Comparative plasma disposition, bioavailability and efficacy of ivermectin following oral and pour-on administrations in horses.

The research evaluates the effectiveness of using pour-on, oral, and intravenous methods to administer the antiparasitic drug ivermectin in horses. It shows that the pour-on method leads to longer-lasting, but overall less absorption of the drug. This could contribute to parasite drug-resistance if the drug concentration in the horse’s system is not high enough to fully eliminate the parasites.

Research Design

• This study involved eighteen female horses, each weighing between 510-610 kg. The horses were divided into three groups receiving ivermectin through three different methods – pour-on, oral (per os), and intravenous.
• Depending on the groups they belonged to, the horses were administered doses of ivermectin paste, pour-on (designed for use in cattle), and injectable formulations at dose rates of 0.2, 0.5, and 0.2mg/kg bodyweight respectively.
• Researchers collected heparinized blood and hair samples at various times ranging from 1 hour to 40 days post-treatment. These samples underwent analysis using high-performance liquid chromatography with a fluorescence detector.
• The researchers also obtained fecal strongyle egg counts (EPG) using a modified McMaster’s technique before and at weekly intervals for 10 weeks post-treatment to assess the efficacy of the drug’s delivery method.

Findings and Implications

• The results indicated that the skin-application (pour-on) method yielded lower plasma concentration and systemic availability of ivermectin, but provided prolonged plasma persistence compared to the oral administration method.
• The oral administration method reduced the strongyle egg counts by over 95% for 10 weeks, while the pour-on method lead to reductions ranging from 82 to 97% with the pour-on group showing lower reduction levels than the per os group.
• The lower bioavailability of ivermectin in horses after pour-on administration could be attributed to degradation at the application site, cutaneous biotransformation, or binding of the drug to the horse’s hair or sebum.
• These results suggest that while pour-on administration is more convenient and lowers the risk of injury during treatment, it could lead to subtherapeutic plasma concentrations of the drug, potentially promoting the development of drug-resistant parasites.