Working and training in University of Iowa Health Care is an opportunity to witness and treat many conditions that are not likely to appear anywhere else. In his first first-author publication since arriving at the University of Iowa, first-year Infectious Diseases fellow Dr. Takaaki Kobayashi demonstrates this fact.

In the most recent issue of BMJ Case Reports, Dr. Kobayashi documents an infection caused by Mycobacterium marinum, which shares a number of features with other diagnoses and is only detectable via a specific kind of culture. The majority of documented cases of M. marinum infections resulted from exposure to contaminated fish-tank water. The same held true for the case Dr. Kobayashi detailed with Clinical Assistant Professor in Infectious Diseases Dr. Poorani Sekar and Clinical Associate Professor of Pathology Dr. Bradley Ford.

You can read the case report and see their photos of the bacterial-induced rash and its culture here. Dr. Kobayashi took the time to answer a few questions and give a little more detail about the case.

There were a lot of options in the differential, how did you know to have the lab culture for M. marinum?

We initially suspected nocardia because the dermatopathology exam showed organisms that were branching and beaded, but we did not have definitive culture evidence of this organism. Our infectious disease team asked Dr. Ford, the microbiology lab director, if there was concern for other organisms, given that nocardia had not grown on culture and the organism was fully acid-fast on staining and not partially/weakly acid-fast (as nocardia usually is). Later that day Dr. Ford noted growth on AFB culture plates and thought that this might be M. marinum (even before the formal identification was back), since it was a photochromogen (i.e., produces a yellow pigment when exposed to light) and there are limited mycobacterial species that have this unique property.

At what point did you ask the patient if he had fish tanks?

After suspicion for M. marinum came up, we asked about history of exposure to fish tanks which the patient had had, which solidified our diagnosis.

Had you ever seen this before?

I had not seen this before. Dr. Sekar had seen one patient while a trainee with fish-tank exposure, but the patient only had a few local nodules on the hand and nothing to this extent.

What prompted you to submit this for publication?

It is not a common disease, and this is an exaggerated presentation of M. marinum infection. Most patients have solitary nodules or multiple nodules that spread through the lymphatic system (causing a sporotrichoid pattern). It often gets misdiagnosed because of the complex methodology needed to detect this organism. It is important to always ask patients with an unknown rash about animal exposures, including pets that patients may have or had (in the past few months), and also about any other animal exposures (farm/wild animal exposures), since this may give us vital clues as to the diagnosis. If we don’t suspect this disease we don’t ask for specific cultures (AFB cultures at low temperature), and the diagnosis may be missed. We thought internists, including trainees, should be aware that painless nodules on upper limbs warrants suspicion for M. marinum in patients with exposure to fish tanks.