Shortened strongylid egg reappearance periods in horses following macrocyclic lactone administration – The impact on parasite dynamics.

The research investigates the impact of shortened strongylid egg reappearance periods (ERPs) on the effectiveness of anthelmintic drugs ivermectin and moxidectin in horses. Using computer simulations, it found a significant increase in parasite egg output and larvae, and a bigger impact on the performance of moxidectin as compared to ivermectin.

Background

• The study explores the recent reports that suggest a shortening of ERPs for anthelmintics, specifically, ivermectin and moxidectin used in horse treatment. Previously, ERPs of these drugs ranged from 8-10 weeks for ivermectin and 12-16 weeks for moxidectin. However, recent studies found that these periods have reduced to about 4-5 weeks. This reduction could lead to a significant impact on parasite control.

Methodology

• A computer simulation model was used to evaluate the impact of shortened ERPs on the anthelmintic performance of these drugs against equine parasites.
• The original ERPs were set at 7.1 weeks for ivermectin and 15.4 weeks for moxidectin, whereas the reduced ERP was set at 4.6 weeks for both.
• Multiple simulations were run, comparing the results between original and reduced ERP scenarios.
• Two different treatment scenarios (2 and 4 treatments annually), two different age groups (yearlings and adults), and four different climates (cold humid continental, temperate oceanic, humid subtropical, and hot/cold semi-arid) were also analyzed in the study.

Results

• Across all the simulations, there was a substantial increase in the parameters: strongylid egg output, infective third stage larvae on herbage (L3h), and encysted early third stage larvae (EL3).
• The study observed a 100-300% increase in these parameters with the use of ivermectin across all the climates, age groups, and treatment intensities.
• For moxidectin, the increase ranged from a few hundred to several thousand percent with the most significant increase observed for L3h in the two cooler climates, reaching as high as 6727%.
• The loss of anthelmintic performance was much higher with moxidectin compared to ivermectin. This loss was influenced by the climate and treatment intensity but was not affected by the age of the horse.

Conclusion

• This is the first study to evaluate the consequences of shortened ERPs in horses and demonstrated a significant loss in anthelmintic performance due to this development.
• The research results highlight that measuring anthelmintic efficacy is more than just reducing fecal egg counts at 14 days post treatment.
• The study concludes that despite the fecal egg count reduction tests (FECRTs) remaining at 100%, significant anthelmintic performance can be lost.