A possible origin population of pathogenic intestinal nematodes, Strongyloides stercoralis , unveiled by molecular phylogeny

Humans and dogs are the two major hosts of Strongyloides stercoralis , an intestinal parasitic nematode. To better understand the phylogenetic relationships among S . stercoralis isolates infecting humans and dogs and to assess the zoonotic potential of this parasite, we analyzed mitochondrial Cox1, nuclear 18S rDNA, 28S rDNA, and a major sperm protein domain-containing protein genes. Overall, our analyses indicated the presence of two distinct lineages of S . stercoralis (referred to as type A and type B). While type A parasites were isolated both from humans and dogs in different countries, type B parasites were found exclusively in dogs, indicating that the type B has not adapted to infect humans. These epidemiological data, together with the close phylogenetic relationship of S . stercoralis with S . procyonis , a Strongyloides parasite of raccoons, possibly indicates that S . stercoralis originally evolved as a canid parasite, and later spread into humans. The inability to infect humans might be an ancestral character of this species and the type B might be surmised to be an origin population from which human-infecting strains are derived.