Dynamic community of strongyle nematodes and ivermectin resistant evidence of domesticated horses in thailand

Strongylids are a group of parasitic nematodes that infect horses and other equids worldwide, with a potential negative impact on their health and performance. There is no available research that has studied strongylids infection in domesticated horse populations nor assessed the effectiveness of ivermectin in treating these parasites in Thailand. This thesis, therefore, aimed to comprehensively investigate strongyle parasite intensity and factors influencing egg shedding patterns. Additionally, it utilized nemabiome metabarcoding approach to identify strongylid species, analyse ecological diversity, and assess co-occurrence patterns within the studied horse populations. Furthermore, it employed faecal egg count reduction (FECR) test to assess the efficacy of ivermectin treatment and determine the strongylid egg reappearance period (ERP). The study found that 50.98% (95% CI: 46.13 - 55.83) of the total 408 examined horses tested positive for strongyle infection, with a mean intensity of 445.67 ± 639.58. Remarkably, only 25.74% of these horses had faecal egg counts of ≥ 200 eggs per gram (EPG), suggesting a priority for implementing selective deworming programs. Significant differences in parasite prevalence were found in yearling group (p-value = 0.001) and for horses housed in outdoor pastures (p-value = 0.0001). Nemabiome metabarcoding identified 14 strongylid species in the examined horse populations, dominated by Cylicostephanus longibursatus and Cylicocyclus nassatus, without any evidence of Strongylus species. Diversity analysis revealed that nemabiome species composition within horse groups (alpha diversity) showed a non-significant difference in terms of species richness and evenness. Beta diversity analysis, conversely, revealed a significant difference (p-value = 0.004) among horse population. This dissimilarity between communities is mainly driven by Cylicostephanus longibursatus, Cyathostomum pateratum, and Cylicostephanus calicatus, which together contribute to over 60% of the variation. Species co-occurrence patterns were dominated by random co-occurrences. The study demonstrated that ivermectin successfully eliminated adult strongyle parasites within two weeks post-treatment. However, the ERP was shortened to 6 weeks post-treatment with a mean FECR of 70.4% (95% CI 46.1–84.0), indicating a change in drug performance and the potential emergence of resistance. In addition, nemabiome metabarcoding revealed that strongyle species have different levels of susceptibility in response to ivermectin treatment. The reduction in ERP was associated with the early reappearance of specific species, dominated by Cylicostephanus longibursatus and Cylicocyclus nassatus, indicating the lower susceptibility of these species. In contrast, Poteriostomum imparidentatum, Triodontophorus nipponicus, and Triodontophorus serratus were not found post-treatment, indicating the high level of susceptibility of these species. This study has provided a practical and applicable understanding of the current status of strongylid infection in Thai horse populations, serving as a valuable tool for future research and informing effective management strategies