Evolution of the efficacy of a combined moxidectin-levamisole treatment against resistant gastrointestinal nematodes in lambs

Nematodicidal combinations can be a valid strategy to achieve effective nematode control in thepresence of drug resistance. The aim of the current trial was to evaluate the pharmaco-parasitological outcome after the continuous use of moxidectin (MXD) and levamisole (LEV) as acombined treatment in lambs naturally parasitized with ivermectin-resistant gastrointestinalnematodes. Forty (40) lambs were divided into four groups (n=10): untreated control andsubcutaneously treated with either MXD (0.2 mg/kg), LEV (8 mg/kg) or with MXD+LEV (0.2and 8 mg/kg, respectively). Blood samples were collected at different times up to 1 (LEV) or 14(MXD) days post-treatment. LEV and MXD plasma concentrations were measured byHPLC. Faecal samples were collected on days 0, 7, and 14 post-treatment to perform the faecalegg count reduction tests (FECRT). No significant pharmacokinetic (PK) adverse changes wereobserved for either MXD or LEV after their co-administration in sheep. The clinical efficacy ofthe MXD+LEV combination was evaluated after its continuous use (3 treatments/year) over five(5) years at the same farm. The initial anthelmintic efficacies (1 st year) were 99% (MXD), 85%(LEV) and 100% (MXD+LEV). Following repeated annual treatments over five years, theclinical response for the combined treatment reached 87% efficacy. The combination reachedefficacies of 100% (1 st year) and 98.5% (5 th year) against Haemonchus contortus. Teladorsagia spp. and Trichostrongylus spp. were the main nematode genera surviving theindividual and combined treatments. The co-administration of MXD+LEV during five yearsresulted in a significant higher anthelmintic effect compared to MXD or LEV given alone. Evenwhen MXD and LEV individual efficacies were reduced during the five-year period, thecombined treatment maintains acceptable efficacy levels against H. contortus. Fil: Luque, Sonia Elisabet. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil. Centro de Investigación Veterinaria de Tandil. Universidad Nacional del Centro de la Provincia de Buenos Aires. Centro de Investigación Veterinaria de Tandil. Provincia de Buenos Aires. Gobernación. Comision de Investigaciones Científicas. Centro de Investigación Veterinaria de Tandil; Argentina Fil: Lloberas, Maria Mercedes. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Buenos Aires Sur. Estación Experimental Agropecuaria Balcarce. Agencia de Extensión Rural Balcarce; Argentina