Mycobacterium marinum Infections in Aquatic Animals and Humans: A One Health Perspective

Introduction

Mycobacterium marinum is a nontuberculous mycobacterium (NTM) that causes systemic granulomatous disease in fish and localized cutaneous infections in humans. The organism is an acid-fast, slow-growing, environmental bacterium that thrives in aquatic ecosystems, including both freshwater and marine environments. From a One Health perspective, M. marinum exemplifies a pathogen that circulates among fish populations, persists in biofilm communities within aquaria and aquaculture systems, and sporadically transmits to humans through direct contact with contaminated water or infected fish. This review integrates veterinary microbiology, molecular diagnostics, and clinical management to provide a comprehensive reference for veterinary professionals and diagnostic laboratory scientists.

Microbiology and Pathogenesis

M. marinum is a member of the Mycobacterium marinum complex, closely related to M. ulcerans and M. tuberculosis. It is a rod-shaped, aerobic, non-spore-forming bacterium with a lipid-rich cell wall that confers acid-fastness and resistance to disinfectants. The optimal growth temperature ranges from 25 to 35 degrees Celsius, with no growth at 37 degrees Celsius, a feature that distinguishes it from M. tuberculosis [1, 2]. In fish, the pathogen enters through the gastrointestinal tract, gills, or skin abrasions. Once inside the host, M. marinum is phagocytosed by macrophages but evades killing by inhibiting phagosome-lysosome fusion and replicating within the phagosomal compartment [3]. This intracellular survival leads to the formation of granulomas, which are the hallmark of mycobacteriosis in fish. Granulomas consist of epithelioid macrophages, multinucleated giant cells, and a peripheral layer of lymphocytes and fibroblasts [4]. In humans, the same mechanism produces a localized granulomatous infection, typically on the extremities, known as "fish tank granuloma" or "swimming pool granuloma" [5].

Epidemiology and Host Range

M. marinum has been isolated from a wide range of aquatic animals, including freshwater and marine fish, amphibians, reptiles, and occasionally invertebrates [6, 7]. In aquaculture settings, outbreaks of mycobacteriosis cause significant economic losses, particularly in ornamental fish species such as zebrafish (Danio rerio), gouramis, and cichlids, as well as in food fish like striped bass and salmonids [8, 9]. The prevalence of M. marinum in wild fish populations is variable but can exceed 50 percent in some estuarine environments [10]. Human infections are considered occupational or recreational zoonoses, occurring most frequently in aquarium hobbyists, fish farmers, fishermen, and marine biologists [11]. The incidence of human M. marinum infection is likely underreported due to misdiagnosis as other granulomatous skin conditions [12].

Clinical Manifestations in Fish

Piscine mycobacteriosis caused by M. marinum is a chronic, progressive disease. Clinical signs are nonspecific and include lethargy, anorexia, emaciation, exophthalmos, skin ulcers, fin erosion, and abdominal distension due to coelomic granulomas [13, 14]. Internally, gross pathology reveals gray-white nodules (granulomas) in the spleen, kidney, liver, and mesentery. Histopathology shows well-organized granulomas with central caseous necrosis and variable mineralization [15]. In advanced cases, the disease can become systemic, leading to high mortality, especially under stress conditions such as poor water quality or overcrowding [16]. Subclinical carriers are common and serve as reservoirs for transmission within the aquarium or farm.

Clinical Manifestations in Humans

Human infection typically occurs through inoculation of the skin by contaminated water or fish spines. The incubation period ranges from two to four weeks. The classic presentation is a solitary, slowly growing papulonodular lesion on the hand or forearm, which may ulcerate and form a sporotrichoid pattern of ascending lymphangitic nodules [17, 18]. Deep infections involving tenosynovitis, arthritis, or osteomyelitis can occur in immunocompromised individuals [19]. Disseminated disease is rare but has been reported in patients receiving immunosuppressive therapy or with underlying HIV infection [20]. The differential diagnosis includes sporotrichosis, leishmaniasis, nocardiosis, and other NTM infections.

Biofilm Formation in Aquaria

M. marinum readily forms biofilms on surfaces in aquatic environments, including aquarium glass, filter media, and plumbing components. Biofilm formation is a critical factor in the persistence and transmission of the pathogen. The biofilm matrix consists of extracellular polymeric substances (EPS) that protect the bacteria from disinfectants and antimicrobial agents [21]. In aquaria, M. marinum biofilms can harbor high bacterial loads and continuously shed planktonic cells into the water column [22]. The presence of organic matter and the warm temperatures typical of tropical aquaria (24 to 28 degrees Celsius) promote biofilm development [23]. Routine cleaning and disinfection of aquaria are often insufficient to eradicate biofilms, and the use of quaternary ammonium compounds or chlorine at appropriate concentrations is recommended, though efficacy is variable [24].

Diagnostic Approaches

Culture

The gold standard for diagnosis of M. marinum infection is isolation of the organism by culture. Samples from fish include swabs of skin lesions, aspirates of coelomic fluid, or tissue homogenates from granulomatous organs. Human specimens include biopsy tissue or pus from lesions. Samples are decontaminated using the N-acetyl-L-cysteine-sodium hydroxide method and inoculated onto Lowenstein-Jensen medium or Middlebrook 7H10 agar [25]. Cultures are incubated at 30 degrees Celsius for up to eight weeks. Colonies appear as smooth, yellow-pigmented (photochromogenic) when exposed to light, a key phenotypic characteristic [26]. The slow growth rate and requirement for specialized media limit the utility of culture for rapid diagnosis.

Molecular Detection

Polymerase chain reaction (PCR) assays targeting the 16S rRNA gene, the heat shock protein 65 gene (hsp65), or the internal transcribed spacer (ITS) region provide rapid and specific detection of M. marinum [27, 28]. Real-time PCR formats with probes such as TaqMan can achieve sensitivities equivalent to culture within 24 to 48 hours [29]. Multiplex PCR panels that differentiate M. marinum from other NTM species are available for clinical and veterinary use [30]. Sequencing of the hsp65 gene or the rpoB gene allows definitive species identification and phylogenetic analysis [31]. For formalin-fixed, paraffin-embedded tissues, PCR can still be performed but with reduced sensitivity due to DNA fragmentation [32].

Histopathology and Acid-Fast Staining

In fish, histopathological examination of granulomas with Ziehl-Neelsen or Fite's acid-fast stain reveals numerous red-stained bacilli within macrophages and necrotic centers [33]. In human biopsies, acid-fast bacilli may be scarce, and the sensitivity of staining is low (less than 30 percent) [34]. Immunohistochemistry using monoclonal antibodies against M. marinum antigens can enhance detection in tissue sections [35].

Serology

Serological assays such as enzyme-linked immunosorbent assay (ELISA) have been developed for detection of antibodies against M. marinum in fish, but cross-reactivity with other mycobacteria limits their specificity [36]. In humans, serology is not routinely used due to poor sensitivity and specificity.

Diagnostic Workflow

The following Mermaid diagram illustrates a recommended diagnostic algorithm for suspected M. marinum infection in fish and humans.

flowchart TD
    A[Clinical suspicion: fish with granulomas or human with nodular skin lesion], > B{Specimen type}
    B, >|Fish tissue/swab| C[Decontamination and culture at 30°C]
    B, >|Human biopsy/aspirate| D[Decontamination and culture at 30°C]
    C, > E[Growth after 2-8 weeks?]
    D, > E
    E, >|Yes| F[Photochromogenicity test]
    F, > G[Positive: M. marinum]
    E, >|No| H[PCR on original sample]
    H, > I[16S rRNA or hsp65 PCR]
    I, > J[Amplicon sequencing]
    J, > K[Species identification]
    G, > L[Confirm with PCR/sequencing]
    K, > M[Report result]
    L, > M

Treatment Challenges

In Fish

Treatment of mycobacteriosis in fish is rarely attempted due to the chronic nature of the disease, the difficulty of achieving therapeutic drug concentrations in aquatic environments, and the risk of generating antimicrobial resistance. Antibiotics such as rifampicin, ethambutol, clarithromycin, and doxycycline have been used experimentally in fish, but efficacy is inconsistent and relapses are common [37, 38]. In aquaculture, culling of infected stocks and disinfection of facilities are the recommended control measures. No licensed vaccine exists for M. marinum in fish, although experimental vaccines using killed or attenuated strains have shown partial protection in laboratory trials [39].

In Humans

Human M. marinum infections require prolonged combination antibiotic therapy. Standard regimens include clarithromycin or azithromycin plus ethambutol, with or without rifampicin [40]. Treatment duration is typically three to six months, and surgical debridement may be necessary for deep infections or abscesses [41]. Resistance to clarithromycin has been reported, necessitating susceptibility testing of isolates [42]. The organism is intrinsically resistant to isoniazid and pyrazinamide, which are first-line drugs for tuberculosis [43]. Relapse can occur if therapy is prematurely discontinued.

One Health Implications

M. marinum is a classic One Health pathogen that links environmental health, animal health, and human health. In aquaculture systems, the presence of M. marinum can compromise fish welfare and productivity, while also posing a zoonotic risk to workers. The biofilm-forming ability of the organism in aquaria and water distribution systems creates a persistent environmental reservoir that is difficult to eliminate [44]. Surveillance of M. marinum in fish populations, particularly in ornamental fish trade and aquaculture, is essential for early detection and risk mitigation [45]. Molecular typing methods such as variable-number tandem repeat (VNTR) analysis and whole-genome sequencing can trace transmission pathways between fish and humans [46, 47]. Public health education targeting aquarium hobbyists and fish handlers about the risks of direct contact with aquarium water and the importance of wearing gloves can reduce the incidence of human infection [48]. Integrated management strategies that combine biosecurity, water quality control, and diagnostic surveillance are needed to control M. marinum at the human-animal-environment interface [49, 50].

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