Reduced efficacy of ivermectin and moxidectin against Parascaris spp. in foals from Argentina.

This research article presents a study that found suboptimal effectiveness of ivermectin and moxidectin, two commonly used drugs in Argentina for controlling a particular type of worm (Parascaris spp.) in horses. An alternative drug, fenbendazole, yielded 100% effectiveness.

Background

• The research focuses on how effective ivermectin and moxidectin are against gastrointestinal nematodes (a type of worm) found in horses’ digestive tracks. These two drugs are widely used in Argentina, primarily because of their broad spectrum anthelmintic properties, which means they can destroy many different kinds of parasitic worms.
• However, according to the study, the effectiveness of these two drugs against the Parascaris spp., a kind of worm infection commonly found in horses, had been largely unknown in the local population, despite certain international reports hinting at possible resistance.

Findings

• The research emerged from a case where treatment results were less-than-optimal. In February 2018, a considerable concentration of Parascaris spp. eggs was discovered in the fecal matter of foals treated with moxidectin about a month prior – this raised the suspicion of potential Parascaris spp. resistance against the treatment.
• The researchers conducted a fecal egg count reduction test in two groups of 12 foals each – one group was given ivermectin, the other moxidectin. The count reduction was found to be suboptimal in both cases, at 48.1% for moxidectin and an even lower 34.8% for ivermectin. Additionally, 25% of the animals experienced a higher egg count after the treatment.

A Better Alternative

• Five days after the aforementioned treatment, the animals were treated with fenbendazole, a different anthelmintic drug. This time, the egg count reduction was a complete 100%, indicating that this drug might serve as a more effective alternative against Parascaris spp. resistance.

Implications and Need for Further Monitoring

• The reduced efficacy of ivermectin and moxidectin against Parascaris spp. necessitates a reevaluation of the current control programs for parasitic worms in horses in Argentina.
• The study concludes that every establishment should monitor the status of susceptibility or resistance to design control programs that hinge on rational, sustainable use of anthelmintics, thus allowing for more effective control of parasitic worm infections in the long run.