Comparison of morphological and molecular Strongylus spp. identification in equine larval cultures and first report of a patent Strongylus asini infection in a horse.

Background: Surveillance of Strongylus vulgaris and other Strongylus spp. in equids is important for targeted intervention in parasite control, requiring reliable routine diagnostic methods.

Objectives: Comparing morphological examination and PCR analyses of larval cultures to identify Strongylus spp. species based on German diagnostic samples from 2018.

Study Design: Method comparison.

Methods: During the routine diagnostic investigations, in total 712 strongyle-egg positive equine faecal samples were cultured. Third-stage larvae (L3) were morphologically differentiated. For molecular validation, samples were examined using S. vulgaris real-time PCR and Strongylus edentatus/Strongylus equinus/Strongylus asini high-resolution melting PCRs.

Results: Based on 28S rRNA PCR, 594 samples positive for nematode DNA were included in the study. The inter-rater reliability to compare morphological and molecular species identification was fair for Strongylus spp. without species identification and for S. edentatus, slight for S. equinus and poor for S. vulgaris. The frequency based on morphological and molecular data in this study were for S. vulgaris 0% and 0.8%, respectively, for S. edentatus 0.3% and 1.5%, respectively, and for S. equinus 2.0% and 0.2%, respectively. Based on molecular analyses, one sample obtained from a domestic horse contained S. asini DNA, which was confirmed by sequencing.

Main Limitations: For many samples, no or only incomplete data regarding clinical history, the exact geographical location and whether samples were obtained on individual or farm level, were available.

Conclusions: Results of morphological and molecular examination methods of strongyle L3 from equine samples can differ substantially. Further evaluation of these methods is required to provide reliable and cost-effective methods of screening equine parasites. Further studies using approaches suitable to detect S. asini are needed to evaluate its clinical and epidemiological relevance.