Mycobacterial Diseases

Key Points

• Mycobacteria are the etiologic agents of three major types of infection:

– 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.

• Spread of leprosy is dependent on: (1) a contagious person (predominantly through nasal/oral droplets); (2) a susceptible person; and (3) close/intimate contact.

• 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.