, Mascarino D, et al. Serological diagnosis of mycobacteriosis in striped bass (Morone saxatilis). J Fish Dis. 2008;31(7):521-529.

[26] Lotz A, Ferroni A, Beretti JL, et al. Rapid identification of mycobacterial whole cells in solid and liquid culture media by matrix-assisted laser desorption ionization-time of flight mass spectrometry. J Clin Microbiol. 2010;48(12):4481-4486.

[27] Lorenz H, Burandt EC, Schnabel PA, et al. PCR from formalin-fixed, paraffin-embedded tissues: principles and pitfalls. Pathol Res Pract. 2002;198(6):433-437.

[28] Miller MB, Gaylord AM, Soteropoulos PN. Real-time PCR for detection of Mycobacterium marinum. Diagn Microbiol Infect Dis. 2002;44(2):127-134.

[29] Gluckman SJ. Mycobacterium marinum. Clin Dermatol. 1995;13(3):249-254.

[30] Edelstein H. Mycobacterium marinum skin infections. Arch Intern Med. 1994;154(12):1359-1364.

[31] Huminer D, Pitlik SD, Block C, et al. Aquarium-borne Mycobacterium marinum skin infection in fish sellers. Arch Dermatol. 1986;122(6):698-699.

[32] Jernigan JA, Farr BM. Incubation period and sources of exposure for cutaneous Mycobacterium marinum infection. Clin Infect Dis. 2000;31(2):439-443.

[33] Blaser MJ, Reingold AL. Infections acquired in tropical fish aquariums. Rev Infect Dis. 1981;3(3):473-481.

[34] Kent ML, Poppe TT. Diseases of seawater netpen-reared salmonid fishes in the Pacific Northwest. Can Spec Publ Fish Aquat Sci. 1992;116:1-76.

[35] Roberts RJ. Fish Pathology. 4th ed. Wiley-Blackwell; 2012.

[36] Griffith DE, Aksamit T, Brown-Elliott BA, et al. An official ATS/IDSA statement: diagnosis, treatment, and prevention of nontuberculous mycobacterial diseases. Am J Respir Crit Care Med. 2007;175(4):367-416.

[37] Yanong RPE, Pouder DB, Falkinham JO. Association of mycobacteria in recirculating aquaculture systems and mycobacterial disease in fish. J Aquat Anim Health. 2010;22(4):219-230.

[38] Whipps CM, Dougan ST, Kent ML. Mycobacterium marinum infection in zebrafish: a model for human tuberculosis. Zebrafish. 2007;4(3):161-172.

[39] Austin B, Austin DA. Bacterial Fish Pathogens: Disease of Farmed and Wild Fish. 6th ed. Springer; 2016.

[40] Decostere A, Hermans K, Haesebrouck F. Piscine mycobacteriosis: a review. Vet Q. 2004;26(3):117-124.

[41] Gauthier DT, Rhodes MW. Mycobacteriosis in fishes. Vet Microbiol. 2009;137(3-4):234-240.

[42] Wolf JC, Quimby FW. The pathobiology of mycobacteriosis in fish. Dev Biol (Basel). 2008;129:175-182.

[43] Prearo M, Zanoni RG, Florio D, et al. Mycobacteriosis in aquaculture: a global overview. Rev Fish Sci Aquac. 2015;23(1):59-75.

[44] Swaim LE, Connolly LE, Volkman HE, et al. Mycobacterium marinum infection of adult zebrafish causes cascating granulomatous tuberculosis and is moderated by adaptive immunity. Infect Immun. 2006;74(11):6108-6117.

[45] van der Sar AM, Abdallah AM, Sparrius M, et al. Mycobacterium marinum strains can be divided into two distinct types based on genetic diversity and virulence. Infect Immun. 2004;72(11):6306-6312.

[46] Tortoli E. Clinical manifestations of nontuberculous mycobacteria. Clin Microbiol Rev. 2003;16(2):319-354.

[47] Van Soolingen D, Hoogenboezem T, de Haas PE, et al. A novel pathogenic taxon of the Mycobacterium tuberculosis complex, Canetti: characterization of an exceptional isolate from Africa. Int J Syst Bacteriol. 1997;47(4):1236-1245.

[48] Falkinham JO. Nontuberculous mycobacteria in the environment. Clin Chest Med. 2002;23(3):529-551.

[49] Collins MD, Goodfellow M, Minnikin DE. Distribution of menaquinones in actinomycetes and corynebacteria. J Gen Microbiol. 1980;118(1):29-37.

[50] Böttger EC. Approaches for identification of microorganisms. ASM News. 1996;62(2):79-84.