62
Mycobacterial Diseases
Key Points
• Mycobacteria are the etiologic agents of three major types of infection:
– Leprosy – Mycobacterium leprae.
– Tuberculosis – Mycobacterium tuberculosis.
– Atypical or nontuberculous infections – e.g. Mycobacterium marinum, Mycobacterium chelonae (Tables 62.1 and 62.2).
Table 62.1
Mycobacteria that cause cutaneous disease.
| Group and Pigment | Rate of Growth | Examples of Pathogens |
| Slow growers | ||
| Photochromogens* | 2–3 weeks | M. kansasii, M. marinum, M. simiae |
| Scotochromogens† | 2–3 weeks | M. scrofulaceum, M. szulgai, M. gordonae, M. xenopi |
| Nonchromogens‡ | 2–3 weeks | M. tuberculosis, M. avium, M. intracellulare, M. ulcerans, M. haemophilum, M. bovis§ |
| Rapid growers | 3–5 days | M. fortuitum, M. chelonae, M. abscessus |
| Noncultured (to date) | M. leprae |
* Capable of yellow pigment formation upon exposure to light.
† Capable of yellow pigment production without light exposure.
‡ Incapable of pigment production.
§ Including bacillus Calmette–Guérin.
Modified classification of Runyon from Hautmann G, Lotti T. Atypical mycobacterial infections of the skin. Dermatol. Clin. 1994;12:657–668; Yates VM, Rook GAW. Mycobacterial infections. In: Burns T, Breathnach S, Cox N, Griffiths C (eds). Rook’s Textbook of Dermatology, 7 edn. London: Blackwell Science, 2004;28.1–39; Neves RG, Pradinaud R. Micobacterioses atípicas. In: Neves RG, Talhari S (eds). Dermatologia Tropical. Rio de Janeiro: Medsi, 1995:283–290.
Table 62.2
Important features of atypical mycobacteria.
Disseminated skin lesions are an HIV-defining criterion, most commonly due to M. avium intracellulare or M. kansasii.
Leprosy
• Slowly progressive disease characterized by granuloma formation in nerves and the skin.
• Affects all ages, but bimodal peak distribution – ages 10–15 years and 30–60 years.
• Currently, the highest incidence is in Brazil (Fig. 62.1).
Fig. 62.1 Leprosy prevalence rates, 2011. The World Health Organization (WHO) has achieved its goal of a prevalence rate of less than 1 case per 10 000 persons in all but a few countries. Reproduced from the World Health Organization, http://www.who.int/lep/situation/prevalence/en.
• Incubation period averages 4–10 years.
• Degree of immunity is reflected in clinical findings (Table 62.3; Figs. 62.2 and 62.3) and histopathologic features; the latter range from macrophages containing numerous bacilli (globi) in lepromatous leprosy to granulomas without organisms in tuberculoid leprosy.
Table 62.3
Classification of leprosy: Ridley–Jopling and operational.
Adapted from A Guide to Leprosy Control, 2nd ed. Geneva: World Health Organization, 1988;27–28.
Fig. 62.2 Tuberculoid leprosy. Note the elevated border and central hypopigmentation. Courtesy, Robert Hartman, MD.
Fig. 62.3 Lepromatous leprosy. A Note the diffuse infiltration with leonine facies and madarosis. B Numerous nodules and thick plaques on the forearms. A, B, Courtesy, Louis A. Fragola, Jr., MD.
• Nerves are often affected, particularly ones close to the surface of the skin (Figs. 62.4 and 62.5); sensations of pain, temperature, and/or touch should be evaluated within skin lesions.
Fig. 62.4 Nerve examination sites.
Fig. 62.5 Leprosy. Enlargement of the radial nerve with a circumscribed area of hypopigmentation. Courtesy, Regional Dermatology Training Centre, Moshi, Tanzania.
• Sequelae of leprosy can be disfiguring (Figs. 62.6 and 62.7).
