1. Please check and comment entries here.
Table of Contents

    Topic review

    Hybrid Schistosomes in Africa

    Subjects: Parasitology
    View times: 11
    Submitted by: Ursula Panzner

    Definition

    Causal agents of schistosomiasis are dioecious, digenean schistosomes affecting mankind in 76 countries.  Natural and anthropogenic changes impact on breaking species isolation barriers favoring introgressive hybridization, i.e., allelic exchange among gene pools of sympatric, interbreeding species leading to instant large genetic diversity. Phylogenetic distance matters, thus the less species differ phylogenetically the more likely they hybridize. 

    1. Schistosomiasis and Parasite Hybridization

    Schistosomiasis affects mankind in 76 countries causing an estimated annual mortality rate of 280,000, 779 million people at risk of infection, 250 million people with active infections, and 440 million people with residual morbidity [1][2][3][4]. The disease burden is largest throughout sub-Saharan Africa accounting for >90% caused by the clinically most relevant human-pathogenic species Schistosoma mansoni (Sm) and S. haematobium (Sh) [1][2][3][4]. Though schistosomiasis is likely underrecognized among the veterinary sector, inclusive of livestock and wildlife animals, it is assumed that in particular ruminants, rodents and non-human primates are carriers of zoonotic species [5].

    The causal agents of this neglected tropical disease are dioecious digenean schistosomes within the Platyhelminthes; species by clade of the Schistosoma genus including their geographical distribution, and intermediate and definitive hosts are illustrated in Figure 1 [5][6][7][8].

    Figure 1. Clades within the Schistosoma genus inclusive of their geographical distribution, Schistosoma species, and species-specific intermediate invertebrate and definitive experimental and natural mammalian hosts ordered from the inside to the outside of the hexagon. Notes: S. = Schistosoma, O. = Orientobilharzia; On. = Oncomelania, T. = Tricula, N. = Neotricula, R. = Robertsiella, I = Indoplanorbis, P. = Planorbis, L. = Lymnaea, Bi. = Biomphalaria, B. = Bulinus, A. = Australorbis, Pl. = Planorbarius, F. = Ferrissia; adapted from Leger et al. [5], Lawton et al. [6], Webster et al. [7], and Pearce et al. [9]; species-specific intermediate and definitive hosts retrieved from the host-parasite database of the Natural History Museum [8].
    The chemotherapeutic at present is the acylated quinoline-pyrazine known as praziquantel (PZQ) that acts against adult schistosomes though poorly against immature schistosome larva by changing irreversibly the permeability and stability of tegumental membranes and needs established host immune defense mechanisms for full efficacy [2][3][10].
    Preventive measures are multifaceted, but need to be complemented by vaccination to achieve long-term protection against transmission, infection, and disease recurrence as acquired naturally in endemic settings [1][11]. The goal is a non-sterilizing immunity with long-term decline in tissue eggs and egg excretion preferably through killing of female worms while preserving concomitant natural immunity induced by less-pathogenic single male worms [12][13].
    Parasite hybridization is an emerging public health concern and may be linked to global natural and anthropogenic changes causing selection pressure; this in turn impacts the dynamics and distribution of schistosomes and their hosts by breaking isolation barriers, thus increases chances of introgressive hybridization [2][14][15][16][17].
    Schistosomes are known to hybridize and natural hybrids are more and more frequently found disorienting baseline laboratory and clinical diagnoses; see Figure 2 for likely natural schistosomal hybridization events across Africa [18].
    Figure 2. Distribution of Schistosoma hybrids across Africa. Notes: S.h. = S. haematobium; S.m. = S. mansoni; S.g. = S. guineensis; S.i. = S. intercalatum; S.b. = S. bovis; S.c. = S. curassoni; S.r. = S. rodhaini; S.ma. = S. mattheei; adapted primarily from Leger et al. [5].
    Recombinants are characterized by heterotic alterations, speciations, neo-functionalization and adaptations, called hybrid vigor [19]. This affects e.g., virulence, transmission, infectivity, pathologies, maturation, fecundity, host spectrum and preference, e.g., extended host range of ShxS. mattheei (Sma) hybrids to sheep, and ShxSb hybrids to non-human primates and rodents, and chemotherapeutic efficacy, e.g., diminished response to oxamniquine if Sma interacts with Sh [5][20][21][16][22][23][24][25][26][27]. Taken together, this may result in the expansion to new geographic habitats, and homogenization or progressive, competitive extinction of species as seen for S. guineensis (Sg) in Cameroon, and ultimately the emergence of new diseases [19].

    2. Schistosome Hybrids across Africa

    2.1. S. haematobium/S. guineensis Hybrids

    Interactions between Sh and Sg are reported from South-West Cameroon; of note is the re-description of S. intercalatum (Si) from Lower Guinea, e.g., Cameroon, Gabon, Equatorial Guinea and São Tomé, as Sg [22][23][28][29][30]. Ecological changes such as forest clearance and agricultural development evolving in the 1960s besides population movement favored the presence of snail hosts, e.g., Bulinus wrighti, B. forskalii and B. truncatus, and species sympatry and interplay; this materialized the complete replacement of Sg by Sh within 25–30 years through introgressive hybridization and competitive extinction abetted by behavioral, reproductive and genetic advantages [31][32][33]. Experiments confirmed greater competitiveness and pairing abilities of ShxSg crosses over parental species, and Sh being dominant over Sg, which supports observations made in Cameroon and elsewhere, e.g., Kinshasa, Democratic Republic of Congo, and Dogon Country, Mali [34][35][36][37]. Retrospective experiments among schistosomes of laboratory maintained urinary ova of Cameroonian children detected hybridization between Sh and Sg among 100%, 33% and 5% isolates from Kumba, Loum (1990), and Loum (1999 and 2000), respectively; the remainder had pure Sh, but no Sg despite established B. forskalii hosts [28]. Findings were confirmed by four-banded single-stranded conformational polymorphism (SSCP) profiles of second nuclear ribosomal internal transcribed spacer (ITS2) fragments. Sequencing isolates from Loum revealed unequivocal intermediate SNPs at the positions 25, 80 and 130 within ITS2 fragments compared to parental strains, i.e., guanine-cytosine-guanine for Sh, and adenine-thymine-adenine for Sg, clearly demonstrating ShxSg recombinants with dominance, thus inheritance, of Sh over Sg [23].

    2.2. S. haematobium/S. bovis Hybrids

    ShxSb hybrids are certainly the most widespread and best studied schistosome hybrids in West African countries. The presence of these hybrids was first discovered in 2009 in the Senegal River Basin (SRB) [16] and attributed to contemporary ecological and anthropogenic changes having affected the SRB since the 1980s. It was pointed out the construction of the Diama Dam on the Senegal River for sea water desalination and expanded irrigation and agriculture, and the Manantali Dam on the Bafing River for hydroelectricity and alterations in water velocity; Biomphalaria pfeifferi and Bulinus spp. hosts of Sm, Sh and Sb got established due to more favorable habitats triggering increases in urogenital and intestinal schistosomiasis in both, humans and livestock, intensified by growing contacts between humans and companion, domestic/livestock and wildlife animals [16][37][38][39][40][41]. These changes have certainly favored the propagation of ShxSb hybrids in the SRB, though recent genome-based studies did show that hybridization events are much older than presumed. Based on parasites recovered from Niger, it has been estimated that introgression occurred around 240.6 years ago (range: −107.8 to −612.5 years) [42]. The ancestral origin of these hybrids was confirmed with parasites originating from several settings across Western Africa [19].
    In Senegal, numerous studies have been performed on ShxSb hybrids following their initial discovery. Investigations on miracidia hatched from urinary and fecal ova of Senegalese schoolchildren from six villages in the SRB adjunct to Senegal River, Lampsar River, and Lac de Guier revealed 15% ShxSb isolates with Sh-ITSxSb-COX1 profiles producing viable offspring; patterns were confirmed by double chromatographic peaks at polymorphic positions of ITS and sequence divergence of COX1 with alignment to Sh and Sb strains from Guede Chantier and St. Louis, Senegal, respectively [16][43][44]. Recombinants evolved from crossing of male Sh with female Sb were capable of infecting humans and both B. globosus and B. truncatus snails indicating loss of host specificity [16]. The reverse, more rare recombinant seems less viable due to biased homogenization towards parental species, asymmetric backcrossing, or concerted evolution. Restriction fragment length polymorphism (RFLP) was applied to COX1 and ITS amplicons of miracidia hatched from urinary and fecal ova of children and adolescents in Pakh, Northern Senegal, and 10 villages of the SRB, followed by back-crossing of fertile hybrid progeny with parental species [43][44].
    Recent investigations in Côte d’Ivoire identified ShxSb hybrids in both, molluscs and humans [45][46]. Sequences of miracidia collected from schoolchildren at Sh and Sm endemic sites in the south of the country, i.e., Agboville, Adzopé, Sikensi and Duekoué, uncovered 35% pure Sh, and very few pure Sb suggesting zoonotic transmission similar to reports from Corsica [45][46].
    Ongoing disease surveillance among children residing in Chickhawa, Nsanje and Mangochi Districts of Malawi, lacking reported evidence of Sb, found primarily pure Sh ova and few eggs with discordant Sh-ITSxSb-COX1 schemes [37][47][48][49][50][51].

    2.3. S. haematobium/S. mansoni Hybrids

    Even before Boon et al. reported miracidial ShxSm-COX1 and ShxSm-ITS patterns, interbreeding between phylogenetic distant Sh and Sm was observed among miracidia collected from schoolchildren in the SRB, Senegal; based on microsatellite DNA amplification, 36% urinary and 2% fecal isolates presented with the hybridale allele profiles 218:239 or 218:236, respectively, whereby allele 218 is specific to Senegalese Sh, Sb and their hybrids, and allele 239 is specific to Senegalese Sm [52]. RFLP applied on COX1 and ITS amplicons of miracidia and ova detected heterozygote Sm-COX1xSmxSh-ITS and Sh-COX1xSmxSh-ITS schemes. Similar to observations made for ShxSb crossbreeds in this setting, a potential zoonotic role of livestock and/or rodents, the latter being capable of serving as host for Sb and ShxSb crossbreeds but not Sh, as seen in the SRB is suspected.

    2.4. S. mansoni/S. rodhaini Hybrids

    Long-term anthropogenic alterations largely affected also the Rift Valley Region in West Kenya leading to SmxSr (S. rodhaini) interbreeding due to directional introgression from male Sr into female Sm with ascendance of Sm. Laboratory passaged miracidia hatched from fecal ova and matured schistosomes were compared for their patterns in ITS, 16S-12SDNA (partial nuclear mitochondrial 12S-16SDNA), and additional microsatellite loci to Sr strains from Burundi and Kenya, and Sm from Puerto Rico. Sympatrically occurring species likely interacted because prezygotic reproductive barriers of differential host use, preferential mating, male–male competition with male Sr dominating male Sm, and circadian cercarial emergence from snail points are imperfect [21][53][54].

    3. Schistosomal Hybridization—Interference on Leading Vaccine Candidates

    Extensive small-scale gene amplifications and alterations are contributing to a steady schistosomal genomic evolution [27]. SNPs emerging through transversional/transitional or synonymous/non-synonymous nucleotide substitutions leading to e.g., alternative splicing or silent mutations are of high importance in particular among sequences serving as drug or vaccine targets; they may lead to structural and functional protein alterations impacting charge changes and residue conservation affecting drug binding sites or antibody recognition of antigenic epitopes [55][56][57]. Hosts react in the following general manners against schistosomal infections, i.e., development of age-dependent partial protective immunity to reinfection from repeated adult worm death, and initiation of immunopathogenic and/or immunoregulatory mechanisms against parasitic antigens released from eggs trapped in tissues [58][59][60].

    Preliminary findings of the phase 2a trial confirmed rSm14+GLA-SE’s safety and strong, long-lasting immunogenicity, and reported 92% seroconversion following the second dose with cellular responses, memory cells and T-cell activation makers [61]. However, Sm14 isoforms demonstrate the potential of structural instability resulting in its aggregation and precipitation and, in turn, to uncertain immunoprotection and functional alterations [57][62].

    rSh28GST+Alhydrogel/Bilhvax® vaccine candidate was tested in a phase 3 safety, efficacy, disease recurrence and immunogenicity trial among Sh infected schoolchildren from the ShxSb and ShxSm hybrid endemic SRB receiving two-dose PZQ pre-treatment, three-dose immunization, and a single-dose PZQ treatment prior to a final vaccine booster dose [63][64][65][66]. rSh28GST+Alhydrogel proved its safety, strong, long-lasting immunogenicity, and anti-worm fecundity with inhibited egg viability despite the fact that the primary endpoint of significant delay in schistosomiasis recurrence was not reached in the trial [63].

    Of interest related to the ShxSm hybrid type could be the potential of the Smp80-GLA-SE/SchistoShield® vaccine candidate to confer robust, balanced Th1/Th2 cross-species prophylactic and therapeutic protection against Sh in addition to Sj as demonstrated in rodents and baboons [67][68]. This observation could be due to 95% and 84% amino acid homology among calpain of Sh and Sj with Sm, respectively. Likewise, an ongoing phase 1 dose-escalation, safety and immunogenicity Sm-TSP-2/Sm-TSP-2Al® trial coupled with a subsequent 2b Sm-TSP-2/Sm-TSP-2Al® trial among Sm and Sh exposed healthy male and female Ugandan adults receiving three-dose immunization could reveal promising cross-species protective effects depending on the level of inter- and/or intra-species polymorphism of the TSP-2 EC2 domain [69].

    4. Conclusions

    A vaccine as the current candidate in advanced to pre-clinical and clinical phases with cross-species protective potential is needed to complement preventive measures, and protect long-term against transmission, infection, and disease recurrence and sequelae. Despite the strong evidence of naturally occurring schistosomal hybrids of large functional and structural diversity, its potential impact on current vaccine candidates is yet unclear and requires further research. Vaccine targets exposed at the schistosome tegumental surface should not evolve rapidly leading to sequence heterogenicity in natural parasite populations, and rendering its use as a vaccine candidate ineffective, but rather highly conserved. Additionally, natural history studies seem of particular importance in areas with known endemicity for hybrid schistosomes that are targeted for clinical trials.

    The entry is from 10.3390/microorganisms9071465

    References

    1. Nelwan, M.L. Schistosomiasis: Life Cycle, Diagnosis, and Control. Curr. Ther. Res. 2019, 91, 5–9.
    2. Webster, J.P.; Neves, M.I.; Webster, B.L.; Pennance, T.; Rabone, M.; Gouvras, A.; Allan, F.; Walker, M.; Rollinson, D. Parasite Population Genetic Contributions to the Schistosomiasis Consortium for Operational Research and Evaluation within Sub-Saharan Africa. Am. J. Trop. Med. Hyg. 2020, 103, 80–91.
    3. Colley, D.G.; Bustinduy, A.L.; Secor, W.E.; King, C.H. Human schistosomiasis. Lancet 2014, 383, 2253–2264.
    4. Molehin, A.J. Schistosomiasis vaccine development: Update on human clinical trials. J. Biomed. Sci. 2020, 27, 1–7.
    5. Leger, E.; Webster, J.P. Hybridizations within the Genus Schistosoma: Implications for evolution, epidemiology and control. Parasitology 2017, 144, 65–80.
    6. Lawton, S.P.; Hirai, H.; Ironside, J.E.; Johnston, D.A.; Rollinson, D. Genomes and geography: Genomic insights into the evolution and phylogeography of the genus Schistosoma. Parasites Vectors 2011, 4, 131.
    7. Webster, B.; Southgate, V.; Littlewood, D. A revision of the interrelationships of Schistosoma including the recently described Schistosoma guineensis. Int. J. Parasitol. 2006, 36, 947–955.
    8. London NHM. Host-Parasite Database. Available online: (accessed on 2 June 2021).
    9. Pearce, E.J.; Macdonald, A.S. The immunobiology of schistosomiasis. Nat. Rev. Immunol. 2002, 2, 499–511.
    10. Vale, N.; Gouveia, M.; Rinaldi, G.; Brindley, P.J.; Gärtner, F.; da Costa, J.M.C. Praziquantel for Schistosomiasis: Single-Drug Metabolism Revisited, Mode of Action, and Resistance. Antimicrob. Agents Chemother. 2017, 61, e02582-16.
    11. Sokolow, S.H.; Wood, C.L.; Jones, I.J.; Lafferty, K.D.; Kuris, A.M.; Hsieh, M.H.; De Leo, G.A. To Reduce the Global Burden of Human Schistosomiasis, Use ‘Old Fashioned’ Snail Control. Trends Parasitol. 2018, 34, 23–40.
    12. Merrifield, M.; Hotez, P.J.; Beaumier, C.M.; Gillespie, P.; Strych, U.; Hayward, T.; Bottazzi, M.E. Advancing a vaccine to prevent human schistosomiasis. Vaccine 2016, 34, 2988–2991.
    13. Dumont, M.; Moné, H.; Mouahid, G.; Idris, M.A.; Shaban, M.; Boissier, J. Influence of pattern of exposure, parasite genetic diversity and sex on the degree of protection against reinfection with Schistosoma mansoni. Parasitol. Res. 2007, 101, 247–252.
    14. King, K.; Stelkens, R.B.; Webster, J.P.; Smith, D.F.; Brockhurst, M.A. Hybridization in Parasites: Consequences for Adaptive Evolution, Pathogenesis, and Public Health in a Changing World. PLoS Pathog. 2015, 11, e1005098.
    15. Léger, E.; Garba, A.; Hamidou, A.A.; Webster, B.; Pennance, T.; Rollinson, D.; Webster, J.P. Introgressed Animal Schistosomes Schistosoma curassoni and S. bovis Naturally Infecting Humans. Emerg. Infect. Dis. 2016, 22, 2212–2214.
    16. Huyse, T.; Webster, B.L.; Geldof, S.; Stothard, J.R.; Diaw, O.T.; Polman, K.; Rollinson, D. Bidirectional introgressive hybridization between a cattle and human schistosome species. PLoS Pathog. 2009, 5, e1000571.
    17. Webster, B.; Diaw, O.T.; Seye, M.M.; Webster, J.P.; Rollinson, D. Introgressive Hybridization of Schistosoma haematobium Group Species in Senegal: Species Barrier Break Down between Ruminant and Human Schistosomes. PLoS Negl. Trop. Dis. 2013, 7, e2110.
    18. Crego-Vicente, B.; Fernández-Soto, P.; Febrer-Sendra, B.; Diego, J.G.-B.; Boissier, J.; Angora, E.; Oleaga, A.; Muro, A. Application of a Genus-Specific LAMP Assay for Schistosome Species to Detect Schistosoma haematobium × Schistosoma bovis Hybrids. J. Clin. Med. 2021, 10, 1308.
    19. Rey, O.; Toulza, E.; Chaparro, C.; Allienne, J.-F.; Kincaid-Smith, J.; Mathieu-Begné, E.; Allan, F.; Rollinson, D.; Webster, B.L.; Boissier, J. Diverging patterns of introgression from Schistosoma bovis across S. haematobium African lineages. PLoS Pathog. 2021, 17, e1009313.
    20. Steinauer, M.L.; Blouin, M.S.; Criscione, C.D. Applying evolutionary genetics to schistosome epidemiology. Infect. Genet. Evol. 2010, 10, 433–443.
    21. Steinauer, M.L.; Hanelt, B.; Mwangi, I.N.; Maina, G.M.; Agola, L.E.; Kinuthia, J.M.; Mutuku, M.W.; Mungai, B.N.; Wilson, W.D.; Mkoji, G.M.; et al. Introgressive hybridization of human and rodent schistosome parasites in western Kenya. Mol. Ecol. 2008, 17, 5062–5074.
    22. Moné, H.; Minguez, S.; Ibikounlé, M.; Allienne, J.F.; Massougbodji, A.; Mouahid, G. Natural interactions between S. haematobium and S. guineensis in the Republic of Benin. Sci. World J. 2012, 2012, 793420.
    23. Webster, B.L.; Tchuenté, L.A.T.; Southgate, V.R. A single-strand conformation polymorphism (SSCP) approach for investigating genetic interactions of Schistosoma haematobium and Schistosoma guineensis in Loum, Cameroon. Parasitol. Res. 2007, 100, 739–745.
    24. Rollinson, D. Biochemical genetics in the study of schistosomes and their intermediate hosts. Parassitologia 1985, 27, 123–139.
    25. Wright, C.; Ross, G. Hybrids between Schistosoma haematobium and S. mattheei and their identification by isoelectric focusing of enzymes. Trans. R. Soc. Trop. Med. Hyg. 1980, 74, 326–332.
    26. Catalano, S.; Sène, M.; Diouf, N.D.; Fall, C.B.; Borlase, A.; Léger, E.; Bâ, K.; Webster, J.P. Rodents as natural hosts of zoonotic schistosoma species and hybrids: An epidemiological and evolutionary perspective from West Africa. J. Infect. Dis. 2018, 218, 429–433.
    27. Wang, S.; Zhu, X.-Q.; Cai, X. Gene Duplication Analysis Reveals No Ancient Whole Genome Duplication but Extensive Small-Scale Duplications during Genome Evolution and Adaptation of Schistosoma mansoni. Front. Cell. Infect. Microbiol. 2017, 7, 412.
    28. Webster, B.; Tchuenté, L.T.; Jourdane, J.; Southgate, V. The interaction of Schistosoma haematobium and S. guineensis in Cameroon. J. Helminthol. 2005, 79, 193–197.
    29. Wright, C.A.; Southgate, V.R.; Van Wijk, H.B.; Moore, P.J. Letter: Hybrids between Schistosoma haematobium and S. intercalatum in Cameroon. Trans. R. Soc. Trop. Med. Hyg. 1974, 68, 413–414.
    30. Webster, B.; Southgate, V.; Tchuenté, L.-A.T. Isoenzyme analysis of Schistosoma haematobium, S. intercalatum and their hybrids and occurrences of natural hybridization in Cameroon. J. Helminthol. 2003, 77, 269–274.
    31. Southgate, V.R.; van Wijk, H.B.; Wright, C.A. Schistosomiasis at Loum, Cameroun; Schistosoma haematobium, S. intercalatum and their natural hybrid. Parasitol. Res. 1976, 49, 145–159.
    32. Tchuem Tchuenté, L.A.; Southgate, V.R.; Njiokou, F.; Njiné, T.; Kouemeni, L.E.; Jourdane, J. The evolution of schistosomiasis at Loum, Cameroon: Replacement of Schistosoma intercalatum by S. haematobium through introgressive hybridization. Trans. R. Soc. Trop. Med. Hyg. 1997, 91, 664–665.
    33. Southgate, V.R. On factors possibly restricting the distribution of Schistosoma intercalatum Fisher, 1934. Parasitol. Res. 1978, 56, 183–193.
    34. Cosgrove, C.L.; Southgate, V.R. Competitive mating interactions between Schistosoma haematobium and S. intercalatum (Lower Guinea strain). Parasitol. Res. 2003, 89, 238–241.
    35. Webster, B.L.; Southgate, V.R. Mating interactions of Schistosoma haematobium and S. intercalatum with their hybrid offspring. Parasitology 2003, 126, 327–338.
    36. Tchuem Tchuenté, L.A.; Southgate, V.R.; Vercruysse, J.; Kaukas, A.; Kane, R.; Mulumba, M.P.; Pagès, J.R.; Jourdane, J. Epidemiological and genetic observations on human schistosomiasis in Kinshasa, Zaire. Trans. R. Soc. Trop. Med. Hyg. 1997, 91, 263–269.
    37. Vercruysse, J.; Southgate, V.R.; Rollinson, D.; De Clercq, D.; Sacko, M.; De Bont, J.; Mungomba, L.M. Studies on transmission and schistosome interactions in Senegal, Mali and Zambia. Trop. Geogr. Med. 1994, 46, 220–226.
    38. Picquet, M.; Ernould, J.C.; Vercruysse, J.; Southgate, V.R.; Mbaye, A.; Sambou, B.; Niang, M.; Rollinson, D. Royal Society of Tropical Medicine and Hygiene Meeting at Manson House, London, 18 May 1995. The epidemiology of human schistosomiasis in the Senegal river basin. Trans. R. Soc. Trop. Med. Hyg. 1996, 90, 340–346.
    39. Southgate, V. Schistosomiasis in the Senegal River Basin: Before and after the construction of the dams at Diama, Senegal and Manantali, Mali and future prospects. J. Helminthol. 1997, 71, 125–132.
    40. Van den Broeck, F.; Maes, G.E.; Larmuseau, M.H.D.; Rollinson, D.; Sy, I.; Faye, D.; Volckaert, F.A.M.; Polman, K.; Huyse, T. Reconstructing colonization dynamics of the human parasite Schistosoma mansoni following anthropogenic environmental changes in Northwest Senegal. PLoS Negl. Trop. Dis. 2015, 9, e0003998.
    41. Boon, N.A.; Mbow, M.; Paredis, L.; Moris, P.; Sy, I.; Maes, T.; Webster, B.; Sacko, M.; Volckaert, F.A.; Polman, K.; et al. No barrier breakdown between human and cattle schistosome species in the Senegal River Basin in the face of hybridisation. Int. J. Parasitol. 2019, 49, 1039–1048.
    42. Platt, R.; McDew-White, M.; Le Clec’H, W.; Chevalier, F.; Allan, F.; Emery, A.M.; Garba, A.; Hamidou, A.A.; Ame, S.M.; Webster, J.P.; et al. Ancient Hybridization and Adaptive Introgression of an Invadolysin Gene in Schistosome Parasites. Mol. Biol. Evol. 2019, 36, 2127–2142.
    43. Boon, N.A.; Van Den Broeck, F.; Faye, D.; Volckaert, F.A.; Mboup, S.; Polman, K.; Huyse, T. Barcoding hybrids: Heterogeneous distribution of Schistosoma haematobium × Schistosoma bovis hybrids across the Senegal River Basin. Parasitology 2018, 145, 634–645.
    44. Boon, N.A.M.; Fannes, W.; Rombouts, S.; Polman, K.; Volckaert, F.A.M.; Huyse, T. Detecting hybridization in African schistosome species: Does egg morphology complement molecular species identification? Parasitology 2017, 144, 954–964.
    45. Boissier, J.; Grech-Angelini, S.; Webster, B.; Allienne, J.-F.; Huyse, T.; Mas-Coma, S.; Toulza, E.; Barré-Cardi, H.; Rollinson, D.; Kincaid-Smith, J.; et al. Outbreak of urogenital schistosomiasis in Corsica (France): An epidemiological case study. Lancet Infect. Dis. 2016, 16, 971–979.
    46. Angora, E.K.; Allienne, J.F.; Rey, O.; Menan, H.; Touré, A.O.; Coulibaly, J.T.; Raso, G.; Yavo, W.; N’Goran, E.K.; Utzinger, J.; et al. High prevalence of Schistosoma haematobium × Schistosoma bovis hybrids in schoolchildren in Cote d’Ivoire. Parasitology 2019, 147, 287–294.
    47. Webster, B.L.; Alharbi, M.H.; Kayuni, S.; Makaula, P.; Halstead, F.; Christiansen, R.; Juziwelo, L.; Stanton, M.C.; LaCourse, J.; Rollinson, D.; et al. Schistosome Interactions within the Schistosoma haematobium Group, Malawi. Emerg. Infect. Dis. 2019, 25, 1245–1247.
    48. Pitchford, R. Observations on a possible hybrid between the two schistosomes S. haematobium and S. mattheei. Trans. R. Soc. Trop. Med. Hyg. 1961, 55, 44–51.
    49. Van Wyk, J.A. The importance of animals in human schistosomiasis in South Africa. S. Afr. Med. J. 1983, 63, 201–203.
    50. Kruger, F.J.; Schutte, C.H.; Visser, P.S.; Evans, A.C. Phenotypic differences in Schistosoma mattheei ova from populations sympatric and allopatric to S. haematobium. Onderstepoort J. Vet. Res. 1986, 53, 103–107.
    51. Kruger, F.J.; Hamilton-Attwell, V.L. Scanning electron microscope studies of miracidia suggest introgressive hybridization between Schistosoma haematobium and S. haematobium × S. mattheei in the Eastern Transvaal. J. Helminthol. 1988, 62, 141–147.
    52. Huyse, T.; Broeck, F.V.D.; Hellemans, B.; Volckaert, F.; Polman, K. Hybridisation between the two major African schistosome species of humans. Int. J. Parasitol. 2013, 43, 687–689.
    53. Norton, A.J.; Webster, J.P.; Kane, R.A.; Rollinson, D. Inter-specific parasite competition: Mixed infections of Schistosoma mansoni and S. rodhaini in the definitive host. Parasitology 2008, 135, 473–484.
    54. Thèron, A. Hybrids between Schistosoma mansoni and S. rodhaini: Characterization by cercarial emergence rhythms. Parasitology 1989, 99, 225–228.
    55. Vilar, M.M.; Barrientos, F.; Almeida, M.; Thaumaturgo, N.; Simpson, A.; Garratt, R.; Tendler, M. An experimental bivalent peptide vaccine against schistosomiasis and fascioliasis. Vaccine 2003, 22, 137–144.
    56. Simões, M.; Bahia, D.; Zerlotini, A.; Torres, K.; Artiguenave, F.; Neshich, G.; Kuser, P.; Oliveira, G. Single nucleotide polymorphisms identification in expressed genes of Schistosoma mansoni. Mol. Biochem. Parasitol. 2007, 154, 134–140.
    57. Ramos, C.R.R.; Figueredo, R.C.R.; Pertinhez, T.A.; Vilar, M.M.; Nascimento, A.L.T.O.D.; Tendler, M.; Raw, I.; Spisni, A.; Ho, P.L. Gene Structure and M20T Polymorphism of the Schistosoma mansoni Sm14 Fatty Acid-binding Protein. J. Biol. Chem. 2003, 278, 12745–12751.
    58. Fukushige, M.; Mutapi, F.; Woolhouse, M.E. Population level changes in schistosome-specific antibody levels following chemotherapy. Parasite Immunol. 2019, 41, e12604.
    59. McManus, D.P.; Bergquist, R.; Cai, P.; Ranasinghe, S.; Tebeje, B.M.; You, H. Schistosomiasis—From immunopathology to vaccines. Semin. Immunopathol. 2020, 42, 355–371.
    60. Panzner, U.; Excler, J.L.; Kim, J.H.; Marks, F.; Carter, D.; Siddiqui, A.A. Recent advances and methodological considerations on vaccine candidates for human schistosomiasis. Front. Trop. Dis. 2021. submitted.
    61. Tendler, M.; Almeida, M.S.; Vilar, M.M.; Pinto, P.M.; Limaverde-Sousa, G. Current Status of the Sm14/GLA-SE Schistosomiasis Vaccine: Overcoming Barriers and Paradigms towards the First Anti-Parasitic Human (itarian) Vaccine. Trop. Med. Infect. Dis. 2018, 3, 121.
    62. Ramos, C.R.R.; Spisni, A.; Oyama, S.; Sforça, M.; Ramos, H.R.; Vilar, M.M.; Alves, A.C.; Figueredo, R.C.R.; Tendler, M.; Zanchin, N.; et al. Stability improvement of the fatty acid binding protein Sm14 from S. mansoni by Cys replacement: Structural and functional characterization of a vaccine candidate. Biochim. Biophys. Acta Proteins Proteom. 2009, 1794, 655–662.
    63. Riveau, G.; Schacht, A.-M.; Dompnier, J.-P.; Deplanque, D.; Seck, M.; Waucquier, N.; Senghor, S.; Delcroix-Genete, D.; Hermann, E.; Idris-Khodja, N.; et al. Safety and efficacy of the rSh28GST urinary schistosomiasis vaccine: A phase 3 randomized, controlled trial in Senegalese children. PLOS Neglected Trop. Dis. 2018, 12, e0006968.
    64. Medicine/ClinicalTrials.gov USNLo. Efficacy and Safety Evaluation of the Therapeutic Vaccine Candidate Sh28GST in Association with Praziquantel (PZQ) for Prevention of Clinical and Parasitological Recurrences of S. haematobium Infection in Children [NCT00870649]. 2012. Available online: (accessed on 19 June 2021).
    65. Medicine/ClinicalTrials.gov USNLo. Safety and Immunogenicity Evaluation of the Vaccine Candidate Sm14 in Combination with the Adjuvant Glucopyranosyl Lipid A (GLA-SE) in Adults Living in Endemic Regions for S. Mansoni and S. haematobium in Senegal. A Comparative, Randomized, Open-Label Trial [NCT03041766]. 2016. Available online: (accessed on 19 June 2021).
    66. Medicine/ClinicalTrials.gov USNLo. Safety and Immunogenicity Evaluation of the Vaccine Candidate Sm14 against Schistosomiasis in Senegalese School Children Healthy or Infected with S. Mansoni and/or S. haematobium. A Comparative, Randomized, Controlled, Open-label Trial [NCT03799510]. 2018. Available online: (accessed on 19 June 2021).
    67. Molehin, A.J.; Sennoune, S.R.; Zhang, W.; Rojo, J.U.; Siddiqui, A.J.; Herrera, K.A.; Johnson, L.; Sudduth, J.; May, J.; Siddiqui, A.A. Cross-species prophylactic efficacy of Sm-p80-based vaccine and intracellular localization of Sm-p80/Sm-p80 ortholog proteins during development in Schistosoma mansoni, Schistosoma japonicum, and Schistosoma haematobium. Parasitol. Res. 2017, 116, 3175–3188.
    68. Karmakar, S.; Zhang, W.; Ahmad, G.; Torben, W.; Alam, M.U.; Le, L.; Damian, R.T.; Wolf, R.F.; White, G.L.; Carey, D.W.; et al. Cross-species protection: Schistosoma mansoni Sm-p80 vaccine confers protection against Schistosoma haematobium in hamsters and baboons. Vaccine 2014, 32, 1296–1303.
    69. Medicine/ClinicalTrials.gov USNLo. Sm-TSP-2 Schistosomiasis Vaccine in Healthy Ugandan Adults [NCT03910972]. 2019. Available online: (accessed on 19 June 2021).
    More