Mycobacterium ulcerans ecological dynamics and its association with freshwater ecosystems and aquatic communities: results from a 12-month environmental survey in Cameroon

Abstract

Background: Mycobacterium ulcerans (MU) is the agent responsible for Buruli Ulcer (BU), an emerging skin disease with dramatic socioeconomic and health outcomes, especially in rural settings. BU emergence and distribution is linked to aquatic ecosystems in tropical and subtropical countries, especially to swampy and flooded areas. Aquatic animal organisms are likely to play a role either as host reservoirs or vectors of the bacilli. However, information on MU ecological dynamics, both in space and time, is dramatically lacking. As a result, the ecology of the disease agent, and consequently its mode of transmission, remains largely unknown, which jeopardizes public health attempts for its control. The objective of this study was to gain insight on MU environmental distribution and colonization of aquatic organisms through time.

Methodology/principal findings: Longitudinal sampling of 32 communities of aquatic macro-invertebrates and vertebrates was conducted from different environments in two BU endemic regions in Cameroon during 12 months. As a result, 238,496 individuals were classified and MU presence was assessed by qPCR in 3,084 sample-pools containing these aquatic organisms. Our study showed a broad distribution of MU in all ecosystems and taxonomic groups, with important regional differences in its occurrence. Colonization dynamics fluctuated along the year, with the highest peaks in August and October. The large variations observed in the colonization dynamics of different taxonomic groups and aquatic ecosystems suggest that the trends shown here are the result of complex ecological processes that need further investigation.

Conclusion/perspectives: This is the largest field study on MU ecology to date, providing the first detailed description of its spatio-temporal dynamics in different aquatic ecosystems within BU endemic regions. We argue that coupling this data with fine-scale epidemiological data through statistical and mathematical models will provide a major step forward in the understanding of MU ecology and mode of transmission.

Figure 1. M. ulcerans spatial distribution in water bodies sampled in Cameroon from June 2012 to Mai 2013.

Maps show regional distribution of M. ulcerans in water bodies sampled in Bankim (Left) and Akonolinga (Right). Each circle is a site and colors represent the type of ecosystem sampled. The size of the circles varies according to the percentage of pools that were qPCR positive to both KR and IS2404. Inlet figures illustrate a map of Cameroon with the location of Yaoundé, the capital city (dark triangle) and locations of Bankim and Akonolinga (dark squares).

Figure 2. Overall distribution of M.ulcerans in aquatic ecosystems.

Bars represent total proportion of M. ulcerans DNA positive sample-pools from each type of ecosystem in Akonolinga (blue) and Bankim (red). Whiskers indicate 95% Confidence intervals for the proportions. Asterisks represent significant differences in positivity between ecosystems within each region (χ² test, p-value<0.0001).

Figure 3. Monthly distribution of M. ulcerans positivity rate in sample-pools from aquatic ecosystems in Akonolinga from June 2012 to May 2013.

Values indicate the proportion of pools of aquatic organisms collected from a specific ecosystem that were positive to M.ulcerans at a given month. The solid line in black represents the total trend (all ecosystems); dashed lines represent trends for pools from each type of ecosystem. Missing information for flooded areas in February and March is due to lack of water in those sites, which prevented sampling.

Figure 4. Seasonal distribution of M. ulcerans positive sample-pools from each type of ecosystem.

Inset figure on top indicates the rainfall patterns in Akonolinga from June 2012 to May 2013 and the cutting of the period sampled into two dry seasons (LD and HD; Rainfall <100 mm) and two rainy seasons (LR and HR; Rainfall >100 mm). Bars indicate the proportion of M. ulcerans positive sample-pools from a given season and ecosystem out of the total number of positive sample-pools from that ecosystem.

Figure 5. Monthly distribution of M. ulcerans positivity rate in pools of aquatic organisms in Akonolinga from June 2012 to May 2013.

Values indicate the proportion of pools of aquatic organisms belonging to a specific taxon that were positive to M.ulcerans at a given month. Only the 5 most abundant taxonomic orders were systematically tested for all sites and months. The positivity dynamics for the rest of sample-pools are grouped as “others”. The solid line in black represents the total trend (all taxonomic groups); dashed lines represent trends for each taxonomic group.