Anthelmintic treatment in horses: the extra-label use of products and the danger of under-dosing.

The research evaluates the effectiveness of various anthelmintic drugs, used to control worms in horses, on a stud farm in South Africa. It particularly focuses on the performance of moxidectin in two different dosages and formulations, documenting the results over two consecutive years.

Objectives and Methodology

• The primary aim was to assess the effectiveness of four different anthelmintic treatments (doramectin, pyrantel pamoate, ivermectin, and moxidectin), hoping to gain insights on worm egg reappearance period and worm resistance.
• The study was conducted on a Thoroughbred stud farm located in South Africa’s Western Cape Province.
• Two formulations of moxidectin were tested using their recommended doses – an injectable form not registered for horses (0.2 mg/kg) and an oral gel authorised for horses (0.4 mg/kg).
• Two mixed-sex groups of horses – 30 yearlings in 2001 and 40 weaners in 2002, were tested. These were further subdivided into smaller groups.
• The faecal egg count reduction test was employed to determine the efficacy of each anthelmintic in both years. Each horse acted as its control, and mean faecal egg count and lower 95% confidence limit were determined for each group.

Findings & Results

• In 2001, moxidectin (0.4 mg/kg, oral administration) caused a 100% reduction in faecal egg counts and also recorded a 100% lower 95% confidence limit, indicating effective anthelmintic performance.
• In 2002, pyrantel pamoate and ivermectin (19 and 0.2 mg/kg respectively, oral administration) led to a 99% and 96% reduction in the worm egg count.
• Unexpectedly, doramectin and moxidectin (administered intramuscularly at 0.2 mg/kg, the dosage registered for other species) had no significant impact on worm egg counts in 2002.
• Among all four drugs tested in 2002, only pyrantel pamoate recorded lower confidence limits greater than 90%.

Conclusions

• The study affirms the effectiveness of certain anthelmintics in controlling worm infections in horses, subject to correct dosage and administration method.
• However, it emphasises the potential for drug resistance, indicating the need for regular reevaluation of these products’ effectiveness.
• Importantly, the study highlights the potential dangers of extra-label usage of anthelmintics (specifically doramectin and injectable moxidectin), which were seen to be ineffective at registered dosages for other species.