Combined administration of ivermectin and fenbendazole against highly resistant Haemonchus contortus: assessment of dose increment.

LUQUE S., LLOBERAS M., ENTROCASSO C., LANUSSE C., ALVAREZ L., LIFSCHITZ A.

Congreso; XVIII Congreso Brasilero de Parasitología Veterinaria,; 2014

Previous studies involving albendazole and ivermectin (IVM), demonstrated that the enhanced systemic exposure achieved after their administration at high doses (therapeutic dose x 5 or x10) correlated to a significant increment in drug efficacy against highly resistant Haemonchus contortus. The combination of nematodicidal anthelminctics from different chemical groups has been proposed as a strategy to delay the development of resistance. The aim of the current trial was to compare the clinical anthelmintic efficacy of fenbendazole (FBZ) and IVM administered at x5 the therapeutic dose, either alone or co-administered to lambs parasitized with an H. contortus strain highly resistant to both compounds. Forty (40) lambs naturally infected with resistant nematodes were allocated into four groups (n=10) and orally treated either with IVM (1 mg/kg), FBZ (25mg/kg) or with a combined formulation of IVM+ FBZ (1 and 25 mg/kg, respectively). An untreated control group (n=10) was included to asses the comparative anthelmintic efficacy (14 days post.treatment) of the different treatments by the faecal egg count reduction test (FECRT). Additionally, faecal samples were cultured to determine nematode genus and species. The Kruskal-Wallis test was applied for the statistical comparison (P0.05) were observed on faecal egg counts among treated animals. The faecal cultures showed H. contortus as the main nematode surviving all treatments. The combined administration of IVM+FBZ at 5 times the therapeutic did not improve the efficacy against a highly resistant H. contortus strain.