Extensive hybridization between pig and human Ascaris identifies a highly interbred species complex infecting humans

Human ascariasis is a major neglected tropical disease caused by the nematode Ascaris lumbricoides. We report a 296 megabase (Mb) reference quality genome comprised of 17902 protein-coding genes derived from a single, representative Ascaris worm collected from 60 human hosts in Kenyan villages where pig husbandry is rare. Notably, the majority of human isolates (63/68) possessed mitochondrial genomes that clustered closer to the pig parasite Ascaris suum than to A. lumbricoides. Comparative phylogenomic analyses identified over 11 million nuclear-encoded SNPs but just two distinct genetic types that had recombined across the genomes analysed. The nuclear genomes had extensive heterozygosity and all samples existed as genetic mosaics with either A. suum-like or A. lumbricoides-like inheritance patterns supporting a highly interbred Ascaris species genetic complex. As no barriers appear to exist for anthroponotic transmission of these “hybrid” worms, a one-health approach to control the spread of human ascariasis will be necessary.