Epidemiology of Buruli ulcer in the Mapé Basin of Cameroon

Buruli ulcer (BU) is a neglected tropical disease of the skin and subcutaneous tissue caused by Mycobacterium ulcerans. The disease has been reported from over 30 countries with most cases coming from West Africa. While it is commonly accepted that BU is not acquired by human-to-human transmission both the environmental reservoir of the pathogen and the mode of transmission to humans remain to be identified. Clinically BU presents with a wide range of forms which includes non-ulcerative lesions and ulcers with undermined edges. Much of the pathology of a M. ulcerans infection is believed to be caused by its unique ability to produce the macrolide toxin called mycolactone which causes tissue necrosis and local immunosuppression. BU can be diagnosed by microscopy, polymerase chain reaction (PCR), culturing and histology, however due to lack of access to laboratory facilities, cases are often diagnosed based on clinical symptoms only. Historically, BU was treated using wide scale excision, but since 2004 the WHO recommends the use of streptomycin and rifampicin daily for 8 weeks to treat the infection.
In the framework of this PhD thesis we have established a new BU field research site in the Mapé Basin of Cameroon and studied various aspects of BU epidemiology, differential diagnosis and transmission at this location. As a basis for our research, we conducted an exhaustive house-by-house survey for BU, leprosy and yaws in the Bankim Health District. Following the survey we closely monitored and studied all BU cases detected in the region. By supporting local laboratory diagnosis with real time-PCR (RT-PCR) and culturing, we were able to identify and describe a case of cutaneous tuberculosis which was initially diagnosed as BU. This patient highlighted the importance of further support to improve clinical differential diagnosis of BU in remote endemic areas. Eighty-eight of all the RT-PCR confirmed BU cases identified in the Mapé Basin in the course of our research were studied in detail. By mapping the patients’ homes we were able to describe the distribution of BU in the area and to identify highly endemic communities. Based on the population age data collected in the survey we were also able to compute the age adjusted cumulative incidence rate of BU, revealing that children below the age of five were underrepresented among cases of BU. By analysing serological responses of patients and community members from BU endemic areas in the Mapé Basin and Ghana against an immunodominant antigen of M. ulcerans, we have observed a similar late onset of seroconversion, indicating that BU transmission intensifies when preschool children start moving further away from home. To identify potential sites of M. ulcerans transmission, we screened sites of environmental contact at the homes and farms of laboratory confirmed BU patients for the presence of M. ulcerans DNA. In this analysis we identified three RT-PCR positive permanent village water sites and by studying one of these sites longitudinally in great detail we obtained evidence that M. ulcerans can persist in underwater detritus. This niche of M. ulcerans may represent an environmental reservoir and a source of infection of the pathogen. To further elucidate BU transmission pathways we have generated a set of clinical isolates of M. ulcerans from the Mapé Basin for a phylogeographic analysis of the distribution of the currently circulating haplotypes of M. ulcerans based on whole genome sequencing. By the routine culturing from clinical specimens and the evaluation of different transport media and decontamination methods we were further able to develop an optimized protocol for the primary isolation of M. ulcerans after long term storage of samples.
Our interdisciplinary research approach including biomedical and social science research elements, including future behavioural studies in young children, may eventually help to elucidate transmission and to improve control of BU.