Sub-optimal efficacy of ivermectin againstParascaris equorumin foals on three Thoroughbred stud farms in the Manawatu region of New Zealand

To ascertain whether resistance was present in Parascaris equorum to the macrocyclic lactone anthelmintic ivermectin, using faecal egg count reduction tests.Thirty-nine foals aged between 11 and 28 weeks on three Thoroughbred stud farms (Farms A; n=20, B; n=5 and C; n=14) were treated with ivermectin (Day 0) and faecal egg counts (FEC) were monitored before and for 21 (Farms A and B) or 14 (Farm C) days after treatment. On Farms A and B, the foals were treated with a macrocyclic lactone/benzimidazole/praziquantel combination on Day 21 and FEC assessed on Day 35. The three farms were all in the Manawatu region in the southern half of New Zealand's North Island.Of the 39 foals, 15 were not shedding P. equorum eggs on the day they were treated with ivermectin, but all 15 did so post-treatment. The FEC on Farms A and B showed no evidence of a reduction at any time between Day 7 and 21; egg output increased steadily over this period. Following combination treatment FEC were reduced by 94% on farm A and 100% on Farm B. On Farm C, there was a 69% reduction in P. equorum FEC on Day 14. Twelve of the 14 foals on Farm C were shedding strongylid eggs on Day 0 and on Day 14 FEC were reduced by 84%.Without comparison to untreated control animals, these results do not allow a reliable estimation of the exact level of efficacy of ivermectin against P. equorum, but they nevertheless show that, within the farms studied, ivermectin no longer achieved a complete kill of egg-laying adults and that foals treated with this drug may continue to shed considerable numbers of eggs after treatment. The results also indicate that ivermectin had sub-optimal efficacy against strongylid nematodes on one of the farms.