Rodents as Natural Hosts of Zoonotic Schistosoma Species and Hybrids: An Epidemiological and Evolutionary Perspective From West Africa

Abstract

The complex multi-host disease dynamics of schistosomiasis and Schistosoma spp., including the emergence of zoonotic parasite hybrids, remain largely unexplored in West Africa. We elucidated the role of wild small mammals as reservoir for zoonotic Schistosoma species and hybrids in endemic areas of Senegal. We identified Schistosoma mansoni, Schistosoma bovis, and a Schistosoma haematobium/S. bovis hybrid, with local prevalence in wild rodents ranging from 1.9% to 28.6%. Our findings indicate that rodents may be an important local reservoir for zoonotic schistosomiasis in endemic areas of West Africa, amplifying transmission to humans and acting as natural definitive hosts of schistosome hybrids.

(See the Editorial commentary by Colley and Loker, on pages 344–6.)

Schistosomiasis is a neglected tropical disease caused by dioecious trematodes of the genus Schistosoma. The spatial distribution of schistosomiasis is highly focal, with transmission restricted to freshwater bodies in which specific snails act as intermediate hosts. Globally, approximately 250 million people are infected, with the highest burden in sub-Saharan Africa. Schistosomiasis is intimately associated with poor and rural communities where access to clean water is limited. Guided by the World Health Assembly resolution 65.21, initiatives to control and eliminate schistosomiasis as a public health issue have been recently catalyzed [1].

Schistosoma haematobium, responsible for human urogenital schistosomiasis, and Schistosoma mansoni, an agent of human intestinal schistosomiasis, are both endemic across sub-Saharan Africa. Although S. haematobium is essentially specific to humans, S. mansoni can naturally infect rodents, nonhuman primates, and other animal hosts [2, 3]. In West Africa, small mammals as potential sources of zoonotic schistosomiasis have been investigated only once during a major human epidemic of S. mansoni across the Senegal River Basin in the early 1990s. During that period, Nile rats (Arvicanthis niloticus) and Hubert’s multimammate mice (Mastomys huberti) were identified as suitable alternative definitive hosts of the circulating S. mansoni [4]. This outbreak of intestinal schistosomiasis followed dramatic anthropogenic land-use changes driven by the completion of the Diama dam in 1985, which generated a rapid agroindustrial growth as a consequence of the guaranteed fresh water supply [5]. Recently, although human intestinal schistosomiasis has remained endemic in the Senegal River Basin (baseline prevalence 1.8%–68.9%), the foci of urogenital schistosomiasis have expanded throughout the region (baseline prevalence 58.3%–100%) (our unpublished data).

Alterations of the natural landscape may have also favored the progressive loss of ecological barriers and the creation of permanent freshwater bodies (ie, perennial sources of schistosomiasis transmission), supporting the co-occurrence and interspecific interactions between S. haematobium, S. mansoni, and other Schistosoma spp. of veterinary importance [3, 6]. Advances in the application of molecular methods have revealed hybridization and introgression (see [3] for definitions) between S. haematobium and animal schistosomes in Senegal and other endemic countries. To date, humans are reported as the only definitive hosts of hybrids between S. haematobium with other Schistosoma spp. [6]. In the Senegal River Basin, the open and permanent water bodies and irrigation canals promote the interplay between human and animal schistosomes and the transmission of parasite hybrids in the area [7, 8]. The outcome is an entangled epidemiology in which agents of human schistosomiasis can exploit a variety of animals as reservoirs and, at the same time, hybridize with animal Schistosoma spp., causing spillover from zoonotic transmission.

Our aim was to elucidate the role of wild small mammals as reservoir hosts of zoonotic schistosome species and/or hybrids in a West African setting subject to dramatic anthropogenic change. We applied a multi-locus molecular approach to test the hypotheses of small mammal populations as follows: (1) alternative definitive hosts and amplifiers of S. haematobium and S. mansoni transmission to humans; and (2) biotic hubs where hybridization between human and animal Schistosoma spp. occurs, generating zoonotic hybrid parasites.

METHODS

Study Area and Sampling of Small Mammals

Between May 2016 and April 2017, we sampled 9 localities in the Senegal River Basin, before and after the rainy season (June–September). The study sites were previously identified as transmission foci for Schistosoma spp. (our unpublished data), and we considered them as independent replicates given that the shortest distance between 2 sites was >3 km, above the average home range recorded for local rodent species [9]. Small mammals were captured using locally made wire-mesh live traps (26 × 10 × 10 cm) baited with peanut butter and placed adjacent to water bodies in crop fields near human dwellings and/or riparian habitat primarily represented by thick reeds (Typha sp.). Each evening, lines of 10–20 traps at regular intervals (5–10 m, depending on the shape and size of the site) were set over 2–3 nights per site. Each morning, the traps were inspected and the number of traps active throughout the night was recorded, considering as misfire any trap found sprung, missing, or not triggered. By-catch (ie, the capture of non-target species) was recorded then immediately released at the point of capture.

Laboratory Analyses

The trapped small mammals were returned live to the laboratory and humanely euthanized (see Supplementary Data). At post mortem, we recorded species, gender, sexual maturity, body weight, and length of each individual. Age classification of rodents as juveniles or adults was based on anatomical and sexual traits [9]. Organs from each small mammal were examined, and the isolated Schistosoma spp. were stored in separate vials containing 95% ethanol at −20 °C. For hosts harboring adult schistosomes, liver and large intestine were dissected and macerated by passing through a 300 µm metal sieve in water for the hatching of miracidia larval stage and their collection onto Whatman FTA indicator cards [10].

Deoxyribonucleic acid (DNA) from individual adult schistosomes was extracted using the QIAGEN DNeasy Blood & Tissue Kit (QIAGEN, Hilden, Germany) following the manufacturer’s instructions. Deoxyribonucleic acid from individual miracidia stored on Whatman FTA cards was extracted as described [10]. Deoxyribonucleic acid extracts were amplified and sequenced for the complete internal transcribed spacer (ITS) region of the nuclear ribosomal DNA using the primers ETTS1 and ETTS2, and for a segment of the cytochrome c oxidase subunit 1 (COI) gene of the mitochondrial DNA using the primers Cox1_Schist_5ʹ and Cox1_Schist_3ʹ (see Supplementary Data). The ITS and COI sequences were compared by alignment with data available in the National Center for Biotechnology Information (NCBI) GenBank database (Supplementary Table 1).

Statistical Analyses

The relative abundance of small mammals was assumed to be reflected in their different capture rates, calculated for each trapping session and site as the proportion between captures and active traps set overnight [11]. Differences in the occurrence of Schistosoma spp. among host species (A. niloticus and M. huberti), host traits (gender and age), seasons (pre- and post-rainy season), trapping sites, and habitats (riparian vegetation and crop field) were assessed using Pearson’s χ² test, significant when P ≤ .05. Statistical analyses were performed using EpiTools (http://epitools.ausvet.com.au).

Ethics Statement

Trapping activities were initiated after explicit consent from local authorities and land owners. Approval for live trapping and euthanasia of small mammals was obtained from the Clinical Research Ethical Review Board of the Royal Veterinary College, University of London (reference number: 2016 1505).

RESULTS

A total of 509 animals were captured over 2531 trap-nights (overall capture rate 20.1%). We trapped 215 A. niloticus rats, 172 M. huberti mice, 27 Crocidura sp. shrews, and 6 Taterillus sp. gerbils. By-catch (n = 89) was mainly represented by 4-toed hedgehogs (Atelerix albiventris) and anuran amphibians. We detected Schistosoma spp. in the portal system and mesenteric vessels out of 7 of 172 M. huberti (prevalence 4.1%; intensity range 2–35; median intensity 4) and 6 out of 215 A. niloticus (prevalence 2.8%; intensity range 1–44; median intensity 5). No schistosomes were isolated during the inspection of the urogenital system. Among study sites, Schistosoma spp. prevalence was estimated to be 28.6% (6 out of 21) and 1.9% (1 out of 52) in M. huberti from the villages of Temey and Nder, respectively, whereas prevalence was 8.7% (4 out of 46) and 2.7% (2 out of 73) in A. niloticus from Djidiery and Richard Toll, respectively (Figure 1).

We identified S. mansoni in 7 M. huberti and 1 A. niloticus, and Schistosoma bovis in 5 A. niloticus based on the ITS (903–911 base pairs) and COI (861–880 base pairs) DNA regions of each adult worm. One schistosome pair parasitizing a M. huberti from Temey was composed of an S. mansoni male with a hybrid female presenting the profile of S. haematobium for ITS and S. bovis for COI (Table 1). Miracidia collected from 2 A. niloticus infected with S. bovis, 1 M. huberti infected with S. mansoni, and 1 M. huberti infected with both S. mansoni and S. haematobium/S. bovis were molecularly identified as either S. mansoni or S. bovis. Alignment of the ITS sequences did not detect any intraspecific variation among the analyzed specimens. In contrast, different haplotypes were found when aligning the COI sequences of S. mansoni (identity ≥98.2%) and S. bovis (identity ≥98.7%). Intraspecific nucleotide polymorphism translated in missense mutations in the amino acid sequence of COI’s protein product for S. mansoni from Temey (Supplementary Table 2). Sequence data from individual schistosomes were deposited in the NCBI GenBank database under the accession numbers MF776585-MF776597 (ITS) and MF919405-MF919428 (COI).

The estimated Schistosoma spp. prevalence was significantly higher in adult rodents when compared with juveniles (χ² = 4.842; d.f. = 1; P = .028) and in the village of Temey when compared with other study sites (χ² = 49.406; d.f. = 11; P < .001). Schistosome prevalence did not significantly vary when tested against host species, gender, season, and habitat (P > .05).

DISCUSSION

A quarter of a century after the finding of S. mansoni in rodents of northern Senegal [4], we show that M. huberti and A. niloticus continue to serve as alternative definitive hosts of S. mansoni. Furthermore, our data demonstrate that A. niloticus can act as a suitable definitive host of S. bovis, a parasite of ruminants previously identified in small mammals, using molecular tools, only in Mastomys natalensis mice of Kenya [12]. In addition, to our knowledge, we provide the first report of S. haematobium/S. bovis hybrids in any non-human vertebrate outside of a laboratory setting, suggesting that both humans and rodents could act as natural definitive hosts of zoonotic S. haematobium hybrids in sub-Saharan Africa. These results, combined with the absence of pure S. haematobium in the examined rodents, highlight the increased host spectrum and adaptive plasticity that hybrid vigour can confer to parasites [3, 8]. Our data indicate the ability by Schistosoma species and hybrids to be true multi-host pathogens on a local scale, capable of crossing species barriers and maintaining transmission via zoonotic routes [3].

The molecular identification of miracidia confirms that both S. mansoni and S. bovis are able to produce viable eggs in the infected rodents. It is interesting to note that the substitutions characterizing the COI nucleotide sequences and amino acid translations of many S. mansoni specimens may indicate potential introgression of S. haematobium and/or S. bovis DNA [7]. Although the described multi-locus molecular analysis remains an invaluable tool for epidemiological studies of Schistosoma spp. and the detection of early hybridization events, the application of innovative genomic technologies would bring added value to research on multi-host zoonotic pathogens within a One Health framework. This integrated approach would provide fine-scale insight into the evolutionary pressures driving mechanisms of spillover, host switching, hybridization, and introgression between animal and human parasites [3].

In China and the Philippines, transmission maintenance and spillover to humans from rodents and other zoonotic reservoirs of Schistosoma japonicum continue to jeopardize control programs, hindering public health efforts to eliminate this disease [13, 14]. In West Africa, wild rodents and other wildlife could be of paramount importance as zoonotic reservoirs of Schistosoma species and/or hybrids [2]. Our findings emphasize that the role of rodents as local reservoirs of Schistosoma spp. in the Senegal River Basin may be 2-fold, locally serving as either spillover hosts for the circulating schistosomes, or as key hosts in the transmission maintenance of schistosomiasis (see [3] for definitions). With rodents projected to become the dominant wildlife in human-driven environments and the main reservoir of zoonotic diseases in tropical zones [15], further data on the ecology of small mammals and schistosome transmission dynamics are necessary. A pressing question is how ongoing changes in landscape and biodiversity in West Africa will affect the future spread of synanthropic rodents and the subsequent risk of zoonotic transmission of schistosomiasis.

CONCLUSIONS

In conclusion, our study suggests that the role of wild rodents as potential local reservoirs of zoonotic Schistosoma species and hybrids in endemic areas of West Africa cannot be overlooked. The breakdown of ecological barriers in the absence of any mitigation measures may support the life cycle and interspecific interactions between human and animal schistosomes [3, 6]. Therefore, a One Health, multi-host framework would better tailor local, setting-specific schistosomiasis control strategies, enhancing public health interventions in many countries where the disease is endemic.

Supplementary Data

Supplementary materials are available at The Journal of Infectious Diseases online. Consisting of data provided by the authors to benefit the reader, the posted materials are not copyedited and are the sole responsibility of the authors, so questions or comments should be addressed to the corresponding author.

Notes

Acknowledgments. We are grateful to Aidan Emery, Bonnie Webster, and David Rollinson for their valuable support and hospitality at the Natural History Museum, London (UK). A very special acknowledgement to Boubacar Bâ, Alassane Ndiaye, Mapaté Gaye, and Cheikh Thiam and his family for their outstanding contribution during fieldwork and for the incredible hospitality. Finally, we thank the communities of the Senegalese villages we worked with for the friendly welcome and collaboration, and, although no acknowledgement can compensate their sacrifice, we honor all the wild animals trapped during this study.

Financial support. This work was funded by the Royal Veterinary College and the Zoonoses and Emerging Livestock Systems research initiative including the Department for International Development, Biotechnology and Biological Sciences Research Council, Economic and Social Sciences Research Council, Medical Research Council, Natural Environment Research Council, and Defence Science and Technology Laboratory (grant numbers BB/L018985/1 and BB/N503563/1).

Potential conflicts of interest. All authors report no potential conflicts of interest. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

References