Geographic Distribution, Age Pattern and Sites of Lesions in a Cohort of Buruli Ulcer Patients from the Mapé Basin of Cameroon

Buruli ulcer (BU), a neglected tropical disease of the skin, caused by Mycobacterium ulcerans, occurs most frequently in children in West Africa. Risk factors for BU include proximity to slow flowing water, poor wound care and not wearing protective clothing. Man-made alterations of the environment have been suggested to lead to increased BU incidence. M. ulcerans DNA has been detected in the environment, water bugs and recently also in mosquitoes. Despite these findings, the mode of transmission of BU remains poorly understood and both transmission by insects or direct inoculation from contaminated environment have been suggested. Here, we investigated the BU epidemiology in the Mapé basin of Cameroon where the damming of the Mapé River since 1988 is believed to have increased the incidence of BU. Through a house-by-house survey in spring 2010, which also examined the local population for leprosy and yaws, and continued surveillance thereafter, we identified, till June 2012, altogether 88 RT-PCR positive cases of BU. We found that the age adjusted cumulative incidence of BU was highest in young teenagers and in individuals above the age of 50 and that very young children (<5) were underrepresented among cases. BU lesions clustered around the ankles and at the back of the elbows. This pattern neither matches any of the published mosquito biting site patterns, nor the published distribution of small skin injuries in children, where lesions on the knees are much more frequent. The option of multiple modes of transmission should thus be considered. Analyzing the geographic distribution of cases in the Mapé Dam area revealed a closer association with the Mbam River than with the artificial lake.


Introduction
Background/rationale 2 Explain the scientific background and rationale for the investigation being reported See the INTRODUCTION.

Objectives 3 State specific objectives, including any prespecified hypotheses
The objectives of the present study were i) to conduct an exhaustive survey for BU, yaws and leprosy in the Bankim HD; ii) to continuously monitor the occurrence of BU in the Mapé Dam area; and iii) to examine the age distribution, geographic origin and distribution of lesions of the RT-PCR confirmed cases of BU to underpin future environmental and social science studies.

Study design 4
Present key elements of study design early in the paper In early 2010 (March 22 to April 19), we conducted an exhaustive cross-sectional house-by-house survey for BU, leprosy and yaws in the 88 villages of the Bankim HD ( Figure 1).
Both BU cases identified in the survey and during the continuous case detection where included in the cohort of patients investigated here.
Setting 5 Describe the setting, locations, and relevant dates, including periods of recruitment, exposure, follow-up, and data collection Location: The study was conducted in the Mapé Dam region of Cameroon ( Figure 1) Figure 1).

Variables 7
Clearly define all outcomes, exposures, predictors, potential confounders, and effect modifiers. Give diagnostic criteria, if applicable.

Outcomes:
To better describe BU epidemiology in the Mapé basin we set out to identify the exact geographic origin of all 88 laboratory confirmed cases in our cohort.
Age and gender distribution of cases

Localisation of BU lesions
Diagnostic Criteria: Both clinically confirmed BU cases identified in the survey and during the continuous case detection where included in the cohort of patients investigated here.
To ensure the reliability of our conclusions we focused the remaining analysis only on the 88 RT-PCR confirmed BU cases.
Data sources/ measurement 8* For each variable of interest, give sources of data and details of methods of assessment (measurement). Describe comparability of assessment methods if there is more than one group.

Geographic origin:
Based on information from the patients or their close relatives we were able to determine the exact origin of the BU cases in our cohort.
Age and gender distribution: In addition to demographic and clinical information, the houses where the patients lived for at least a year before disease onset were mapped using a GPS device.

Lesion localisation:
Details of the location of the lesions on the patient's bodies were also collected and documented by photographs.

Bias 9
Describe any efforts to address potential sources of bias In the course of the survey, a total of 48 962 individuals in 9 344 households ( Figure 1) were interviewed.
Overall, our study identified 157 clinically confirmed cases of BU of which 88 (56%) could be confirmed by RT-PCR. These cases were analyzed in detail.

Spatial distribution of BU cases in the Mapé basin:
Based on information from the patients or their close relatives we were able to determine the HD of origin for 86 (98%) of the cases ( Figure S1). For the 62 cases that originated from within the Bankim HD we were also able to determine their HA of origin. Finally, for more detailed spatial analysis, the exact domiciles of 79 (89.8%) of the confirmed BU cases were mapped ( Figure 3B).

Age and gender distribution of cases:
Information from all patients.

Localisation of BU lesions:
Information from all patients.  As shown in Figure 4B, we observed a low age adjusted cumulative incidence rate of BU in individuals aged below four years. The rate then peaked in children aged between four and < 14 years of age, with the 12 to <14 year olds particularly affected (34.4 cases per 10'000 inhabitants). Interestingly, the age adjusted cumulative incidence rate peaks again in the over 50 year olds (27.0 cases per 10'000 inhabitants; Figure 4B). This BU density map shows that most of the cases occur in the southern part of the Bankim HD, particularly along the Mbam River and in the area between the Mapé Dam reservoir and that river. Interestingly, most of the lesions (52.3%) occurred in close proximity to joints with clusters around the ankles (19.2 %) and elbows (15.9 %; Figure 5A, 5B and Table S2).
Limitations 19 Discuss limitations of the study, taking into account sources of potential bias or imprecision. Discuss both direction and magnitude of any potential bias Indeed it is possible that, because of the differences in case finding strategy inside and outside of the Bankim HD, our findings from outside the HD under represent the true degree of BU endemicity in the areas surrounding the Bankim HD. Further studies are therefore needed to investigate BU endemicity in the entire Mapé basin in more detail.
Interpretation 20 Give a cautious overall interpretation of results considering objectives, limitations, multiplicity of analyses, results from similar studies, and other relevant evidence This may indicate that compared to older children the very young children are less exposed to risk factors due to a smaller movement radius away from the house [10]. Based on the lesion distribution data, the option of multiple modes of transmission should be considered.
Generalisability 21 Discuss the generalisability (external validity) of the study results With only few cases living in the immediate proximity of only the Mapé Dam reservoir, our data does not support the suspected direct importance of this man-made lake. This does not exclude that environmental changes associated with the damming of the Mapé River may have had a more indirect effect on the spread of BU in the wider area.

Other information
Funding 22 Give the source of funding and the role of the funders for the present study and, if applicable, for the original study on which the present article is based The work was funded by the Medicor Foundation (http://www.medicor.li/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
*Give information separately for cases and controls in case-control studies and, if applicable, for exposed and unexposed groups in cohort and cross-sectional studies.