Occupational Mycobacterium marinum Infection Following Fish and Aquaculture Exposure: A Case Report and Literature Review

Abstract

Objective

To describe the clinical characteristics, and treatment duration and outcomes associated with different therapeutic regimens for Mycobacterium marinum infection.

Methods

We report a case caused by Mycobacterium marinum, presenting with erythema and erosions on the left forearm and left fingers, which was confirmed by histopathological examination and polymerase chain reaction (PCR)–based gene sequencing. A narrative literature review was also conducted to summarize reported antimicrobial regimens, treatment durations, and clinical outcomes.

Results

In the present case, the diagnosis was supported by positive acid-fast staining and PCR-based gene sequencing of the lesional specimen. No culture-positive isolate was obtained. The patient was treated with a combination of clarithromycin and rifampicin for four months, resulting in marked regression of skin lesions. The literature review indicated that monotherapy is mainly applied in patients with localized and superficial disease, with a typical treatment duration of 4–6 months. Dual-drug regimens represent the most commonly used therapeutic approach, with treatment courses generally lasting 3–5 months. Triple-drug therapy is primarily reserved for disseminated or deep-seated infections, with treatment durations ranging from 2 to more than 8 months.

Conclusion

Mycobacterium marinum infection is prone to misdiagnosis due to its nonspecific clinical presentation. Definitive diagnosis relies on histopathological examination and molecular biological techniques. Treatment strategies should be individualized according to the extent and severity of lesions. Standardized combination therapy administered for an adequate duration is associated with favorable clinical outcomes.

Introduction

Mycobacterium marinum is a slow-growing nontuberculous mycobacterium widely distributed in freshwater and saltwater environments. Human infection usually occurs after contaminated water, fish, or aquatic materials are inoculated through minor skin injuries, producing cutaneous and subcutaneous lesions known as “swimming pool granuloma” or “fish tank granuloma”.^1,2 Because the hands and forearms are commonly exposed during aquatic activities, more than 90% of reported infections involve the upper extremities.³ Delayed diagnosis may allow infection to extend into deeper structures, causing tenosynovitis, arthritis, bursitis, or osteomyelitis.⁴ Population-based studies indicate that M. marinum infection is rare, with reported annual incidences of approximately 0.04–0.15 cases per 100,000 population.^3,5 However, its true burden may be underestimated because of its indolent course, low clinical awareness, and incomplete microbiological confirmation. Infection is strongly associated with recreational and occupational exposure to aquaria, fishing, aquaculture, fish handling, and seafood processing.^2,3 Occupational outbreaks among fish-farm and fish-market workers further demonstrate the increased risk associated with repeated contact between injured skin and contaminated fish or water.^6,7 The clinical manifestations are nonspecific and may resemble sporotrichosis, cutaneous tuberculosis, fungal infection, or foreign-body granuloma. Consequently, diagnosis is frequently delayed, and patients may receive repeated antibacterial or antifungal treatment without improvement. The median diagnostic delay reported in previous studies ranged from several months to more than six months.^3,4 Diagnosis requires a compatible aquatic exposure history together with histopathological examination, mycobacterial culture under appropriate temperature conditions, and species identification using biochemical or molecular methods. Acid-fast staining may support the diagnosis but has limited sensitivity.⁸ Because randomized comparative trials are lacking, the optimal antimicrobial regimen and treatment duration remain uncertain. Localized superficial lesions may respond to a single active agent, whereas extensive, sporotrichoid, recurrent, or deep infections generally require combination therapy and, in selected cases, surgical intervention.⁹ Observational studies have reported cure rates of approximately 87%–90%, although deep infection is associated with longer treatment and a greater risk of treatment failure.^2,5 In this report, we describe a case of M. marinum infection in a middle-aged man with long-term occupational exposure to fish and aquaculture products. Persistent erythema and erosions involving the left forearm and fingers had continued for eight months despite multiple previous treatments. We also reviewed the relevant literature to evaluate antimicrobial strategies and treatment durations, aiming to provide practical evidence for the recognition and management of this uncommon occupational infection.

Case Presentation

Patient Information

A middle-aged male patient presented with an 8-month history of recurrent ulcerative skin lesions on the left upper limb. Eight months prior to presentation, an erythematous patch initially appeared on the left arm (as shown in Figure 1Ai–Aiii). The lesion repeatedly developed erosion and ulceration with a small amount of exudate. Pruritus was absent, and only occasional mild pain was reported.

Figure 1.

(Ai–Aiii) Clinical appearance at the initial visit showing two erythematous plaques on the left arm with surface erosion and partial crust formation. (Bi–Biii) After treatment, the erosive lesions show obvious contraction and reduction in size compared with baseline. (Ci–Ciii) The erosions have nearly resolved, leaving residual erythema. (Di–Eiii) Follow-up examination demonstrates reticular scarring at the two lesion sites on the left arm.

The patient had previously received topical medications and oral antibiotics at a local hospital; however, the lesions recurred. Three months before admission, surgical debridement and dressing changes were performed, but wound healing was poor. New lesions subsequently developed on the fingers of the left hand, and oral cefaclor therapy produced no significant improvement.

The patient denied systemic symptoms such as fever, night sweats, or weight loss. On further inquiry, he reported long-term occupational exposure to fish and aquatic products through aquaculture breeding and transportation, although he denied a definite history of fish-spine injury. His past medical history was unremarkable, and he denied previous tuberculosis or other infectious diseases.

Dermatological Examination

Physical examination revealed two coin-sized dark-red plaques located on the left arm and fingers. The lesions were well demarcated and showed surface erosion and ulceration, partially covered with crusts. Subjective symptoms were mild.

Laboratory and Histopathological Findings

Routine laboratory tests, including complete blood count, urinalysis, and liver and renal function tests, were within normal limits. Serological tests for human immunodeficiency virus (HIV) antibody, Treponema pallidum particle agglutination assay (TPPA), rapid plasma reagin (RPR), and hepatitis B surface antigen were all negative. Direct microscopic examination and culture for fungi from the lesions were negative.

Histopathological examination of a skin biopsy demonstrated papillomatous epidermal hyperplasia with hyperkeratosis and parakeratosis, along with focal ulceration. Granulomatous inflammation was observed in the dermis, with nuclear debris in the center of the granulomas and dense superficial lymphocytic infiltration (as shown in Figure 2A and B). Ziehl–Neelsen staining revealed acid-fast bacilli.

Figure 2.

Histopathological findings of the skin lesion. (A) (×20)Hematoxylin–eosin staining shows papillomatous epidermal hyperplasia with hyperkeratosis and parakeratosis and focal ulceration. (B) (×100)Granulomatous inflammation is present in the dermis, with central nuclear debris within the granulomas and dense superficial lymphocytic infiltration.

Polymerase chain reaction (PCR) gene sequencing identified Mycobacterium marinum.

Diagnosis

Cutaneous Mycobacterium marinum infection associated with occupational exposure to fish and aquaculture products.

Treatment and Outcome

The patient was treated with oral clarithromycin 250 mg twice daily and rifampicin 450 mg twice daily. Locally, boric acid wet dressings were applied, and topical mupirocin ointment was used to prevent secondary bacterial infection. Complete blood count and liver and renal function tests were monitored monthly.

After 1 month of therapy, the lesions had markedly decreased in size (as shown in Figure 1Bi–Biii). After 2 months, most lesions had resolved (as shown in Figure 1Ci–Ciii). Treatment was continued for a total of 4 months, after which the lesions had nearly completely regressed, leaving residual hyperpigmentation and mild atrophic scarring (as shown in Figure 1Di–Eiii).

Literature Review Methods

A narrative literature search was conducted using PubMed/MEDLINE, Web of Science, Embase and CNKI, up to 012026, with the terms “Mycobacterium marinum”, “treatment”, “treatment duration”, and “clinical outcome”. English- and Chinese-language case reports, case series, and retrospective studies reporting antimicrobial regimens, treatment duration, and outcomes were included. Studies without identifiable treatment or outcome data were excluded. Because of substantial clinical and methodological heterogeneity, the results were summarized descriptively.

Discussion

Mycobacterium marinum is a slow-growing nontuberculous mycobacterium commonly found in fresh and salt water environments, aquariums, and fish-handling settings. Human infection typically occurs after minor trauma that allows contaminated water to be inoculated into the skin, and the upper extremities are most frequently affected. Because the disease progresses slowly and symptoms are often mild, diagnosis is frequently delayed. Patients are commonly treated empirically for bacterial infection, sporotrichosis, or cutaneous tuberculosis before the correct diagnosis is established. In the present case, the patient experienced persistent lesions for approximately eight months despite topical therapy, oral antibiotics, and surgical intervention before appropriate identification was achieved. Such prolonged clinical courses are characteristic of this infection and contribute to underrecognition in routine practice.

Clinical manifestations are variable and nonspecific. Lesions typically present as papules, nodules, plaques, or ulcerations and may spread along lymphatic channels in a sporotrichoid pattern. Epidemiological exposure therefore represents an important diagnostic clue. Occupational or recreational aquatic exposure markedly increases the likelihood of infection, particularly when lesions are located on the hands or forearms. The patient in this report had long-term occupational exposure to aquatic products, supporting the clinical suspicion. Histopathology usually demonstrates granulomatous inflammation with or without necrosis, whereas acid-fast bacilli are often sparse. Culture may require prolonged incubation and may be negative in some cases. For these reasons, molecular diagnostic techniques such as polymerase chain reaction and sequencing are particularly valuable in chronic granulomatous dermatoses suspected to be caused by nontuberculous mycobacteria.

Treatment selection primarily depends on the extent and depth of infection rather than the duration of symptoms. Superficial localized cutaneous disease represents the most common presentation. Limited lesions may respond to monotherapy using tetracyclines, macrolides, or trimethoprim–sulfamethoxazole. Treatment courses generally last approximately three to four months, and favorable outcomes are common, although clinical improvement rather than complete cure may be reported.⁵ Relapse appears more frequent after monotherapy, and therefore single-drug therapy is typically reserved for strictly localized disease.

When lesions are multiple, progressive, or distributed in a sporotrichoid pattern, combination antimicrobial therapy is preferred. Frequently used regimens include a macrolide combined with rifampicin, rifampicin combined with ethambutol, or other macrolide-based dual therapies. Treatment duration usually ranges from three to six months (Table 1). In the present patient, therapy with clarithromycin and rifampicin for four months resulted in marked regression of lesions, consistent with reported therapeutic experience in persistent cutaneous disease.

Table 1.

Treatment and Outcomes of Mycobacterium marinum Infection

Study	Country	Study Design (n)	Antimicrobial Regimen	Treatment Duration (Months)	Outcome
Weinstein 199710	USA	Case report	Clarithromycin	0.9	Cure
Ang 200011	Singapore	Retrospective (38)	TMP-SMX; minocycline; combination regimens	Mean 3.5	Improvement 68%; failure 5%; LTFU 26%
Holden 20185	Denmark	Nationwide retrospective (55)	Tetracyclines; macrolides; rifampicin + clarithromycin	Median 3.7	Improvement 75%; failure 4%; LTFU 11%
Hendrikx 20223	Netherlands	Retrospective cohort (40)	Doxycycline; minocycline; clarithromycin; TMP-SMX; EMB + macrolide; RIF + EMB	Mean 5.8 ± 3.3	Cure 90%; relapse/failure 7.5%
Veraldi 201412	Italy	Case series (14)	Clarithromycin	5.0	Complete remission 86%
Bouricha 201413	France	Case report (1)	Gentamicin + clarithromycin	1.9	Improvement
Esteves 201914	USA	Case report (1)	Clarithromycin + rifampicin	2.0	Cure
Huang 201215	Taiwan, China	Case report (1)	Rifampicin + clarithromycin + amikacin	6.0	Cure
Krooks 201816	USA	Case report (1)	Clarithromycin + ethambutol	2.8	Cure
Holden 20185	Denmark	Nationwide retrospective	Clarithromycin + ethambutol (± rifampicin)	Median 8.0	Improvement
Johnson 201517	USA	Retrospective (28)	EMB; RIF; macrolides	Median 5.0	Improvement
Cheung 201218	Hong Kong, China	Retrospective (166)	Antibiotics ± surgery	Mean 7.2	Functional satisfaction 82%
Pop 201919	Netherlands	Case report (1)	Clarithromycin + ethambutol	6.0	Cure
Tsai 200620	Japan	Literature review (166)	Various regimens	Range 1–14	Cure/improvement 85%
Jernigan 200321	USA	Review	Multidrug therapy	Mean 11.4	Prolonged therapy required
Castillo 202022	USA	Retrospective cohort (46)	TMP-SMX; tetracyclines; macrolides; dual regimens	Median 4.5	Cure in most; relapse 2
Meybeck 202123	France	Case report (1)	Multidrug regimen	12.0	Cure
Bell 202324	UK	Case report (1)	Azithromycin + ethambutol + rifampicin	12.0	Cure

Abbreviations: TMP-SMX, trimethoprim-sulfamethoxazole; EMB, ethambutol; RIF, rifampicin; LTFU, lost to follow-up.

Deep infections involving tendon sheaths, joints, or bone represent a distinct clinical entity. Compared with superficial disease, these patients require longer antimicrobial therapy and frequently require surgical intervention. Treatment durations commonly range from five to eight months and may extend beyond this period. Regimens generally include a macrolide combined with ethambutol, often with the addition of rifampicin. Surgical debridement is frequently necessary because organisms may persist within poorly vascularized synovial tissue, and delayed recognition can result in functional impairment.

Disseminated infection is uncommon but occurs mainly in immunocompromised individuals. These patients typically require multidrug therapy for at least six to twelve months, and relapse may occur if therapy is discontinued prematurely. Severe systemic involvement and unfavorable outcomes have been described, highlighting the importance of evaluating host immune status when determining treatment intensity and duration.

Although localized infection may occasionally respond to monotherapy, combination therapy appears to provide more reliable outcomes in most patients. Dual-drug regimens reduce relapse risk, improve antimicrobial efficacy, and decrease the likelihood of resistance development. In this case, clarithromycin combined with rifampicin achieved near-complete resolution, supporting the effectiveness of macrolide-based combination therapy for persistent cutaneous infection.

Determining the optimal duration of therapy remains challenging. Current clinical experience suggests that treatment should be continued for one to two months beyond clinical resolution to minimize recurrence. In general, superficial disease requires approximately three to four months of therapy, sporotrichoid or extensive disease three to six months, deep infection five to eight months or longer, and disseminated infection at least six to twelve months. The patient in this report received a four-month treatment course and demonstrated lesion regression with residual post-inflammatory pigmentation and mild scarring, consistent with typical outcomes.

In conclusion, Mycobacterium marinum infection is an important cause of chronic cutaneous granulomatous disease that is frequently misdiagnosed. Careful assessment of exposure history, recognition of characteristic clinical features, and appropriate laboratory testing facilitate early diagnosis. Treatment should be individualized according to disease severity, and adequate com antimicrobial therapy administered for a sufficient duration. In this patient, clarithromycin combined with rifampicin resulted in a favorable clinical response. However, the optimal regimen and treatment duration remain uncertain because current evidence is based mainly on observational studies and case reports.

Limitations

The findings of the literature review should be interpreted cautiously because most included publications were retrospective studies, small case series, or case reports. Differences in disease severity, treatment regimens, outcome definitions, and follow-up duration limited direct comparisons among monotherapy, dual-drug therapy, and multidrug therapy.

Funding Statement

This work was supported by Jiading District Central Hospital’s Outstanding Young Personnel Support Program.

Data Sharing Statement

Data are available to request from the Corresponding Author.

Ethics Approval and Consent to Participate

This study has passed the ethical review (2026-27) of the Ethics Office of Jiading District Central Hospital Affiliated Shanghai University of Medicine &Health Sciences. The institutional approval for the publication of the case details was also obtained from Jiading District Central Hospital Affiliated Shanghai University of Medicine &Health Sciences. The patient provided informed consent to participate in the study, in accordance with the Declaration of Helsinki.

Consent for Publication

Written informed consent was obtained from the patient for publication of this case report and the accompanying images.

Disclosure

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.