Horse health by numbers
Journal: Equine Health
Summary
Psoroptic mange, caused by the mite *Psoroptes ovis*, represents a significant welfare and economic concern across equine, cattle and sheep populations, yet treatment options are becoming compromised as resistance to macrocyclic lactone acaricides emerges. Belgian researchers sequenced full chromosomal genomes from both susceptible and resistant mite populations and measured gene expression before and after ivermectin exposure to identify the genetic mechanisms underpinning treatment failure. The resistant mites showed constitutive overexpression of a cytochrome P450 (CYP) detoxification gene alongside UDP-glucuronosyltransferase (UGT) genes, with copy number variation at these loci clearly associated with resistance status—the CYP gene was absent in susceptible populations but present in multiple copies across all resistant strains, suggesting active selection pressure. Importantly, the researchers identified two distinct amplification mechanisms at work: the CYP gene showed sharp population-level differentiation (high Fst values) indicating classical resistance evolution, whilst UGT genes were massively amplified in both susceptible and resistant populations with weaker differentiation, suggesting a background variation unrelated to resistance. Understanding these specific genetic mechanisms enables more targeted approaches to resistance management, including potential diagnostic screening before treatment and informed selection of alternative acaricide classes where cross-resistance patterns differ, ultimately informing your clinical decision-making when psoroptic mange presents in equine patients.
Read the full abstract on PubMed
Practical Takeaways
- •Ivermectin resistance in psoroptic mange is driven by specific genetic amplifications; monitor treatment efficacy closely and consider alternative acaricides if MLs fail
- •Different resistance mechanisms (CYP versus UGT amplification) may require different management strategies—genetic testing of mite populations could help guide treatment selection
- •Rotational use of non-ML acaricides and integrated pest management may help slow the spread of resistance alleles in psoroptic mange populations
Key Findings
- •Cytochrome P450 monooxygenase gene is amplified in all resistant P. ovis populations but not in susceptible populations, showing strong selection pressure
- •UDP-glucuronosyltransferase genes are massively amplified in all populations including susceptible strains, indicating a distinct resistance mechanism
- •Gene expression analysis revealed constitutive over-expression of CYP and UGT genes in resistant mites before and after ivermectin exposure
- •Distinct mechanisms of amplification and gene regulation operate at different resistance loci in P. ovis populations across Belgium