Mycobacterial Diseases: Past, Present and Future
Moderator: Gary W. Procop
Section 3 -
Clinicopathologic classification of Mycobacterium ulcerans disease (Buruli ulcer)
W. M. Meyers, M. Maleombho-Usher, F. M. Abalos, J. Aguiar, R. C. Johnson,
M. Debacker, D. S. Walsh, and F. Portaels
Armed Forces Institute of Pathology, Washington DC, USA;
Centre Sanitaire et Nutritionnel Gbemoten, Benin;
Programme National de Lutte contre l'Ulcère de Buruli, Cotonou, Bénin;
Walter Reed Army Institute of Research, Silver Spring, Maryland, USA,
and Institute of Tropical Medicine, Belgium
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Definition and Etiology
M. ulcerans is a slow-growing environmental mycobacterium
that infects the skin, subcutaneous tissues, and bone, often giving rise to indolent ulcers. After
tuberculosis and leprosy, Buruli ulcer (BU) is the third most common mycobacteriosis. M. ulcerans grows optimally at 32° C and elaborates a necrotizing
immunosuppressive cytotoxin called mycolactone. The etiologic agent was isolated in 1948 in Australia by
MacCallum et al.
Sources of the etiologic agent are closely associated with
tropical wetlands. Most likely, human skin becomes contaminated with M.
ulcerans by contact with wetland environment (mud, water, plants, fish, etc.) or putatively by
insects (waterbugs). Trauma to skin at sites of contamination may introduce the etiologic agent into
skin and/or subcutaneous tissue. Size of inoculum, depth of inoculation and virulence of M. ulcerans strain may influence disease form. For example, superficial
inoculation may provoke strong cell-mediated immunity (CMI), leading to localized disease; deep
inoculation of large numbers of M. ulcerans organisms may lead to early
necrosis, limiting local CMI. Innate host variables may determine type of disease; for example, HLA type
and immunologic conditioning by other environmental mycobacteria.
Spectrum of M. ulcerans Disease
Based on clinicopathologic studies of several thousand M. ulcerans disease patients accessioned at the AFIP, we propose the following
schema for the spectrum of active forms of the disease and their possible natural history:
Clinical features of the forms of M. ulcerans
Nodular: movable, painless but often pruritic, subcutaneous nodule approximately
2-4 cm in diameter.
Minor ulcerative: small, sharply limited ulcer with slight undermining, self-healing.
Major ulcerative: small to large ulcer, wide undermining, well demarcated but
initially with edematous perimeter, with late healing but occasional dissemination.
Contiguous disseminated: small or large edematous indurated plaque that may
Metastatic: spread from initial lesion to distant site, usually in skin and/or bone.
Note: The early pustular form described in Australia has a natural
to major ulcerative forms; however, we have no examples of this type and do not
include it in this classification.
Clinicohistopathologic correlations of forms of M.
ulcerans disease include:
Nodular: coagulation necrosis of lower dermis and subcutaneous tissue, with acid-fast
bacilli (AFB) in center.
Minor ulcerative: ulcer small, AFB in central necrotic slough, with scarring of
Major ulcerative: AFB in necrotic base and adjacent areas, with wide undermining.
Contiguous disseminated: widespread contiguous coagulation necrosis, with AFB in
panniculus and fascia.
Metastatic: typical changes of M. ulcerans disease at distant
skin sites, or M. ulcerans-
Healing (early organization): loosely arranged lymphocytes, epithelioid and giant
AFB scarce or absent.
Healing (late organization): well organized delayed-type hypersensitivity granuloma,
After inoculation into the skin, M. ulcerans proliferates and elaborates
a toxin(s) causing necrosis of dermis, panniculus, and fascia. One well-defined toxin of M. ulcerans is a plasmid related, polyketide-derived macrolide (mycolactone) that
is a virulence factor for Buruli ulcer. M. ulcerans promotes apoptosis in
lesions. Early lesions are closed, but as necrosis spreads, the overlying skin ulcerates, leaving
undermined margins and a necrotic slough in the ulcer base. Histopathologic sections reveal contiguous
coagulation necrosis of the deep dermis and panniculus, with destruction of local nerves, appendages, and
blood vessels. Clumps of extracellular AFB are plentiful and frequently limited to the base of the ulcer
and adjacent necrotic subcutaneous tissue. In early infections and in cellular infiltrates at the
periphery of developed lesions, M. ulcerans may be found intracellularly.
In contiguous disseminated cutaneous lesions, the M. ulcerans organisms tend
to spread along the fascial planes.
The etiologic agent may spread, presumably by lymphatic and hematogenous pathways, to distant foci,
usually to skin and bone. Specific, severe osteomyelitis develops in 10% TO 15% of BU patients.
Hematogenous spread to bones seems to start at the epiphyseal/metaphyseal level. Histologically there
may be local and regional necrotizing lymphadenitis with invasion by M.
ulcerans. In active lesions, inflammatory cells are conspicuously few, perhaps as a result of the
immunosuppression by the toxin. With healing, there are granulomas, and the ulcer is eventually replaced
by a depressed scar.
While information on the topic remains limited, HIV infection may not predispose to M. ulcerans infection, but renders infection with M.
ulcerans more aggressive.
Histopathologic Stages of BU
We propose the following histopathologic classification of stages of M.
ulcerans infection: I, Necrosis; II, Organizing; III, Healing; IV, Combined stages.
In a histopathologic study of biopsy specimens of skin from more than 1000 patients, using
the above staging plan, we obtained the following results in laboratory confirmed lesions (AFB in the
specimen, and PCR or culture positive for M. ulcerans): Necrosis 70%,
organizing 2%, healing 8%, combined (necrosis, organizing and healing) 20%. Ten percent had
Complex grading systems have been proposed for lesions suspected of being M. ulcerans disease, but we prefer a simple 3-tiered system:
- Not M. ulcerans disease
- Consistent with M. ulcerans disease
- M. ulcerans disease
We have noted the following primary diagnoses in specimens submitted to the AFIP with a
clinical impression of, or suspicious for, M. ulcerans disease:
Infections : staphylococcal abscess, tropical phagedenic ulcer, tuberculosis, atypical
mycobacteriosis (other than M. ulcerans), Majocchi's granuloma,
phycomycosis, phaeohyphomycosis, African histoplasmosis, filariasis, leishmaniasis; necrotizing
Malignant: carcinoma of breast, squamous cell carcinoma, basal cell carcinoma,
Kaposi's sarcoma, lymphoma, osteosarcoma; Benign: dermatofibroma, lipofibroma, lipoma,
leiomyoma, angiolipoma, hidradenoma, pilomatricoma;
Miscellaneous: stasis ulcer, calcinosis cutis, epidermal cyst, gouty tophus,
foreign body granuloma.
Specimen Preparation for Histopathologic Diagnosis
Biopsy specimens that include the necrotic base, the undermined edge of lesions,
subcutaneous tissue and fascia are optimal for diagnosis. Specimens from nonulcerated disease are best
obtained from the presumed center of the lesion. Punch specimens are often nonproductive. Specimens
should be approximately 1 cm thick and as long as is convenient. Specimens are best fixed in buffered
10% formalin (4% formaldehyde). If molecular studies are planned, an additional specimen should be fixed
in 70% ethanol.
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