Egg reappearance periods associated with anthelmintic treatments given to horses in winter and summer over two years.

This research examines the cycle of egg reappearance following anthelmintic treatments in horses, exploring potential seasonal variations and the possible implications for the development of drug resistance. The efficacy and egg reappearance period of several common deworming drugs were tested over two years in different seasons.

Study Design and Methodology

• The study was conducted over two years on a single herd of horses.
• The effects and egg reappearance periods (ERP) of four commonly used anthelmintic (deworming) drugs – ivermectin, moxidectin, abamectin, and fenbendazole – were assessed.
• The study involved administering the drugs during winter and summer and then observing the number of parasite eggs in fecal samples.
• The researchers tracked egg counts before and for 6-7 weeks after treatment to determine the point when counts returned to at least 10% of pre-treatment values.

Results

• Treatment with ivermectin, moxidectin, and the first abamectin product tested resulted in a >99% reduction in egg counts for 4 weeks after treatment.
• The egg reappearance period (ERP) for these three treatments was 5-7 weeks, with little variation among the treatments.
• A possible pattern of more rapid egg reappearance in summer was observed, as well as an accelerated egg reappearance in the second year of testing compared to the first.
• The second abamectin product and the fenbendazole were found to be ineffective, with immediate post-treatment egg count reductions of 68% and 52%, respectively.

Implications and Conclusions

• The apparent shortening of ERP could signal an early warning of the development of anthelmintic resistance in cyathostomins, a common intestinal parasite in horses.
• In this scenario, the anthelmintic would still be effective against adult, egg-laying parasites but less effective against later larval stages, leading to an earlier resumption of egg shedding after treatment.
• The seasonal and yearly variations in ERP may have significant implications for the development of sustainable and effective deworming protocols.
• The findings also highlight the variability in effectiveness between different deworming drugs, with potential implications for their use in managing horse parasites.