Atypical mycobacterial infection

Author: Vanessa Ngan, staff writer. Reviewed and updated by Dr Amanda Oakley Dermatologist, Hamilton, New Zealand; and Vanessa Ngan, Staff Writer; June 2014.

What is 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; and Mycobacterium leprae, the cause of leprosy.

Atypical mycobacteria may cause many different types of infections, which are divided into the following four clinical syndromes:

Skin infection tends to result in crusted nodules and plaques. Abscesses may develop in skin and bone infection.

What causes atypical mycobacterial infection?

There are many different species of mycobacterium. To date at least 30 species of mycobacteria that do not cause tuberculosis or leprosy have been identified. Some of those causing atypical mycobacterial infections include:

Mycobacterium avium-intracellulare and Mycobacterium kansasii primarily cause lung disease similar to pulmonary TB, whilst Mycobacterium marinum, Mycobacterium ulcerans, Mycobacterium fortuitum 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.

MycobacteriaClinical features
Mycobacterium avium-intracellulare
  • Also known as MAC (Mycobacterium avium complex)
  • Most common non-tuberculous mycobacterial infection associated with AIDS
  • Symptoms include fever, swollen lymph nodes, diarrhoea, fatigue, weight loss and shortness of breath
  • May develop into pulmonary MAC
  • Skin lesions are uncommon
Mycobacterium kansasii
  • May cause a chronic infection of the lungs similar to pulmonary TB
  • Second most common non-tuberculous mycobacterial infection associated with AIDS
  • Symptoms include fever, swollen lymph nodes and lung crackles and wheezing
  • Skin lesions may occur either alone or as part of a more widespread disease
Mycobacterium marinum
  • Also known as fishtank granulomas
  • Uncommon infection that occurs most often in people with recreational or occupational exposure to contaminated freshwater or saltwater
  • Usually a single lump or pustule that breaks down to form a crusty sore or abscess
  • Other lumps may occur around the initial lesion, particularly along the lines of lymphatic drainage (sporotrichoid forms)
  • Most often affects elbows, knees, feet, knuckles or fingers
  • Multiple lesions and widespread disease may occur in immunocompromised patients
  • Rarely causes red, swollen and tender joints
Mycobacterium ulcerans
  • Also known as Buruli ulcer
  • Infection most common in Central and West Africa around areas of lush vegetation and swamps but may also occur in Australia
  • Solitary, painless and sometimes itchy nodule of 1–2 cm develops about 7-14 days after infection through broken skin
  • Over one to two months the nodule may break down to form a shallow ulcer that spreads rapidly and may involve up to 15% of the patient's skin surface
  • Severe infections may destroy blood vessels, nerves, and invade bone
Mycobacterium chelonae
  • Worldwide distribution: found in tap water and other water sources
  • May cause lung disease, joint infection, eye disease and other organ infections
  • May result in non-healing wound, subcutaneous nodule or abscess
  • Immunosuppression may cause disseminated lesions throughout the body
Mycobacterium abscessus
  • Subspecies of M. chelonae
  • Found in water, soil, dust, animals
  • Rarely causes illness in humans
  • Can cause skin infection after puncture wounds
  • May cause lung infection and disseminated infection in immunosuppressed people
Mycobacterium fortuitum
  • Worldwide distribution: found in natural and processed water sources, sewage and dirt
  • Local cutaneous disease, osteomyelitis, joint infections and ocular disease may occur after trauma
  • Severe immunosuppression, especially AIDS, may cause disseminated skin and soft tissue lesions
  • Often the cause of wound and surgical-site infections from contaminated water sources
  • Causes a non-healing ulcerative skin lesion and/or subcutaneous nodules

More images of atypical mycobacterial infections ...

How is atypical mycobacterial infection diagnosed?

Atypical mycobacteria are diagnosed on culture of tissue. Specific conditions are required, such as cool temperature, so the laboratory must be informed of the clinician's suspicion of this diagnosis. The infections have specific pathological features on skin biopsy.

Other diagnostic tools used include radiographic imaging studies and more recently, polymerase chain reaction (PCR) testing on swabs of ulcers or tissue biopsies.

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.

Consider the following points when treating atypical mycobacterial infections with antibiotics:

Surgical removal of infected lymph nodes and aggressive debridement of infected skin lesions is sometimes necessary. In severe cases, skin grafts may be necessary to repair the surgical wound.

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