Symbiotic bacteria are associated with ivermectin resistance in the parasitic nematode Haemonchus contortus

Abstract Anthelmintic resistance in gastrointestinal nematodes poses a severe threat to global livestock production and sustainability. While the genomic adaptations of parasites have been extensively characterized, the role of the parasite-associated microbiome in modulating drug susceptibility remains largely unexplored. In this study, we investigated the symbiotic microbiota of ivermectin-resistant and ivermectin-susceptible strains of the blood-feeding parasitic nematode Haemonchus contortus and assessed its potential association with ivermectin resistance. Comparative microbiome analysis revealed Stenotrophomonas maltophilia as a potential resistance biomarker, with a 3.4-fold higher relative abundance in resistant strains. Furthermore, functional prediction analysis revealed that pathways related to genetic information processing and cell motility were significantly enriched in the microbiota of resistant strains. Larval mortality assays demonstrated that gentamicin treatment significantly increased the susceptibility of the larvae to ivermectin. These findings suggest that symbiotic bacteria are correlated with ivermectin resistance in Haemonchus contortus , possibly in combination with genetic factors of the parasite.