Clinical efficacy assessment of the albendazole-ivermectin combination in lambs parasitized with resistant nematodes.

Combination of anthelmintic drugs from different chemical groups has been proposed as alternative parasite control strategies where failure of individual drugs is documented. The main goal of the current trial was to compare the clinical anthelmintic efficacy of albendazole (ABZ) and ivermectin (IVM) given either separately or co-administered to lambs naturally infected with gastrointestinal nematodes resistant to both molecules. Seventy (70) Corriedale lambs naturally infected with multiple resistant gastrointestinal nematodes were involved in the efficacy trial: the animals were allocated into 7 experimental groups (n=10) and treated with either ABZ intravenously (iv) (ABZ(IV)), IVM(IV), ABZ(IV)+IVM(IV), ABZ intraruminally (ir) (ABZ(IR)), IVM subcutaneously (sc) (IVM(SC)) and ABZ(IR)+IVM(SC) or kept as untreated controls. The indirect estimation of the efficacy of the different treatments was performed by the faecal egg count reduction test (FECRT). Additionally, four animals randomly chosen from the untreated control and ABZ(IV,) IVM(IV) and ABZ(IV)+IVM(IV) experimental groups were sacrificed 15 days post-treatment to evaluate the efficacy against different adult resistant nematode parasites. The results were statistically compared by a non-parametric ANOVA (Kruskal-Wallis test). The following egg output reduction values were obtained: 73.4% (ABZ(IV)), 79.0% (IVM(IV)), 91.9% (ABZ(IV)+IVM(IV)), 43.5% (ABZ(IR)), 79.8% (IVM(SC)) and 70.8% (ABZ(IR)+IVM(SC)). The efficacy against Haemonchus spp. was 95.1 (ABZ(IV)), 99.3 (IVM(IV)) and 99.9% (ABZ(IV)+IVM(IV)), while the efficacy against Trichostrongylus colubriformis for the same treatment groups was 79.6, 100 and 99.9%. The data obtained on the assessment of the ABZ-IVM combination indicates that no potentiation synergism is observed. This work is complementary to a parallel study that demonstrated the lack of negative pharmacokinetic interactions between the two anthelmintics acting by different mode of action. Thus, an additive effect may be achieved against nematodes resistant to both compounds. Further work is required to understand the implications of potential pharmacokinetic/pharmacodynamic interactions between anthelmintics before drug combined formulations are developed to be introduced into the pharmaceutical market.