Buruli ulcer (BU) caused by Mycobacterium ulcerans is considered to be the third most common mycobacterial infection after tuberculosis and leprosy. Since it is one of the most devastating neglected infectious diseases, the World Health Assembly adopted in May 2004 a resolution on BU calling for increased surveillance and control and for intensified research to develop tools for diagnosis, treatment and prevention of BU.

Distribution
The disease has been reported in more than 30 countries worldwide, but children living in rural communities in sub-Saharan Africa are affected the worst. BU is a chronic necrotizing skin disease mainly affecting subcutaneous and adipose tissue. The unique pathology of BU is primarily attributed to a plasmid-encoded macrolide toxin, mycolactone, which has cytopathic and apoptotic activity. A histopathological hallmark of progressing BU is poor inflammatory response despite the presence of clusters of extracellular acid-fast bacilli surrounded by areas of necrosis.

Clinical Findings and Treatment
Clinical lesions usually start as painless subcutaneous nodules that may develop into plaques or oedema. If left untreated, extensive ulcerations with typical undermined edges of the dermis develop. Until recently, surgery has been the only WHO recommended treatment for BU. Wide excision margins reaching into the healthy tissue are necessary to prevent recurrences and often subsequent skin grafting is required. In most endemic areas access to surgery is very limited for the majority of BU patients. Moreover, the costs for treatment and prolonged hospital stays are often prohibitive. Recently, WHO published provisional guidelines recommending treatment with a combination of rifampicin and streptomycin for eight weeks. More than 50% of BU cases are children under the age of 15 years. Potential long-term side effects of streptomycin in this population limit the duration of the antibiotic treatment. While no antibiotic therapy has been formally proven effective in BU, there is evidence that the treatment with rifampicin and streptomycin reduces recurrence rates and helps to avoid surgery or at least limits its extent. Since both rifampicin and streptomycin are front line drugs for tuberculosis treatment, potential development of antibiotic resistance in both M. ulcerans and M. tuberculosis due to the short course treatment is a concern.

Research on BU at STI
Driven by demands of colleagues from BU endemic regions of Africa, the Molecular Immunology unit started to work on BU in 2000. In accordance with the priorities identified by the WHO Technical Advisory Group for BU the goals of our research are to (i) improve understanding of the transmission of M. ulcerans; (ii) develop methods for early diagnosis; and (iii) investigate prospects for improvement of therapy and vaccine development. We have established strong research partnerships on BU with colleagues from Ghana and from Cameroon, that allows us to carry newly developed treatment options and laboratory research tools that we are developing into the field.

Genetic Diversity
BU often occurs in focalised areas close to stagnant or slow-moving waters. The mode of transmission is not fully understood, partly because no molecular typing method has sufficiently high resolution for micro-epidemiological analyses. Standard molecular typing methods have revealed an apparent lack of genetic diversity in M. ulcerans within individual geographical regions, indicating a clonal population structure. We are developing molecular-biological tools which should allow us to differentiate between closely related M. ulcerans isolates coming from the same area and to map the spread of genetic variants in time and space. Our comparative genomic analysis of 30 M. ulcerans clinical isolates of diverse geographic origin with a novel plasmid-based microarray technology revealed extensive large sequence polymorphisms. The identified transposable element-associated insertional/deletional (InDel) recombination events are indicative for progressing genome shrinking in M. ulcerans, which has emerged from the environmental mycobacterium M. marinum by acquisition of a large virulence plasmid. Categorization of the deleted genes according to their biological functions indicates that M. ulcerans is adapting to a more stable environment. Analysis of the large InDel polymorphism allowed us to distinguish between two distinct lineages: (i) the “classical” lineage representing the most pathogenic genotypes – those that come from Africa, Australia and South East Asia; and (ii) an “ancestral” lineage comprising strains from China/Japan, South America and Mexico. Results indicate that M. ulcerans has passed through at least two major evolutionary bottlenecks since divergence from M. marinum. The classical lineage shows more pronounced reductive evolution than the ancestral lineage, suggesting that there may be differences in the ecology between the two lineages. In a different genetic fingerprinting approach, variable number of tandem repeats typing based on newly identified polymorphic loci has enabled us to differentiate for the first time between genotypes in an African country. These results suggest the emergence and spreading of new genetic variants of M. ulcerans within Ghana.

Perspectives
More research is needed to develop evidence-based differentiated recommendations how to combine antibiotic and surgical therapy. Our pilot studies of lesions from antibiotic-treated BU patients provide evidence for initiation of vigorous immune responses. Results indicate that the relatively short antibiotic treatment reverses local immunosuppression and that the curative effect may be sustained by immune defence mechanisms. Three different general types of infiltration, diffuse mixed infiltrates, granulomas and dense lymphocyte aggregation in the vicinity of vessels were observed. Mycobacterial material was primarily located inside phagocytes. Some patients appear to show no adequate clinical response to the antibiotic treatment. This has raised the question, whether lack of response is due to antibiotic resistance of certain strains of M. ulcerans, inherited or acquired host factors or merely to lack of compliance to the treatment. On the other hand we have also growing evidence, that some patients develop signs of immunopathology after antibiotic treatment. In close collaboration with the WHO and treatment centers in West-Africa this is currently investigated further.
As an alternative to chemotherapy, we have started to explore in collaboration with Dr. Junghanss (University of Heidelberg, Germany) and Dr. UmBoock (Aide aux Lépreux Emmaüs-Suisse, Cameroon), whether thermotherapy is an alternative. An ongoing first pilot trial in Cameroon with an innovative heat application device is showing very promising results, encouraging us to pursue optimisation of this treatment option.

Prof. Dr. Gerd Pluschke
contact

back to Mainpage

 

photo left: surgical treatment of a BU lesion
photo in the middle: Nyong basin in Cameroon, a BU endemic region
photo right: Sand digging and collection of drinking water from the river Nyong

 

please click to enlarge