Hidden in plain sight - Multiple resistant species within a strongyle community

Area of Science:

• Veterinary Parasitology
• Molecular Epidemiology
• Antimicrobial Resistance

Background:

• Ovine parasitic gastroenteritis is managed with anthelmintics, but resistance is a significant challenge.
• Teladorsagia circumcincta is a primary pathogen in temperate regions, often exhibiting resistance.
• Accurate diagnosis of anthelmintic resistance is crucial for effective parasite control in sheep.

Purpose of the Study:

• To investigate benzimidazole and ivermectin resistance in sheep parasites on a commercial farm.
• To assess the impact of parasite species diversity on the diagnosis of anthelmintic resistance.
• To correlate anthelmintic resistance test results with the genetic basis of resistance in Teladorsagia circumcincta.

Main Methods:

• Faecal egg count reduction tests (FECRT) and in vitro bioassays were used to determine anthelmintic efficacy.
• PCR speciation was employed to identify parasite species composition before and after treatment.
• Pyrosequencing of the beta-tubulin isotype-1 gene was used to quantify benzimidazole resistance-associated genotypes in T. circumcincta.

Main Results:

• FECRT indicated moderate resistance to albendazole (65% efficacy) and ivermectin (77% efficacy).
• PCR speciation revealed that susceptible species masked a highly resistant T. circumcincta population, which became dominant post-treatment.
• Pyrosequencing identified a high frequency and diversity of benzimidazole resistance mutations in T. circumcincta, even after ivermectin treatment.

Conclusions:

• Parasite species diversity can significantly impact the diagnosis of anthelmintic resistance in sheep.
• Dual-resistant Teladorsagia circumcincta populations can be strongly selected by common anthelmintic treatments.
• Effective parasite control strategies must account for species composition and the genetic basis of resistance.