A population genetic analysis of the nematode Strongyloides stercoralis in Asia shows that human infection is not a zoonosis from dogs.

Abstract

Gut nematode worms are important parasites of people and other animals. The parasitic nematode Strongyloides stercoralis infects an estimated 600 million people worldwide, and is one of the soil-transmitted helminthiases, a WHO-defined neglected tropical disease. It has long been suggested that human S. stercoralis infection may be a zoonosis from dogs. We investigated this by whole genome sequence analysis of S. stercoralis from sympatric human and dog populations in Asia. We find that human- and dog-derived S. stercoralis have genetically distinct nuclear genomes, but we also find evidence of rare cross infection. Analysis of the S. stercoralis mitochondrial genome reveals evidence of historical introgression between human- and dog-derived parasites. Based on these data we suggest that S. stercoralis was originally a parasite of canids, that began to infect humans when people domesticated dogs, since when human- and dog-derived parasites have differentiated, but have not become separate species. Significance StatementNematode worms infect approximately a quarter of the worlds human population; Strongyloides stercoralis infects some 600 million people. It has been suggested that S. stercoralis in people and dogs is the same parasite, such that some human infection is acquired zoonotically from dogs. It is important to understand the source of human Strongyloides infection to be able to control it and the harm that it causes. Our population genomic analyses of human- and dog-derived S. stercoralis in Asia show that these parasites are distinct, but also reveal rare cross infection events. Our results are consistent with S. stercoralis ancestrally being a parasite of dogs that began infecting people when dogs became domesticated.