Anthelmintic therapy of equine cyathostomin nematodes – larvicidal efficacy, egg reappearance period, and drug resistance.

The research is about studying the efficacy of two drugs, moxidectin and fenbendazole, in treating cyathostomin nematodes in horses. The study also sought to identify the species of this parasite associated with the fast reappearance of eggs after treatment and those showing decreased susceptibility to the medications tested.

Research Methodology

• Two groups of 18 ponies each were administered either the drug moxidectin (0.4 mg/kg) or fenbendazole (10 mg/kg daily for five consecutive days).
• Half of the ponies in each group were euthanized after 2 weeks for examination, while the remainder were necropsied after 5 weeks.
• The number of luminal and mucosal worms in the ponies, as well as the count of strongyle eggs, were studied at weekly intervals.

Treatment Efficacy

• After two weeks, fenbendazole treatment reduced the count of early L3s (a larval stage of the parasite) by 50.4%, while moxidectin reduced it by 73.8%. At five weeks, the respective decrease recorded was 51.3% and 71.8%.
• Two-week efficacies against nematode larvae in later stages (LL3s/L4s) were measured as 70.8% for fenbendazole and 74.6% for moxidectin. At five weeks, low numbers were detected across all three groups.
• The counts of mucosal worms, which inhabit the mucus secreting surface of the gastrointestinal tract, showed no significant difference among the treatment groups.
• Worms in the ponies’ lumina (cavity within tubular structures) were reduced by 93.2% for fenbendazole and by 98.3% for moxidectin at two weeks post-treatment, with adult counts in the moxidectin group being significantly lower. However, the counts in both groups increased three weeks later.

Species Associated with Fast Egg Reappearance and Reduced Drug Susceptibility

• The study identified species that survived in the ponies’ lumina despite treatment, contributing to a shortened egg reappearance period (ERP) of four weeks post-treatment with moxidectin. These species were Cyathostomum catinatum, Cylicostephanus longibursatus, Cylicocyclus ashworthi, and Cylicocyclus nassatus.
• The larvicidal efficacy of fenbendazole was reported to be lower than historical standards, indicating an increased resistance of the cyathostomins to this drug. The study also found survival of luminal immature stages (L4) following moxidectin administration, suggesting potential resistance to this drug as well.