Derm Cases · Dermatology / Infectious disease / Soft-tissue infection
OSCE — fish-tank nodules and post-procedure abscesses: nontuberculous mycobacteria
An 8-minute OSCE station on recognising M. marinum fish-tank granuloma with sporotrichoid spread, culture at 30°C, combination therapy, rapid-grower post-procedure infections, Buruli ulcer clues, and why standard RHZE is wrong for most NTM.
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NEET-PGINICETUSMLEPLABMRCP
Prompt
An 8-minute OSCE station on recognising M. marinum fish-tank granuloma with sporotrichoid spread, culture at 30°C, combination therapy, rapid-grower post-procedure infections, Buruli ulcer clues, and why standard RHZE is wrong for most NTM.
Brief (to candidate)
A 35-year-old aquarium hobbyist has a chronic finger nodule with sporotrichoid ascending lesions after a minor cut cleaning a fish tank. A second patient has multiple abscesses weeks after a cosmetic injection. You have 8 minutes to diagnose NTM skin infection, order the right cultures, and start species-aware dual therapy — not RHZE alone.
[4]Candidate instructions
- Suspect M. marinum with aquatic exposure + sporotrichoid spread.
- Request biopsy with AFB stain, culture at 30°C and 37°C, and PCR/speciation.
- Treat marinum-type disease with ≥2 drugs (e.g. clarithromycin + ethambutol) for months.
- Recognise rapid growers (M. abscessus, fortuitum, chelonae) after procedures.
- State that standard anti-TB RHZE is usually wrong for NTM.
Examiner checklist (mark each domain / 10)
| Domain | Key actions expected |
|---|---|
| M. marinum | Fish-tank/swimming-pool exposure; hand/arm nodules; sporotrichoid lymphocutaneous spread; optimal growth ~30–32°C (37°C culture may be false-negative)[1][3] |
| Work-up | Tissue for AFB, mycobacterial culture at both temperatures, PCR/16S/speciation; do not rely on standard bacterial culture alone[1][2] |
| Therapy principles | Never monotherapy (resistance); base regimen on species + susceptibilities; M. marinum: commonly clarithromycin + ethambutol for ~3–4 months (± alternatives); surgery for deep tenosynovitis/osteomyelitis[4][3] |
| Rapid growers | Post-procedure/tattoo/injection nodules-abscesses weeks later — M. abscessus complex etc.; prolonged multi-drug regimens; abscessus often hard (erm41 macrolide issues)[6][2] |
| Buruli clue | Painless undermined ulcer in endemic areas (M. ulcerans, mycolactone) — different from marinum; WHO-style rifampicin + clarithromycin frameworks + surgery as indicated |
| Not RHZE | Most cutaneous NTM do not respond to standard TB RHZE; misdiagnosis delays cure[1][2] |
| Communication | Expect slow response; avoid further procedures with non-sterile water exposure counselling |
Model key actions
- Diagnose fish-tank granuloma and culture at 30°C.[3]
- Start dual therapy (clarithromycin + ethambutol framework) after/while confirming speciation.[4]
- Do not treat as ordinary TB with RHZE alone.[2]
Common errors
- Bacterial antibiotics only for chronic sporotrichoid hand nodules.
- Mycobacterial culture only at 37°C (miss marinum).
- Single-drug macrolide monotherapy.
- Automatic RHZE “because AFB positive.”
- Missing iatrogenic rapid-grower NTM after cosmetic procedures.
References5ShowHide
- [1]Gonzalez-Santiago TM, Drage LA. Nontuberculous Mycobacteria: Skin and Soft Tissue Infections. Dermatologic Clinics, 2015.PMID 26143432
- [2]Franco-Paredes C, Marcos LA, Henao-Martínez AF, et al. Cutaneous Mycobacterial Infections. Clinical microbiology reviews, 2018.PMID 30429139
- [3]Aubry A, Mougari F, Reibel F, et al. Mycobacterium marinum. Microbiology spectrum, 2017.PMID 28387180
- [4]Medel-Plaza M, Esteban J. Current treatment options for Mycobacterium marinum cutaneous infections. Expert opinion on pharmacotherapy, 2023.PMID 37145964
- [6]Sepulcri C, Vena A, Bassetti M. Skin and soft tissue infections due to rapidly growing mycobacteria. Current Opinion in Infectious Diseases, 2023.PMID 36718980