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

In the present study, an anthelmintic treatment regimen with reduced treatment frequency was evaluated in horses on two study sites in Belgium during three consecutive summer pasture seasons. Historically, the horses on both study sites were treated up to 6 times a year with ivermectin (IVM) or up to 4 times a year with moxidectin (MOX), and previous efficacy evaluations indicated a reduced egg reappearance period in some of the treated horses for both IVM (28 days) and MOX (42 days). In the present study, all horses were treated with IVM or MOX in the spring and in autumn. Faecal egg counts (FEC) were conducted every two weeks during the summer pasture season and whenever the individual FEC exceeded 250 eggs per gram of faeces, the specific horse was treated with pyrantel embonate. No increase in parasitic disease over the three-year period of the study was observed. The FEC data collected in the study as well as the age of the animals and local weather data were then imported into a cyathostomin life-cycle model, to evaluate long term effects of the newly applied treatment regimen on the selection pressure for anthelmintic resistance, and compare to the previous high frequency treatment regimen. The model simulations indicated that the whole-herd treatment regimen with at least 4 macrocyclic lactone treatments annually led 2-3 times faster resistance development than any of the alternative treatment regimens evaluated under the specific conditions of these two study sites. Further lowering the treatment frequency or applying even more selective treatments enhanced the delay in resistance development, but to a lesser extent.