Parascaris univalens—a victim of large-scale misidentification?

Abstract

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.

Evidence‐based considerations for control of Parascaris spp. infections in horses

“…One abattoir-based prevalence survey conducted in Italy in the 1970s indicated an overwhelming dominance of P. univalens among more than 2000 investigated specimens (Bullini et al 1978). A recent karyotype investigation of worm specimens and ascarid eggs obtained from faecal samples collected from foals in Central Kentucky identified only P. univalens (Nielsen et al 2014a). Further, a recent study of the population structure among about 200 equine ascarid specimens collected in Sweden, Norway, Germany, Iceland, Brazil and USA concluded that all specimens were genetically homogenous (Tyden et al 2013), and one of the US isolates collected for this study was later identified to be P. univalens.…”

“…A recent karyotype investigation of worm specimens and ascarid eggs obtained from faecal samples collected from foals in Central Kentucky identified only P. univalens (Nielsen et al . ). Further, a recent study of the population structure among about 200 equine ascarid specimens collected in Sweden, Norway, Germany, Iceland, Brazil and USA concluded that all specimens were genetically homogenous (Tyden et al .…”

“…As the name implies, P. univalens has only one chromosome pair, whereas P. equorum has 2 (Goday and Pimpinelli 1986). Contrary to the general assumption, available evidence now suggests that P. univalens is the main equine ascarid, whereas P. equorum is rarer (Nielsen et al 2014a). One abattoir-based prevalence survey conducted in Italy in the 1970s indicated an overwhelming dominance of P. univalens among more than 2000 investigated specimens (Bullini et al 1978).…”

Control of helminth parasites in juvenile horses

“…Recent work suggests that the predominant equine ascarid species is Parascaris univalens and not Parascaris equorum, as commonly assumed (Nielsen et al . ). Even foals of the youngest age group are likely to ingest ascarid eggs as they suckle the mare and explore their immediate environs.…”

Monitoring the avermectin and pyrantel resistance status of nematode parasites of horses in the Czech Republic

“…The ascarid is most often found in the small intestine of foals and less commonly in weanlings and young horses. Slow growth, weight loss, cough and colic are all signs of potential massive gastrointestinal infection (Reinemeyer 2009;Nielsen et al 2014). Cyathostomins are found in the large intestines of horses; aggregation of the developmental stages in the mucosa of the large intestine can lead to clinically apparent disease which manifests itself as diarrhoea, hypoproteinaemia, subcutaneous oedema and metabolic acidosis (Lyons et al 2000).…”