Future schistosome hybridizations: Will all Schistosoma haematobium hybrids please stand-up!

Interrogating the genetic make-up of schistosome larvae (i.e., eggs, miracidia, and cercariae) originating from definitive or intermediate snail hosts with molecular DNA methods has, by noting unexpected interspecies hybrids, started a revolution in our appraisal of African schistosomiasis [1-4].Here, two dominant species of human schistosome exist, Schistosoma haematobium and S. mansoni, which are transmitted by specific intermediate freshwater snails, Bulinus spp.for the former and Biomphalaria spp.for the latter.The two schistosomes cause either urogenital or intestinal schistosomiasis, respectively [5], and depending on local snail distributions, schistosome transmission zones in the aquatic habitat may or may not overlap [6].Within the S. haematobium group, a further eight sister species are described with S. intercalatum and S. guineensis of medical importance, causing intestinal schistosomiasis, while others, such as S. bovis, S. curassoni, and S. mattheei occur in livestock, with the remaining species infecting wildlife.S. mattheei is also of medical interest for occasional infection and associated disease [7].In contrast, S. mansoni has a single sister species, S. rodhaini, typically found in small rodents which can hybridize with S. mansoni, if given sufficient opportunity [2].Across Africa, whilst the continental burden of coinfection with urogenital and intestinal schistosomiasis is not formally reported, at the local level, it often is [5,8].Inside a coinfected definitive host, an enigmatic but dynamic set of plausible worm-pairings can take place; these include a combination of both homo-specific (e.g., S. haematobium [♂] and S. haematobium [♀]) and various hetero-specific (e.g., S. mansoni [♂] and S. haematobium [♀], etc.) worm couplings [9].Cross-specific couplings depend on worm competition, mating preference, and anatomical location(s) in the vasculature surrounding hepatoportal, urogenital, and intestinal systems [9].As further hybrid variants come to light [2], some hetero-specific worm-pairing possibilities are of particular significance for currently known genetic introgression (e.g., S. haematobium and S. intercalatum) or more critically as yet unknown interactions (e.g., amongst various S. haematobium-hybrids themselves with or without S. haematobium or S. mansoni couplings).In contrast with experimental schistosomiasis [9], in which laboratory manipulation and direct dissection of adult worms can provide a sentry insight into interspecific pairings and hybridization potentials, our understanding of worm-pairing dynamics in the human host is entirely inferred by genotyping schistosome retrieved ova and/or miracidia [2].