A 4 year observation of gastrointestinal nematode egg counts, nemabiomes, and the benzimidazole resistance genotypes of Teladorsagia circumcincta on a Scottish sheep farm.

Anthelmintic resistance threatens the sustainability of sheep production globally.
Advice regarding strategies to reduce the development of anthelmintic resistance incorporates the outcomes of modelling exercises. Further understanding of gastrointestinal nematode (GIN) species diversity, and population dynamics and genetics (which may vary between species) is required to refine these models; and field studies combining faecal egg outputs, species composition and resistance genetics are needed to calibrate them. In this study, faecal samples were taken from ewes and lambs on a commercial farm in south-east Scotland at approximately 3 to 4 week intervals between spring and autumn over a period of 4 years. Faecal egg counts (FECs) were performed on these samples, and third stage larvae (L 3 ) were collected from pooled coprocultures. Deep amplicon sequencing was used to determine both the species composition of these L 3 , and the proportions of benzimidazole (BZ) resistant SNPs in the isotype-1 β-tubulin locus of the predominant species, Teladorsagia
circumcincta L 3 . Despite consistent management throughout the study, the results show variation in GIN species composition with time and between age groups, that was potentially associated with weather conditions. The F200Y BZ resistance mutation is close to genetic fixation in the T. circumcincta population on this farm. There was no evidence of variation in isotype-1 β-tubulin SNP frequency between age groups, and no genetic evidence of reversion to BZ susceptibility, despite targeted BZ usage. This study highlights the need to include speciation when investigating GIN epidemiology and anthelmintic resistance, and serves as an example as to how genetic data may be
analysed alongside species diversity and FECs, when markers for other anthelmintic classes are identified.