Comparative assessment of ivermectin, abamectin and moxidectin efficacies against resistant nematodes in lambs

Abstract

The macrocyclic lactones (ML) (ivermectin (IVM), abamectin (ABM) and moxidectin (MXD) are extensively used in parasite control in ruminants. Resistance has emerged in different gastrointestinal nematodes after several years of their intensive use. An advantageous efficacy pattern against resistant nematodes was previously obtained for the oral administration of ML compared to the subcutaneous route. Although the three ML acts by the same mechanism, some differences in the efficacy against resistant nematodes have been observed particularly after their oral administration to infected sheep. The aim of the current work was to compare the efficacy of IVM, ABM and MXD against IVM-resistant nematodes following their intraruminal administration to infected lambs. Twenty four (24) lambs naturally infected with IVM-resistant gastrointestinal nematodes were used. Animals were allocated into four (4) experimental groups (n=6), including an untreated control and those intraruminally treated (0.2 mg/kg) with either IVM, ABM or MXD. Individual fecal samples for nematode egg counts were collected at 0, 6, 9 and 15 days post treatment. Post-mortem examination was done at day 15 after treatment. The recovered adult gastrointestinal worms were identified by species and counted. The faecal count reduction test (FECRT) and percentage of efficacy on each nematode species were obtained according the WAAVP guidelines. The Kruskal Wallis test was applied to the statistical comparison. A high level of resistance against the ML compounds was observed. The FECRT showed a complete failure for both IVM and ABM (0% reduction), while the egg reduction after the MXD treatment reached 86.8%. The overall efficacy against adult nematodes showed differences among ML, ranging from 84.8% (IVM) and 90.2% (ABM) to 98.0% (MXD). Haemonchus contortus appeared as the most resistant species, where the failure of IVM (20% efficacy) and ABM (39%) contrasted with the 89.6% efficacy obtained after the MXD treatment. The advantage of the oral/intraruminal compared to the parenteral route and the differential efficacy patterns observed among different ML should be seriously considered to optimize nematode control in the early stages of resistant development before a complete therapeutic failure is observed.