Anthelmintic resistance of intestinal nematodes to ivermectin and pyrantel in Estonian horses.

This research paper addresses a study carried out in Estonia regarding the resistance of horse intestinal parasites, namely nematodes, to commonly used anthelmintic treatments, specifically ivermectin and pyrantel. The results found high rates of resistance, indicating a need for revised treatment strategies.

Study setup

• The researchers selected 41 horses for the study, all of whom were either trotting or riding horses under three years of age.
• These horses were housed in four different stables in Estonia.
• To identify the horses in need of treatment, faecal samples were collected and analysed. The horses with nematode egg counts exceeding 200 per gram of faeces were selected to receive treatment.

Treatment Process

• 32 of the participants, all of whom were shedding eggs of a nematode known as strongyles, were treated using the anthelminthic drug pyrantel.
• Those animals who tested positive for the presence of Parascaris equorum nematodes were treated with a different anthelminthic, ivermectin.
• Ultimately, almost 80% of the selected horses ended up requiring anthelminthic treatment.

Efficacy Evaluation

• The effectiveness of the treatments was evaluated by comparing the nematodes’ egg counts in horse faeces before and after treatment.
• The researchers noted resistance to ivermectin in half of the horses treated for P. equorum, and resistance to pyrantel treatment in 27% of the horses with strongyles.
• For the horses suffering from strongyle infection, ivermectin managed to achieve a 100% reduction, while pyrantel only achieved an 88% reduction.
• Meanwhile, for the P. equorum infections, ivermectin attained an 89% reduction.

Potential Causes for Resistance

• The paper highlights that this study is the first to indicate resistance towards pyrantel, and further highlights the need for additional studies into ivermectin resistance.
• According to questionnaires completed by the horse owners, lack of evidence-based treatment strategies, an over-reliance on ivermectin, and potentially inaccurate evaluation of horse body weight may be contributing factors to the identified resistance problems.