Atypical mycobacterial infection
Atypical mycobacterial infections are infections caused by a species of mycobacterium other than Mycobacterium tuberculosis, the causative bacteria of pulmonary TB and extrapulmonary TB including cutaneous TB.
Atypical mycobacteria may cause many different types of infections such as septic arthritis, abscesses and skin and bone infection. They may also affect the lungs, gastrointestinal tract, lymphatic system and other parts of the body.
Skin infection tends to result in crusted nodules.
What causes atypical mycobacterial infection?
There are many different species of mycobacterium. Those that cause atypical mycobacterial infections include:
- Mycobacterium avium-intracellulare
- Mycobacterium kansasii
- Mycobacterium marinum
- Mycobacterium ulcerans
- Mycobacterium chelonae
Mycobacterium avium-intracellulare and Mycobacterium kansasii primarily cause lung disease similar to pulmonary TB, whilst Mycobacterium marinum, Mycobacterium ulcerans and Mycobacterium chelonae cause skin infections.
What are the clinical features of atypical mycobacterial infection?
The clinical features of atypical mycobacterial infection depend on the infecting mycobacteria.
Image provided by Alastair Hazell
How are atypical mycobacteria diagnosed
Atypical mycobacteria are diagnosed on culture of tissue. Specific conditions are required, so the laboratory must be informed of the clinician's suspicion of this diagnosis. The infections have specific pathological features on skin biopsy.
What is the treatment of atypical mycobacterial infection?
Treatment of atypical mycobacterial infections depends upon the infecting organism and the severity of the infection. In most cases a course of antibiotics is necessary. These include rifampicin, ethambutol, isoniazid, minocycline, ciprofloxacin, clarithromycin, azithromycin and cotrimoxazole. Usually treatment consists of a combination of drugs. Some points to consider when treating atypical mycobacterial infections:
Mycobacterium marinum species are often resistant to isoniazid. Treatment with other antibiotics should be for at least two months.
Mycobacterium kansasii should be treated for at least 18 months.
Mycobacterium chelonae is best treated by clarithromycin in combination with another agent, Sometimes surgical excision is the best approach.
AIDS patients on HIV protease inhibitor drugs cannot be treated with rifampicin because rifampicin significantly increases the breakdown of these drugs. Rifabutin is a suitable alternative. Antibiotics are usually ineffective in treating large skin lesions caused by Mycobacterium ulcerans. Rifampicin may promote healing of pre-ulcerative lesions. Most lesions eventually spontaneously heal after 6-9 months but may leave behind extensive scarring and disfigurement.
Surgical removal of infected lymph nodes and skin lesions is sometimes necessary. In severe cases, skin grafts may be necessary to repair the surgical wound.
- Textbook of Dermatology. Ed Rook A, Wilkinson DS, Ebling FJB, Champion RH, Burton JL. Fourth edition. Blackwell Scientific Publications.
On DermNet NZ:
- Buruli ulcer
- Mycobacterium marinum – pathology
- Cutaneous tuberculosis
- Bacterial infections online course for health professionals
- Mycobacterium ulcerations World Health Organization (WHO)
- Medscape Reference:
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