Three-year study to evaluate an anthelmintic treatment regimen with reduced treatment frequency in horses on two study sites in Belgium.

This research looked to see if fewer treatments with certain anti-worming medications (anthelmintics) would be as effective as regular treatments in managing parasitic disease in horses in Belgium over three years. The study also estimated the impact of lower treatment frequencies on the development of drug-resistance in parasites using a mathematical model.

Research Context

• Horses on these study sites in Belgium had historically been treated with Ivermectin (IVM) or Moxidectin (MOX), two types of anthelmintics used to combat parasitic infections. Previously, the horses had been treated up to six times a year with IVM or up to four times a year with MOX. However, there was evidence that parasites were beginning to become resistant to these drugs, reducing the time it took for eggs to reappear post-treatment.
• The researchers wanted to see if less frequent treatments of these drugs, combined with a third anthelmintic, Pyrantel embonate, could effectively manage parasitic loads in the horses.

Methodology

• In the present study, all horses were treated with IVM or MOX in the spring and autumn, reducing the treatment frequency to twice a year. Then, every two weeks during the summer pasture season, the horses’ faecal matter was examined for the presence of parasitic eggs. If a sample contained more than 250 eggs per gram, the horse was treated with Pyrantel embonate.
• Alongside the field study, a mathematical model was constructed to estimate the impact of the altered treatment frequency on the development of anthelmintic resistance in the parasites. The model included data such as local weather conditions, horse ages, and faecal egg count data collected during the study.

Findings

• The lowered treatment frequency did not increase the occurrence of parasitic disease among the investigated horse population over the study’s three-year duration.
• The mathematical model suggested that whole-herd treatment with at least four annual treatments of IVM or MOX led to 2-3 times faster resistance development, compared to lower frequency or more selective treatment strategies under the conditions observed at the study sites.
• Although reducing the treatment frequency or using selective treatment strategies helped delay resistance development, the effect was less substantial than would be ideal.

Conclusion

• The results of the study suggest that reducing the frequency of treatments with anthelmintics can manage parasitic diseases in horses while slowing the development of drug-resistant parasites. Nevertheless, more comprehensive strategies may be required to significantly curb resistance growth in parasites.