Mycobacterium ulcerans, a slow-growing acid-fast bacillus (AFB), arose from Mycobacterium marinum and acquired a virulence plasmid coding for mycolactone, a necrotizing, apoptotic, immunosuppressive toxin that mediates pathogenesis. BU is characterized clinically by indolent necrotizing skin and subcutaneous and sometimes bone lesions, often with severe residual scarring. It is an important public health issue.
The incidence of BU is highest in West Africa, particularly Benin, Nigeria, Ivory Coast, and Ghana, although increased severity and spread of M ulcerans have been reported in Southeastern Australia between 1998 and 2017. Far fewer cases have been reported in China, Japan, Mexico, Papua, and in some South American countries such as French Guiana, Brazil, and Peru. In industrialized countries, the disease is occasionally imported. A history of travel to endemic regions, especially West Africa, is important. Geographically, multiple strains of M ulcerans exhibit variable pathogenicity and immunogenicity.
In endemic regions, exposure to stagnant water, especially swampy areas, may be a predisposing factor. The mode of transmission in humans remains unknown. In Australia, BU is considered a zoonosis transmitted by mosquitoes from possums to humans.
Somewhat similar to tuberculosis, exposure of cutaneous tissues to M ulcerans may lead to 1 of 3 outcomes:
- Clearance of the infection,
- Clinical disease relatively soon after infection (primary BU), or
- Subclinical or asymptomatic infection (latent BU) that may subsequently reactivate and produce disease.
After inoculation of the skin, followed in general by a 2-3 month incubation period, the extracellular bacilli proliferate and release mycolactone, which causes apoptosis, necrosis, and local immunosuppression.
BU typically presents as one or more painless solitary lesions. In Africa, about 13% of patients develop bone disease (osteitis, osteomyelitis), either subjacent or distant to a skin lesion. Distant lesions are probably caused by lymphohematogenous spread of M ulcerans.
Because M ulcerans is an environmental organism, it is generally accepted that person-to-person spread is uncommon or does not occur. In limited disease, there are few systemic manifestations. Lymphadenopathy and fever are rare unless secondary bacterial infection develops. Small ulcers may either self-resolve or progress to chronic, nonhealing lesions over a period of months.
The Centers for Disease Control and Prevention (CDC) provides useful information about BU, as does the World Health Organization (WHO). A confirmed or highly suspected case in a new region should be reported to WHO.
Related diagnosis: Cutaneous tuberculosis