Ecology and Feeding Habits Drive Infection of Water Bugs with Mycobacterium ulcerans

A need for null models in understanding disease transmission: the example of Mycobacterium ulcerans (Buruli ulcer disease)

Understanding the interactions of ecosystems, humans and pathogens is important for disease risk estimation. This is particularly true for neglected and newly emerging diseases where modes and efficiencies of transmission leading to epidemics are not well understood. Using a model for other emerging diseases, the neglected tropical skin disease Buruli ulcer (BU), we systematically review the literature on transmission of the etiologic agent, Mycobacterium ulcerans (MU), within a One Health/EcoHealth framework and against Hill's nine criteria and Koch's postulates for making strong inference in disease systems. Using this strong inference approach, we advocate a null hypothesis for MU transmission and other understudied disease systems. The null should be tested against alternative vector or host roles in pathogen transmission to better inform disease management. We propose a re-evaluation of what is necessary to identify and confirm hosts, reservoirs and vectors associated with environmental pathogen replication, dispersal and transmission; critically review alternative environmental sources of MU that may be important for transmission, including invertebrate and vertebrate species, plants and biofilms on aquatic substrates; and conclude with placing BU within the context of other neglected and emerging infectious diseases with intricate ecological relationships that lead to disease in humans, wildlife and domestic animals.

Understanding the transmission of Mycobacterium ulcerans: A step towards controlling Buruli ulcer

Mycobacterium ulcerans is the causative agent of Buruli ulcer, a rare but chronic debilitating skin and soft tissue disease found predominantly in West Africa and Southeast Australia. While a moderate body of research has examined the distribution of M. ulcerans, the specific route(s) of transmission of this bacterium remain unknown, hindering control efforts. M. ulcerans is considered an environmental pathogen given it is associated with lentic ecosystems and human-to-human spread is negligible. However, the pathogen is also carried by various mammals and invertebrates, which may serve as key reservoirs and mechanical vectors, respectively. Here, we examine and review recent evidence from these endemic regions on potential transmission pathways, noting differences in findings between Africa and Australia, and summarising the risk and protective factors associated with Buruli ulcer transmission. We also discuss evidence suggesting that environmental disturbance and human population changes precede outbreaks. We note five key research priorities, including adoption of One Health frameworks, to resolve transmission pathways and inform control strategies to reduce the spread of Buruli ulcer.

Buruli ulcer is a debilitating skin and soft tissue disease characterised by large ulcerative wounds that are treated with antibiotics or with adjunctive surgery for advanced cases. Found predominantly in West Africa and Southeast Australia, the causative agent is the environmental bacterial pathogen Mycobacterium ulcerans. Lack of understanding of transmission pathways, combined with the absence of a vaccine, has hindered efforts to control the spread of M. ulcerans. Here, in order to identify probable transmission pathways and inform future studies, we review literature linking M. ulcerans to environmental reservoirs, mammalian hosts, and potential invertebrate vectors. We also summarise factors and behaviours that reduce the risk of developing Buruli ulcer, to inform effective prevention strategies and further shed light on transmission pathways.

Identification of blood-feeding sources in Panstrongylus, Psammolestes, Rhodnius and Triatoma using amplicon-based next-generation sequencing

BACKGROUND

Triatomines are hematophagous insects that play an important role as vectors of Trypanosoma cruzi, the causative agent of Chagas disease. These insects have adapted to multiple blood-feeding sources that can affect relevant aspects of their life-cycle and interactions, thereby influencing parasitic transmission dynamics. We conducted a characterization of the feeding sources of individuals from the primary circulating triatomine genera in Colombia using amplicon-based next-generation sequencing (NGS).

METHODS

We used 42 triatomines collected in different departments of Colombia. DNA was extracted from the gut. The presence of T. cruzi was identified using real-time PCR, and discrete typing units (DTUs) were determined by conventional PCR. For blood-feeding source identification, PCR products of the vertebrate 12S rRNA gene were obtained and sequenced by next-generation sequencing (NGS). Blood-meal sources were inferred using blastn against a curated reference dataset containing the 12S rRNA sequences belonging to vertebrates with a distribution in South America that represent a potential feeding source for triatomine bugs. Mean and median comparison tests were performed to evaluate differences in triatomine blood-feeding sources, infection state, and geographical regions. Lastly, the inverse Simpson's diversity index was calculated.

RESULTS

The overall frequency of T. cruzi infection was 83.3%. TcI was found as the most predominant DTU (65.7%). A total of 67 feeding sources were detected from the analyses of approximately 7 million reads. The predominant feeding source found was Homo sapiens (76.8%), followed by birds (10.5%), artiodactyls (4.4%), and non-human primates (3.9%). There were differences among numerous feeding sources of triatomines of different species. The diversity of feeding sources also differed depending on the presence of T. cruzi.

CONCLUSIONS

To the best of our knowledge, this is the first study to employ amplicon-based NGS of the 12S rRNA gene to depict blood-feeding sources of multiple triatomine species collected in different regions of Colombia. Our findings report a striking read diversity that has not been reported previously. This is a powerful approach to unravel transmission dynamics at microgeographical levels.

Aquatic Hemiptera in Southwest Cameroon: Biodiversity of Potential Reservoirs of Mycobacterium ulcerans and Multiple Wolbachia Sequence Types Revealed by Metagenomics

Buruli ulcer (BU), caused by Mycobacterium ulcerans, is a neglected tropical disease associated with freshwater habitats. A variety of limnic organisms harbor this pathogen, including aquatic bugs (Hemiptera: Heteroptera), which have been hypothesized to be epidemiologically important reservoirs. Aquatic Hemiptera exhibit high levels of diversity in the tropics, but species identification remains challenging. In this study, we collected aquatic bugs from emerging foci of BU in the Southwest Region of Cameroon, which were identified using morphological and molecular methods. The bugs were screened for mycobacterial DNA and a selection of 20 mycobacteria-positive specimens from the families Gerridae and Veliidae were subjected to next-generation sequencing. Only one individual revealed putative M. ulcerans DNA, but all specimens contained sequences from the widespread alpha-proteobacterial symbiont, Wolbachia. Phylogenetic analysis placed the Wolbachia sequences into supergroups A, B, and F. Circularized mitogenomes were obtained for seven gerrids and two veliids, the first from these families for the African continent. This study suggests that aquatic Hemiptera may have a minor role (if any) in the spread of BU in Southwest Cameroon. Our metagenomic analysis provides new insights into the incursion of Wolbachia into aquatic environments and generated valuable resources to aid molecular taxonomic studies of aquatic Hemiptera.

Modelling the spatial distribution of aquatic insects (Order Hemiptera) potentially involved in the transmission of Mycobacterium ulcerans in Africa

Background

Biting aquatic insects belonging to the order Hemiptera have been suggested as potential vectors of Mycobacterium ulcerans in endemic areas for Buruli ulcer (BU). If this is the case, these insects would be expected to co-exist with M. ulcerans in the same geographical areas. Here, we studied the geographical distribution of six aquatic Hemiptera families that are thought to be vectors of M. ulcerans and explored their potential geographical overlapping with communities reporting BU cases in endemic countries.

Methods

We have developed ensemble ecological models of predicted distribution for six families of the Hemiptera (Naucoridae, Belostomatidae, Notonectidae, Nepidae, Corixidae and Gerridae) applying a robust modelling framework over a collection of recorded presences and a suite of environmental and topographical factors. Ecological niche factor analysis (ENFA) was first used to identify factors that best described the ecological niches for each hemipteran family. Finally, we explored the potential geographical co-occurrence of these insects and BU in two endemic countries, Cameroon and Ghana.

Results

Species of the families Naucoridae and Belostomatidae, according to our models, are widely distributed across Africa, although absent from drier and hotter areas. The other two families of biting Hemiptera, the Notonectidae and Nepidae, would have a more restricted distribution, being more predominant in western and southern Africa. All these four families of biting water bugs are widely distributed across coastal areas of West Africa. They would thrive in areas where annual mean temperature varies between 15–22 °C, with moderate annual precipitation (i.e. 350–1000 mm/annual) and near to water courses. Species of all hemipteran families show preference for human-made environments such as agricultural landscapes and urbanized areas. Finally, our analysis suggests that M. ulcerans and species of these aquatic insects might coexist in the same ecological niches, although there would be variation in species diversity between BU endemic areas.

Conclusions

Our findings predict the geographical co-existence of some species of aquatic hemipteran families and BU. Considering the existing biological evidence that points to some of these aquatic insects as potential phoretic vectors of M. ulcerans, its presence in BU endemic areas should be considered a risk factor. The ecological models here presented may be helpful to inform future environmental based models intended to delineate the potential geographical distribution of BU in the African region.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3066-3) contains supplementary material, which is available to authorized users.

A protocol for culturing environmental strains of the Buruli ulcer agent, Mycobacterium ulcerans

Contaminations and fastidiousness of M. ulcerans may have both hamper isolation of strains from environmental sources. We aimed to optimize decontamination and culture of environmental samples to circumvent both limitations. Three strains of M. ulcerans cultured onto Middlebrook 7H10 at 30 °C for 20 days yielded a significantly higher number of colonies in micro-aerophilic atmosphere compared to ambient atmosphere, 5% CO₂ and anaerobic atmosphere. In a second step, we observed that M. ulcerans genome uniquely encoded chitinase, fucosidase and A-D-GlcNAc-diphosphoryl polyprenol A-3-L-rhamnosyl transferase giving M. ulcerans the potential to metabolize chitine, fucose and N-acetyl galactosamine (NAG), respectively. A significant growth-promoting effect of 0.2 mg/mL chitin (p < 0.05), 0.01 mg/mL N-acetyl galactosamine (p < 0.05), 0.01 mg/mL fucose (p < 0.05) was observed with M. ulcerans indicating that NAG alone or combined with fucose and chitin could complement Middlebrook 7H10. Finally, the protocol combining 1% chlorhexidine decontamination with micro-aerophilic incubation on Middlebrook 7H10 medium containing chitin (0.2%), NAG (0.01%) and fucose (0.01%) medium and auto-fluorescence detection of colonies allowed for the isolation of one mycolactone-encoding strain from Thryonomys swinderianus (aulacode) feces specimens collected near the Kossou Dam, Côte d'Ivoire. We propose that incubation of chlorhexidine-decontaminated environmental specimens on Middlebrook 7H10-enriched medium under micro-aerophilic atmosphere at 30 °C may be used for the tentative isolation of M. ulcerans strains from potential environmental sources.