a 35-year - old man was diagnosed with ulceration on the dorsal surface of the right hand in our dermatology consultation . the lesion had started 15 days earlier as a small erythematous nodule that developed into ulceration . subsequently , a large erythematous and warm patch appeared on the forearm , leading the patient to consult his general practitioner . amoxicillin with clavulanic acid was prescribed in combination with mupirocin cream , but without any improvement . on physical examination , the patient presented ulceration on the back of the right hand ( fig . 1 ) , in addition to a light erythema with an indurated patch , 2 cm in diameter , in the elbow crease . the patient had no other lesions or lymphadenopathies , nor any fever or systemic symptoms . the patient 's medical history revealed pericarditis and gastritis ; he did not take any medication or suffer from allergies . he had not travelled abroad , however , it turned out that he has a cat and fish . a smear was taken from the ulceration , antibiotics were discontinued , and a local treatment based on alginate enzymes ( flaminal hydro ) was initiated . several weeks later , there was no improvement in the ulceration , and a number of red subcutaneous nodules appeared along the lymphatic vessels , extending from the wrist to the top of the arm ( fig . routine laboratory investigations included hiv serology , with normal results , and negative hiv serology . histopathological examination showed ulceration with granulation tissue and polymorphic infiltrates containing lymphocytes , neutrophils , and giant cells . cultures were also negative with the exception of one culture placed in a 30c environment , which was identified as an atypical mycobacterium , namely mycobacterium marinum ( fig . a treatment comprising 200 mg doxycycline and 1 g clarithromycin was initiated , resulting in a favourable clinical outcome for the patient . this acid - alcohol fast bacillus was first isolated from fish carcasses by aronzon in 1926 , and in 1951 , it was recognised as a human pathogen by linell and norden . the bacteria are considered to have a worldwide distribution , but fish handlers and fish tank hobbyists seem to be at the highest risk of developing the infection . for this reason , the condition was first known as  swimming pool granuloma or  fish tank granuloma. the overall incidence of m. marinum infection rates is estimated at 0.27 cases per 100,000 inhabitants . with an incubation period ranging from 2 to 6 weeks , the lesions generally occur after a skin wound or fish bite , located on the upper limb in 95% of cases . in two - thirds of cases , the lesions are nodular , and in the remaining third , the distribution is sporotrichoid , being characterised by multiple nodules along the lymph vessels . although the infection is mostly limited to the skin , it may spread to deeper structures . : type i is characterised as self - limiting with a single nodule , type ii as single or multiple granulomas with or without ulceration , and type iii as a deep infection , such as tenosynovitis , arthritis , or osteitis . the number of lesions , depth of the extension , and systemic involvement depend on the immunological status of the patient . disseminations are very rare and occur in immunocompromised patients treated with corticosteroids or immunosuppressive therapies , or presenting aids [ 6 , 7 ] . histological examination reveals granulomatous inflammation , without caseation , while there may be fibrinoid necrosis . the epidermis is parakeratotic , acanthotic , or ulcerated . in the dermis , lymphohistiocytic infiltrates and langhans the bacteria may be identified using ziehl - neelsen stains , but the lesions show only few bacilli except in the case of immunocompromised hosts . therefore , the absence of bacteria on this coloration does not exclude the diagnosis of m. marinum . in two - thirds of cases , diagnosis is made based on culture , with appropriate culture tissues on lowenstein - jenson medium at 30 - 32c . m. marinum is a photochromogen bacillus that produces yellow colonies on this culture medium after 6 weeks . the polymerase chain reaction may lead to a faster diagnosis , although only a few studies to date have demonstrated the cost - effectiveness of this technique . antibiotics are the first - line therapy , but in the scientific literature , there is a significant heterogeneity in types , doses , and duration of antibiotics administered . according to aubry et al . [ 4 , 12 ] , rifabutin and rifampin present the best minimal inhibitory concentrations ( mic ) , followed by clarithromycin . alkhodair and al - khenaizan   recommend 100 mg minocycline with 160 - 800 mg trimethoprim - sulfamethoxazole twice daily . deeper infections , such as tenosynovitis and osteitis , are more resistant to antibiotherapy and may require surgical treatment . there is no pathognomonic manifestation of m. marinum infection , with the average delay from presentation to diagnosis being 14 months . the diagnosis is clinical , being based on a very detailed anamnesis and supported by histopathology and cultures . this aetiology must be suspected in the case of a patient with a chronic lesion and regular contact with water or fish tanks .
we report the case of a 35-year - old man with a chronic ulceration of the hand in whom an infection with mycobacterium marinum was diagnosed . 
 clarithromycin and doxycycline were prescribed , resulting in a slow resolution of the ulceration . 
 m. marinum is a nontuberculous mycobacterium that causes skin lesions such as nodules , ulcerations , and sporotrichoid spread , but may also be responsible for osteoarticular lesions . in this case report , we discuss the clinical characteristics of this condition , as well as its diagnostic methods and treatments .