Different but overlapping populations of Strongyloides stercoralis in dogs and humans - dogs as a possible source for zoonotic strongyloidiasis

Infections of humans with the nematode Strongyloides stercoralis can persist for a very long time, due to the capacity of this pathogen to undergo an autoinfective life cycle and re-infect the same host over and over again. Clinical manifestation, known as human stongyloidiasis, may be fatal and can arise many years and generations of worms after the initial infection occurred. Although Strongyloides stercoralis has been known as the causative agent of strongyloidiasis for a very long time, some key questions about its biology and epidemiology remain open. Here we address two of them. Firstly, it has long been known that dogs can serve as experimental hosts for S. stercoralis but it is a matter of debate if Strongyloides spp. found in dogs in the wild are human pathogenic S. stercoralis, and if dogs therefore are a source of zoonotic transmission of this parasite. Here we show that dogs carry two genetically different populations of Strongyloides spp. one of which is shared with humans. This demonstrates that dogs represent a possible reservoir for zoonotic strongyloidiasis. Secondly all parasitic S. stercoralis are female and reproduce by parthenogenesis. Parasitic generations may alternate with single facultative free-living generations, which consist of females and males. In spite of the presence of both sexes, it had been postulated that males do not contribute genetic material to the progeny and that sperm are merely required to trigger parthenogenetic embryonic development. Here we show that the free-living adults of S. stercoralis reproduce sexually.