146 Fecal egg count reduction in juvenile and mature horses administered fenbendazole or moxidectin/praziquantel dewormer

Parasite resistance is a concern in horses and can be monitored by performing fecal egg counts (FECs). Unfortunately, FECs are not performed regularly, which could contribute to developing anthelmintic resistance as it may lead to deworming too frequently or not using an effective dewormer for the horse. This study aims to evaluate the ability of pellet and paste fenbendazole (FBZ) and a paste moxidectin/praziquantel (MOX/PZQ) dewormer to reduce parasite populations in juvenile and mature horses in River Falls, Wisconsin. To evaluate dewormers, horses with an initial FEC ≥50 eggs per gram (EPG) were used. For FBZ, horses were blocked by shedding level as low ( 500 EPG) shedders and assigned to the pellet (n = 21) or paste (n = 23) FBZ treatment. Two-weeks post-treatment, another FEC was taken and a fecal egg count reduction (FECR) was calculated. Eight-weeks following this study, another FEC was performed and horses with ≥50 EPG were administered a MOX/PZQ paste (n = 46). Two-weeks post-treatment, another FEC was taken and subsequent FECR was calculated. Results from this study indicated no differences were observed for horses receiving pellet or paste FBZ with a FECR of 57.4% and 65.7%, respectively (P ≥ 0.05). Additionally, no difference in the FECR for juvenile versus mature horses or shedding level was observed regardless of the dewormer used (P ≥ 0.05). While both FBZ and MOX/PZQ reduced FECs when comparing the pre- and post-treatment EPG (P ≤ 0.01), the average FECR was 60.4% and 98.5% for horses receiving FBZ and MOX/PZQ, respectively. These results demonstrate parasite resistance to FBZ was observed in this herd while MOX/PZQ was still effective in reducing FECs.