Parascaris univalens—a victim of large-scale misidentification?

The equine ascarid parasite Parascaris equorum is well known as a ubiquitous parasite infecting foals. A sibling species, Parascaris univalens, was first described over 130 years ago, but very little attention has been given to its existence and possible implications for anthelmintic resistance, clinical disease, or host age spectrum. P. univalens only possesses one germ line chromosome pair as opposed to two for P. equorum, but the two species are otherwise considered morphologically identical. For the present study, live worms obtained from the University of Kentucky parasitology horse herd were dissected and identified using karyotyping techniques. With no exception, all specimens (n = 30) were identified to be P. univalens. Further, the karyotyping technique was adapted to ascarid eggs derived from fecal samples and carried out on samples collected from 25 Thoroughbred foals from three farms in Central Kentucky. P. equorum was not identified among these, whereas P. univalens was found in 17 samples, with the remaining being inconclusive. The mitochondrial genome was sequenced, assembled, and annotated from one male worm identified as P. univalens, and comparison with available sequence reads labeled as P. equorum revealed only 0.16 % nucleotide differences. However, it is unlikely that the sequences available in public databases have been unequivocally identified to species level by karyotyping. Taken together, these data suggest that P. univalens is likely the main species now observed in equines and that perhaps the designation Parascaris spp. should be used unless cytological characterization has confirmed the species.