Cylicocyclus species predominate during shortened egg reappearance period in horses after treatment with ivermectin and moxidectin.

The normal time interval between treatment of horses with a macrocyclic lactone (ML) and reappearance of strongyle eggs in the feces, or 'Egg Reappearance Period (ERP)', is at its shortest 8 weeks for ivermectin (IVM) and 12 weeks for moxidectin (MOX). Nowadays, it is not uncommon to find shorter ERPs, potentially indicating the beginning of the development of drug resistance. Whether all cyathostomin species contribute equally to a shortened ERP is not known. In the present study a Reverse Line Blot (RLB) on individual infective larvae was used to compare species composition before and after ML-treatment in horses with either a normal or a shortened ERP. Species composition was determined for a total of 21 horses on 4 farms and was based on differentiation of approximately 40 larvae per horse per sampling day. After IVM treatment the shortest ERP was found to be 28-42 days. After MOX treatment the shortest ERP was found to be 42-56 days. The RLB showed that early egg shedding was dominated by species of the genus Cylicocyclus (Cyc.) (Cyc. ashworthi, Cyc. insigne, Cyc. leptostomum and Cyc. nassatus). Before treatment, species composition was generally more diverse with species from several other genera also present. Interestingly, on the farm with the overall shortest ERP and where all horses showed a shortened ERP, more species/genera were present during early egg excretion than on other farms. Results suggest that cyathostomin species within the genus Cylicocyclus contribute to a shortened ERP more than species from other genera. However, if shortening of ERP overall becomes more pronounced and widespread within a farm, species from other genera will also be present during early egg shedding.