Evaluating efficacy and strongyle egg reappearance period after ivermectin treatment in a Hungarian stud farm.

Research Overview

• This study assessed how effective ivermectin is at treating strongyle worm infections in horses and measured how long after treatment worm eggs reappear in horse feces on a Hungarian stud farm.

Background

• Strongyle infections, particularly caused by small strongyles known as cyathostomins, are a widespread health risk in horses worldwide.
• Routine monitoring of anthelmintic (wormer) effectiveness is critical, as resistance can develop with frequent treatments.
• Previous studies established egg reappearance periods (ERPs), which indicate the time it takes for worm eggs to be detectable again following treatment. Changes in ERP can signal developing resistance or reduced drug efficacy.
• Prior to this study, no data existed on ivermectin efficacy and ERP in Hungarian horses.

Study Design and Methods

• Researchers collected fecal samples from 57 Thoroughbred mares at a Hungarian National Stud Farm.
• Samples were collected before ivermectin treatment and at 2, 4, 5, 6, and 7 weeks after treatment to monitor egg counts over time.
• Fecal Egg Counts (FECs) were measured using the Mini-FLOTAC technique, a sensitive method for detecting parasite eggs in feces.
• The Fecal Egg Count Reduction (FECR) percentage was calculated 2 weeks post-treatment using a Bayesian hierarchical model. This statistic indicates the drug’s effectiveness by comparing egg counts before and after treatment.
• Strongyle egg reappearance period (ERP) was estimated according to established guidelines, identifying how many weeks pass before eggs are detectable again post-treatment.
• Data were analyzed overall and also segmented into two age groups (younger and older mares) to check if age influenced ERP.

Main Findings

• FECR analysis showed no evidence of ivermectin resistance in the strongyle populations at the stud farm, indicating the drug was still effective at the time of the study.
• The estimated ERP was 6 weeks, which is shorter than historically reported ERPs for ivermectin, where egg reappearance often took longer.
• A notably longer ERP was observed in the older mares (ages 12-20), suggesting age or host factors might influence how quickly worms recolonize after treatment.
• The reduction in ERP over time is considered a sign of decreasing drug performance, likely related to intensive deworming practices worldwide that exert selection pressure on parasites.

Implications and Recommendations

• The shortening ERP signals a potential decline in ivermectin’s long-term effectiveness due to widespread treatment protocols.
• Regular monitoring using fecal egg count-based strategies is recommended to better manage worm infections and preserve efficacy of anthelmintics.
• Implementing targeted deworming based on fecal egg counts rather than routine blanket treatments can help reduce infection pressure on pastures and slow resistance development.
• This study provides the first Hungarian evidence on ivermectin efficacy and ERP, serving as a baseline for future monitoring programs in the country.

Conclusion

• Ivermectin remains effective against strongyles on this Hungarian stud farm, but the observed shortening of the ERP highlights the need for sustainable parasite control practices to maintain drug efficacy.
• Using fecal egg count-based management can optimize deworming timing and intensity, helping combat the global challenge of anthelmintic resistance.