Mycobacterium marinum hand infection masquerading as tinea manuum: a case report and literature review Katarina Trčko1 ✉, Jurij Plaznik1, Jovan Miljković2 1Department of Dermatology and Venereal Diseases, University Medical Centre Maribor, Maribor, Slovenia. 2Faculty of Medicine, University of Maribor, Maribor, Slovenia. 91 2021;30:91-93 doi: 10.15570/actaapa.2021.23 Introduction Mycobacterium marinum is a nontuberculous mycobacterium that is present as a saprophyte in fresh and salt water. It can cause infections in fish and sporadically in humans. The organism is especially prevalent in natural or inadequately chlorinated swim- ming pools, and in heated aquariums. Infection in humans is uncommon and occurs when contaminated water or infected fish or shellfish come in contact with open skin wounds or abrasions. Since the first reported case of M. marinum skin infection in 1951 in swimmers that had swum in a contaminated pool in Sweden, several case reports and case series have been published to date (1–3). The majority of M. marinum infections are aquarium-relat- ed today, and therefore instead of swimming pool granuloma the terms aquarium granuloma and fish tank granuloma are now used (4). The typical presentation of infection is a solitary or multiple nodules or pustules, and about 25% of patients have a sporo- trichoid distribution of nodular lesions (5–6). According to the literature, presentation with erythematous scaling plaques does not occur often. Here we present the case of a 37-year-old male with M. marinum skin infection with an unusual clinical presenta- tion. Current diagnostic and treatment recommendations are also reviewed. Case report A 37-year-old man presented with painless livid plaques on the back of his right hand that had appeared 1 year earlier (Fig. 1). The first skin lesions had appeared on the nail fold of the fourth finger as a small warty papule. The lesion grew larger, and after few months a new lesion appeared on the back of his hand and the extensor side of the right wrist. He had been treated with lo- cal antifungal and antibiotic cream. The patient was otherwise healthy. He had no history of tuberculosis or any other systemic illness. In a detailed medical history, he reported that he had had an aquarium with tropical fish, exotic snakes, and lizards for the past 5 years. Physical examination revealed two polycyclic livid plaques with irregular hyperkeratotic elevated borders and central re- gression on the back of his right hand and the extensor side of the right wrist, and two hyperkeratotic papules on the extensor surface of the fourth and third fingers of his right hand. The dif- ferential diagnoses included superficial and subcutaneous fungal infections, atypical granuloma annulare, tuberculosis verrucosa cutis, verrucous lupus vulgaris, atypical mycobacterial infection, and vegetative pyoderma. Clinical examination did not reveal other skin or mucosal lesions, lymphadenopathy, or associated systemic manifestations. The Mantoux test was negative. Skin bi- opsies were taken for histopathological examination and culture Abstract Fish tank granuloma is a rare skin infection caused by Mycobacterium marinum. It occurs after exposure of skin abrasions to con- taminated water or infected fish. The majority of M. marinum infections today are fish tank–related. The most common presenta- tion is a solitary nodule, often with sporotrichoid spread. Other presentations do not occur often. The diagnosis is often delayed because of lack of suspicion, nonspecific histopathological findings, and frequently unsuccessful cultivation. Here we present the case of a 37-year-old male with M. marinum skin infection, presenting as erythematous scaling plaques. Because the initial results of laboratory and histopathological examinations were negative for a fungal infection or nontuberculous mycobacteria, the patient was treated empirically with several systemic antibiotics and antifungals without any success. Finally, the diagnosis of fish tank granuloma was confirmed 3 months after the initial presentation of the patient. After the introduction of treatment with rifampicin and clarithromycin, complete clinical remission was observed after 6 months of therapy. Keywords: Mycobacterium marinum, fish tank granuloma, hand infection, nontuberculous mycobacteria Acta Dermatovenerologica Alpina, Pannonica et Adriatica Acta Dermatovenerol APA Received: 30 March 2021 | Returned for modification: 31 May 2021 | Accepted: 3 June 2021 ✉ Corresponding author: katarina.trcko@gmail.com Figure 1 | Fish tank granuloma: polycyclic livid plaques with irregular hyper- keratotic elevated borders and central regression on the back of the right hand and extensor side of the right wrist, and two hyperkeratotic papules on the extensor surface of the fourth and third fingers of the right hand. 92 Acta Dermatovenerol APA | 2021;30:91-93K. Trčko et al. for bacteria, fungi, and mycobacteria. The histopathology showed pseudoepitheliomatous hyperplasia with a nonspecific mixed in- flammatory infiltrate with intraepidermal abscess and suggested chronic pyodermic inflammation (Fig. 2). Bacterial culture of the tissue identified Staphylococcus aureus. The results of the culti- vation of Mycobacterium tuberculosis and atypical mycobacteria were negative, as was polymerase chain reaction (PCR) for M. tuberculosis. For suspected bacterial skin infection, the patient started treatment with oral clindamycin for 4 weeks without any improvement; after that, he was treated empirically with doxycy- cline for 4 weeks and then with itraconazole for 4 weeks due to suspicion of deep fungal infection without any success. Although the histopathology findings and culture of a biopsy specimen were negative, our suspicion was aroused by his work with tropi- cal fish in an aquarium. We repeated skin biopsies and, after the third examination, the biopsy sent for culture grew M. marinum, confirming the diagnosis of fish-tank granuloma. The patient started treatment with rifampicin at a dose of 750 mg daily in combination with clarithromycin at a dose of 1,000 mg once daily, and after 6 months pronounced regression of the lesions was ob- served. Follow-up after 12 months was negative. Discussion M. marinum infections are uncommon, and estimates of the an- nual incidence vary from 0.04 per 100,000 in France to 0.27 cases per 100,000 in the United States (7, 8). In the past, outbreaks have been reported among individuals frequenting swimming pools, but, after proper chlorination of this reservoir, the main source of infection has been fish tanks; outbreaks have also been described in fish farms (9). Keeping and cleaning out tanks with infected fish or contaminated water are the main predisposing factors to- day. In a French study, 84% of cases of M. marinum disease were linked to fish tank exposure, and in the majority of cases the infec- tion was located on the hands (5). The incubation period ranges from 5 days to 9 months, but it is usually 2 to 3 weeks (10). Be- cause the optimal temperature for growth of M. marinum is 30 to 32 °C, the infection is limited to the skin; less commonly, the in- fection spreads to deeper structures, such as the joints, tendons, and bones (11–13). Immunocompromised patients may develop disseminated lesions, and a more aggressive course may occur in transplant recipients and subjects on corticosteroid or biologic therapy (14–18). The clinical presentations are polymorphous. Skin lesions are most commonly asymptomatic, and some patients occasionally present with pain, pruritus, tenderness, or paresthesia (19). The most common presentation in an immunocompetent patient, as described in about 60% of the cases, is a single granulomatous papule or nodule on a finger or hand, which may ulcerate and discharge a suppurative mass (5). Sporotrichoid dissemination is possible and has been reported in 25% of cases of M. marinum infection (20, 21). Less often, skin lesions may present as verru- cous plaques, erythematous plaques with scaling, or eczema-like lesions without a significant inflammatory component and in- filtration or interstitial granuloma annulare (3, 22, 23). Unusual clinical presentations have also been reported, such as destruc- tive nasal lesions mimicking extranodal natural killer (NK)/T-cell lymphoma (24). The most common site of infection in a patient with aquarium exposure are the upper extremities, such as a hand or fingers (25). A history of preceding minor trauma is common; however, because the incubation period can be very long, many patients cannot re- call skin trauma. Establishing a diagnosis of M. marinum infection can be a great challenge, probably due to the rarity and lack of awareness of this infection. If the key medical history of aquatic exposure is not obtained, the diagnosis is frequently delayed (22). The average time from clinical presentation to correct diagnosis varies from 1 to 27 months with a mean interval of 7 months (26). Diagnosis is eventually made through histopathologic examina- tion and the culture of a biopsy sample. Histopathologic findings are important for diagnosis, but they depend on the duration of the lesion. During the first 6 months, a non-specific inflammatory infiltrate may be observed, but acid-fast stains occasionally reveal bacilli. After such a period, lesions show tuberculoid granulomas with fibrinoid masses rather than caseation. The epidermis fre- quently shows hyperkeratosis, acanthosis, pseudoepithelioma- tous hyperplasia, intraepidermal neutrophilic abscesses, and ul- ceration. Dermal fibrosis and small blood vessel proliferation can be observed (26). The diagnosis must be confirmed by culture on Löwenstein–Jensen medium at 28 to 32 °C. According to the litera- ture data, the positivity rate of cultures ranges from 70 to 80% (5); in the series of 15 cases described by some authors, M. marinum was isolated in 13 out of 15 patients (93.3%) (5, 28). The PCR am- plification technique is a fast, sensitive, and specific diagnostic tool for diagnosing M. marinum infection directly in the biopsy sample. However, it should be interpreted with caution because false-positive results are possible (29, 30). There is no absolute consensus on the treatment of infections with M. marinum, and so therapeutic decisions should be based on reported outcomes in case reports and case series or suscepti- bility testing (6, 31, 32). In localized and uncomplicated skin in- fection, monotherapy with various antibiotics is recommended. M. marinum is susceptible to clarithromycin, trimethoprim/sul- famethoxazole, sulphonamides, rifampin, rifabutin, and etham- butol, and variably susceptible to doxycycline, minocycline, and streptomycin (26, 33, 34). Clarithromycin seems to be superior to other drugs in terms of drug susceptibility testing and due to fewer side effects (33). Combination therapy, often with clarithromycin plus rifampin and/or ethambutol, is preferred in cases of exten- sive infection or deep tissue involvement (16). Antibiotic suscep- tibility testing is routinely not recommended except in cases of treatment failure or relapse (31, 35). According to the literature, Figure 2 | The histopathology showed pseudoepitheliomatous hyperplasia with nonspecific mixed inflammatory infiltrate with intraepidermal abscess. 93 Acta Dermatovenerol APA | 2021;30:91-93 Mycobacterium marinum hand infection the mean duration of the treatment is 4 months (2, 16). Treatment should be continued for 1 to 2 months after the resolution of symp- toms (16, 20). In the case of mild disease, the infection can resolve spontaneously in several months to years (34). Small lesions may be treated with cryotherapy, curettage, or excision (35). In the case presented, the diagnosis was confirmed 3 months after the first visit to our department, and appropriate treatment was introduced 15 months after the initial lesions appeared. De- spite the unusual clinical presentation, a high index of suspicion for fish tank granuloma at the patient’s initial visit was present, mainly because of his medical history. The low positivity rate of cultures, which ranges from 70 to 80%, can often result in false negative results (5). Negative results of initial cultures can be explained by inadequate biopsy samples or technical problems with transporting and processing the specimen. Cultivation re- quires low temperatures for growth and also takes several weeks to become positive. Although the initial results of laboratory and histopathological examinations were negative for a fungal infec- tion or nontuberculous mycobacteria, the patient was treated em- pirically with clindamycin for 4 weeks and then with doxycycline. Because of no response to therapy after 4 weeks of treatment, an alternative diagnosis was considered, and treatment with itracon- azole was introduced. After 4 weeks, no response was observed, but finally the culture of the biopsy sample confirmed infection with M. marinum. The patient received a course of clarithromy- cin and rifampin, and complete resolution was observed after 6 months of therapy. Conclusions Infection with M. marinum is rare and has no pathognomonic clinical presentation. 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