A middle-aged man spent eight months cycling through treatments that did not work. Red patches on his left arm kept breaking open, oozing, and crusting over. Doctors gave him topical medications and oral antibiotics. The lesions came back. Three months before he reached a specialist center, surgeons cut out the damaged tissue and changed the dressings. The wound healed poorly, and new sores appeared on the fingers of his left hand. Another antibiotic did nothing.
The answer came from a question about his work. He had spent years in aquaculture, breeding and transporting fish and aquatic products. A skin biopsy showed granulomatous inflammation and acid-fast bacilli, and genetic sequencing named the culprit: Mycobacterium marinum, an environmental bacterium that lives in fresh and salt water and infects people through small breaks in the skin.
The case was published in Clinical, Cosmetic and Investigational Dermatology by Jiazhen Chen and Hongli Lv of the Department of Dermatology at Jiading District Central Hospital, affiliated with Shanghai University of Medicine and Health Sciences.
Eight Months of Wrong Treatments
The delay is the story. The bacterium grows slowly and causes lesions that resemble several far more common problems, including ordinary bacterial infection, fungal infection, sporotrichosis, and cutaneous tuberculosis. Patients are repeatedly treated for the wrong thing.
The authors write that the infection is "prone to misdiagnosis due to its nonspecific clinical presentation." Published reports describe median diagnostic delays ranging from several months to more than six months. Delay is not harmless. If the infection moves deeper, it can reach tendon sheaths, joints, and bone, causing tenosynovitis, arthritis, bursitis, or osteomyelitis, which then require longer treatment and often surgery.
In this patient, standard blood work was normal. Tests for HIV, syphilis, and hepatitis B were negative, and fungal cultures came back negative. He had no fever, night sweats, or weight loss. He also denied any specific fish spine injury, a reminder that patients often cannot recall the small cut that let the bacterium in.
Aquarium Owners and Seafood Handlers Share the Same Exposure
This case involved occupational aquaculture work, but the exposure route is identical for the large group of Americans who keep home fish tanks. The organism has been known in the United States for a century. It was first isolated in 1926 from saltwater fish that died in a Philadelphia aquarium, according to the reference maintained by the National Library of Medicine.
The infection remains uncommon, though how uncommon depends on where you look. Population studies cited in the case report, including a Danish national review and a Dutch treatment analysis, report an annual incidence of roughly 0.04 to 0.15 cases per 100,000 people. The National Library of Medicine reference cites a higher United States figure of 0.27 per 100,000. Either way, the authors argue that the numbers understate reality because the disease progresses slowly, clinicians rarely consider it, and laboratory confirmation is often incomplete.
More than 90 percent of reported infections involve the upper extremities, which follows directly from how people expose themselves. Hands and forearms go into tanks, handle fish, and clean equipment. Outbreaks have been documented among fish farm and fish market workers, including an outbreak of 32 infections traced to a single case in China.
The University of California, Los Angeles, which manages occupational risk for researchers working with zebrafish and other aquatic animals, advises wearing "sturdy waterproof gloves" whenever hands and forearms enter fish tanks or when handling fish and marine animals. Its occupational health guidance urges particular caution for anyone with scrapes or open sores and recommends washing hands and forearms with soap and water afterward.
Slow Growth Makes the Laboratory Work Difficult
Diagnosis here rested on tissue, not culture. Microscopic examination of the biopsy showed thickened skin with focal ulceration and granulomatous inflammation in the deeper layer. A Ziehl-Neelsen stain revealed acid-fast bacilli. Polymerase chain reaction sequencing was then used to identify the species. No culture-positive isolate was ever obtained.
That pattern is typical. Cultures may require prolonged incubation at temperatures below body temperature and can still come back negative; acid-fast bacilli are often sparse in tissue. Molecular testing has become the practical route to a species-level answer in chronic granulomatous skin disease.
Treatment consisted of two oral antibiotics, clarithromycin and rifampicin, along with boric acid wet dressings and topical mupirocin to prevent secondary bacterial infection. Blood counts and liver and kidney function were checked monthly, since these drugs require monitoring. Lesions shrank markedly after one month and mostly resolved by two. Treatment continued for four months total, ending with near-complete regression, leaving darkened skin and mild atrophic scarring.
Gloves, Wound Care, and When to Ask About a Biopsy
The paper's review of published regimens found that treatment length tracks with how deep the infection runs. Localized superficial disease has been managed with a single drug for roughly three to four months, though relapse appears more common after single-drug therapy. Two-drug combinations, the most frequently used approach, generally run three to six months. Deep infections involving tendon, joint, or bone commonly require 5 to 8 months and often surgery. Disseminated infection, largely in people with weakened immune systems, may require multiple drugs for six to twelve months.
Observational studies report cure rates around 87 to 90 percent, so the outlook is generally good once the diagnosis is made. No randomized trials exist comparing regimens, which the authors identify as the central gap in the evidence.
For readers, the actionable part is short. People who keep aquariums, work with seafood, or fish regularly should cover cuts, wear waterproof gloves, and wash thoroughly afterward. Anyone with a bump, nodule, or sore on the hand or forearm that has not healed within a few weeks, or that keeps returning after antibiotics, should tell a clinician specifically about water and fish exposure and ask whether a biopsy with culture and molecular testing is warranted. Mentioning the exposure is often what redirects the workup.
The work was supported by a hospital young personnel program, and the authors declare no competing financial interests. Their literature review drew on retrospective studies and case reports, which they note limits direct comparison between treatment approaches.
Key Questions Answered
What is Mycobacterium marinum?
It is a slow-growing bacterium related to the organisms that cause tuberculosis, found in fresh and salt water worldwide. In people, it typically causes skin and soft tissue infection after contaminated water enters through a small wound, producing what clinicians call fish tank granuloma or swimming pool granuloma.
Who is most at risk?
People with repeated exposure to water and fish through broken skin. That includes aquarium owners, aquaculture and fish market workers, seafood processors, anglers, and laboratory staff working with fish. People with weakened immune systems face higher risk of deeper or spreading infection.
What does the infection look like?
Papules, nodules, plaques, or ulcers, most often on the hands and forearms. Lesions may spread in a line along lymphatic channels. They tend to progress slowly, cause little pain or itching, and fail to improve with standard antibiotics.
Why does diagnosis take so long?
The appearance mimics common bacterial and fungal skin infections, the bacterium grows slowly and may not culture, and clinicians rarely consider it. Published reports describe median delays from several months to more than six months.
How is it treated?
With antibiotics chosen according to how deep the infection runs, usually for several months. Superficial disease may need three to four months, while deep infection can require five to eight months plus surgery. Treatment requires a prescription and monitoring of blood counts, liver function, and kidney function.
How can exposure be reduced?
Wear sturdy waterproof gloves when reaching into tanks or handling fish, avoid submerging hands with open cuts or scrapes, and wash hands and forearms with soap and water afterward. Cover existing wounds before any contact with water.
