Using a computer model, researchers from the University of Kentucky have been able to predict the course of anthelmintic resistance in small strongyles over the next 40 years.

Incorporating the life-cycle of the cyathostomin species of small strongyles as well as weather station data, the computer model allowed the team to run simulations evaluating the use of the anthelmintic (dewormer) ivermectin, in a variety of circumstances.

The purpose of the study was to determine whether selective (targeted) deworming programs (where only horses that shed a high number of parasite eggs are treated) can slow down resistance to anthelmintic drugs in small strongyles.

To start, the impact of time of the year was evaluated when a single ivermectin treatment was administered once, in any of the 12 months. The next simulations looked at the effects of treatment intensity, varying between two and six treatments per year. Finally, they compared treatment schedules consisting of a combination of traditional strategic treatments administered to all horses and additional treatments administered to high shedders.

They found that the month of treatment had a large effect on resistance development in colder climates, but little or no impact in subtropical and tropical climates. Resistance development was affected by treatment intensity, but was also strongly affected by climate. Selective therapy delayed resistance development in all modelled scenarios, but, again, was climate dependent, with the largest delays observed in the colder climates.