Cyathostomin species richness and alpha diversity at egg reappearance following fecal egg count-based selective ivermectin treatment.

Overview

• This study investigated how the community of cyathostomin nematode species in horses changes before and after treatment with ivermectin, an anthelmintic drug.
• Researchers used DNA sequencing techniques to assess species richness and diversity at egg reappearance following treatment and found that overall species richness and diversity remained stable despite treatment.

Background

• Cyathostomin nematodes are widespread intestinal parasites commonly found in grazing horses.
• Frequent use of anthelmintics (deworming drugs) has led to resistance issues, especially against drugs like benzimidazoles and pyrantel.
• Macrocyclic lactones (ML), such as ivermectin, are currently effective, but signs of reduced efficacy, including shorter egg reappearance periods (ERP), are emerging.
• ERP is the time period after treatment when parasite eggs start reappearing in fecal samples, indicating surviving or reinvading parasites.
• Understanding how different cyathostomin species respond to treatment is important to manage resistance and maintain treatment efficacy.

Objective

• To compare the composition of cyathostomin species in horses before ivermectin treatment and at the point of egg reappearance (ER) following treatment.
• Specifically, to assess species richness (number of species) and alpha diversity (diversity within a sample) changes associated with treatment.

Methods

• Samples were collected from 35 horses on Swedish farms before treatment and from 11 horses at egg reappearance after treatment (6-8 weeks later).
• Larvae (L3 stage) were cultured from fecal samples to extract nematode DNA.
• ITS2 rDNA metabarcoding was used to amplify and sequence DNA from the nematode community, utilizing the PacBio next-generation sequencing platform.
• Species identification relied on bioinformatic analysis of sequence variants matched against a curated ITS2 reference database and verified with BLAST.
• Statistical analyses examined species richness, alpha diversity indices (Inverse Simpson and Shannon-Weiner), and correlations with fecal egg counts (FEC) and horse age.

Key Results

• Ivermectin treatment was 100% effective in reducing fecal egg counts initially.
• No significant changes were found in species richness before treatment compared to at ER: median species numbers were 4 before treatment and 3 at ER (p = 0.71).
• Alpha diversity indices at ER did not significantly differ from pre-treatment values (Inverse Simpson p = 0.41; Shannon-Weiner p = 0.29).
• The two dominant species, Cylicocyclus nassatus and Cyathostomum catinatum, remained dominant both before treatment and at ER.
• Minor shifts in the relative abundance of less common species were observed post-treatment.
• Species richness was not significantly correlated with fecal egg count or horse age in pretreatment samples.
• The species Cylicocyclus insigne was not detected at ER following treatment, suggesting possible increased sensitivity to ivermectin or lack of early reappearance.

Conclusions

• Selective ivermectin treatment based on fecal egg counts effectively reduces parasite burden, but the diversity and composition of cyathostomin species remain largely stable at egg reappearance.
• This suggests that the community structure of adult cyathostomins is resilient to treatment and that egg reappearance may arise from surviving or quickly recovered populations of dominant species.
• The absence of Cylicocyclus insigne post-treatment could indicate species-specific responses to ivermectin, warranting further study for resistance management.
• Overall, this study enhances understanding of how equine cyathostomin populations respond at a community level to ivermectin treatment, which is critical for sustainable parasite control strategies.