A mixed amplicon metabarcoding and sequencing approach for surveillance of drug resistance to levamisole and benzimidazole in Haemonchus spp.

Chromosomal genome assembly resolves drug resistance loci in the parasitic nematode Teladorsagia circumcincta

“…A single peak on the X chromosome unique to the S inbred :RS 3 comparison contained the acetylcholine receptor acr-8 (TCIR_10183160) (peak range: 1.5-5 Mb; gene position: 1.830-1.857 Mb), previously identified as associated with levamisole resistance (Figure 1B). In H. contortus, the S168T variant of acr-8 has been strongly implicated as being causal, having been functionally validated (34) and found in several levamisole-resistant populations around the world (16,35,36); here, we identify the homologous variant S167T found at high frequency in RS 3 (tci2_wsi3.0_chr_X, pos: 1,846,323, freq S167T = 96.4%). This variant was, however, not found in the remaining levamisole-resistant isolates; the only other sample to contain it in a single sequencing was Farm 2 (freq S167T = 2%).…”

Functional validation of novel levamisole resistance marker S168T in Haemonchus contortus

“…Recently, a forward genetic cross between multidrug-resistant and drug-susceptible strains of H. contortus revealed a non-synonymous single nucleotide polymorphism (SNP) in acr- 8 that was strongly associated with LEV resistance ( Doyle et al, 2022 ). This SNP conferred a serine-to-threonine substitution (S168T) in exon 4, which was present in all LEV-resistant populations examined to date (including USA, South Africa, and Australia) and absent in all sensitive populations with available sequencing data ( Sallé et al, 2019 ; Antonopoulos et al, 2022 ; Francis et al, 2024 ). Furthermore, a serine-to-threonine substitution was present at the analogous position of acr-8 in LEV-resistant T. circumcincta after re-analysis of existing data ( Choi et al, 2017 ; Doyle et al, 2022 ).…”