"As I was leaving the room," she says, "I said to her parents, 'I have one more question. Do you have a fish tank?' Mom replied, 'No. But we have a fish bowl. She helps us clean it every weekend.'"
"Yea, slimy things did crawl with legs"
The consultant orders a battery of laboratory tests, including a complete blood count. The white blood cell count is 11.4 X 103/mL, with a differential count of 39% polymorphonuclear neutrophils, 50% lymphocytes, 9% monocytes, and 2% eosinophils; hemoglobin, 12.3 g/dL; platelets, 343 X 103/mL. The C-reactive protein level is 0.1 mg/dL.
Biopsy of the skin in the affected area reveals a focal area of necrosis that is consistent with acute and chronic inflammation. A Gram stain is negative for bacteria, with 3+ polymorphonuclear neutrophils. Acid-fast staining is weakly positive for bacilli. Specimens are taken for bacterial and fungal cultures.
Those cultures eventually return negative. However, the acid-fast culture grows Mycobacterium marinum—and that jibes with the parents' comment about their fish bowl! The organism is sensitive to clarithromycin, rifampin, trimethoprim-sulfamethoxazole, and minocycline. The ID specialist is still smiling!
A course of clarithromycin and rifampin is started and, initially, the finger appears to improve. Approximately one month later, however, the nodule at the base of the finger enlarges and continues to do so. A three-month course of antibiotics ensues, but does not succeed in eliminating the nodule. The girl undergoes incision and drainage. Staining of material from the nodule reveals rare acid-fast organisms. A culture no longer grows M marinum, however.
She continues antibiotic therapy for a total of six months. One year later, your patient continues to do well and has no problem with recurrence.
"I fear thee, ancient Mariner!"
Like all Mycobacterium species, M marinum resides in macrophages. Hosts sequester the pathogen in granulomas, leading to long-term infection. M marinum infects fish and frogs, and was first isolated in saltwater fish in a Philadelphia aquarium in 1926. In 1951, the organism was identified in humans. The great majority of diagnoses are associated with fish tank, lake, pond, and swimming pool exposures. The annual incidence is 0.27/100,000 persons.
M marinum infection is most often limited to skin, where manifestations develop eight to 30 days after exposure at a site of minor skin disruption. Erythematous papules, nodules, or plaques erupt with or without purulent drainage. Infection may extend proximally in a nodular pattern. Although complications are unlikely, tenosynovitis, arthritis, bursitis, and osteomyelitis have been reported; disseminated infection is rare in an immunocompetent host. Treatment is based on the extent of disease. Generally, a prolonged course of an antimicrobial agent is administered. Surgical debridement may also be required for treatment to be effective.
In fish, M marinum infects numerous organs, resulting in a wasting syndrome, nodular lesions, ulceration and loss of scales, necrotic fins, discoloration, and rapid breathing. Because 25% of cases of M marinum infection occur in people who work with aquariums, a recent survey examined fish sales representatives' knowledge of M marinum infection (so-called fish tank granuloma) and their strategies for preventing infection. Of 40 salesmen, only six reported that they "knew the disease well"; 30 said they "knew about it, but ignored it." Thirty-three percent were introduced to the condition during their training to become an aquarium worker. Only eight were "concerned about human disease"; three of those had acquired an infection and five knew someone who had. Importantly, the majority of sales representatives immersed their ungloved hands in fish tanks daily, admitting that they discount the importance of fish tank granuloma.
"Water, water, everywhere"
When you see a patient with an erythematous skin lesion and fever, think of a staphylococcal or streptococcal infection first, of course, and treat the patient appropriately. In this time of increasingly common MRSA infection, remember to obtain material for culture whenever possible (see "Fighting a rising tide of MRSA infection in the young"). But if your patient fails to improve as expected, or if fever is absent, move to clarify the history and consider unusual organisms. That might help you swim toward the correct diagnosis—"alone on a wide wide sea!"
DR. YOUTH is director of the pediatric residency program at The Barbara Bush Children's Hospital at Maine Medical Center, Portland, Maine, where DR. JEWELL is a pediatric hospitalist and DR. MCCARTHY is director of the division of pediatric infectious diseases.