Are macrocyclic lactones useful drugs for the treatment of Parascaris equorum infections in foals?

A study was carried out to assess the field efficacy of moxidectin (MOX) against Parascaris equorum in foals. A total of 70 foals from 2 Italian thoroughbred (Farms 1 and 2) stud farms and one trotter stud farm (Farm 3), aged 3-5 months and with faecal egg counts (FEC) f≥ 300 eggs per gram (EPG) were included in the study. On each farm, foals were divided into 2 groups (of 10, 8 and 10 foals, respectively) and treated with either moxidectin (MOX, 400 mcg/kg b.w.) or ivermectin (IVM, 200 mcg/kg b.w.) on day 0 and examined for faecal count reduction (FECR) on day 14. Five, 4 and 5 foals, respectively, were kept as untreated controls. An individually based estimation of efficacy was assessed by a bootstrap simulation to evaluate the percent reduction of FECRs. Two thousand bootstrap resamples were constructed from individual FECRs and the parasite population was considered susceptible for FECs ≥ 90% and 95% lower confidence limit (CL) > 90%, suspected resistant for FECRs ≥ 90% and 95% LCL < 90% and resistant when FECR < 90% and 95% LCL < 90%. On day 14, foals with FECRs categorized as resistant or suspected resistant were treated with pyrantel pamoate (PYR, 13.2 mg/kg b.w.) and examined for FECR on days 28 and 35. On day 14, MOX and IVM treatments were ineffective in all foals from both thoroughbred farms, while MOX ad IVM were effective (FECRs > 90%) in 7 and 5 trotter foals, respectively, and remained >90% until day 35. PYR treatment given on day 14 to remaining 8 foals (3 MOX-treated and 5 IVM-treated) was fully efficacious at faecal examinations carried out on days 28 and 35.