P-gp modulating drugs greatly potentiate the in vitro effect of ivermectin against resistant larvae of Haemonchus placei.

Since its production in the 1980s, ivermectin (IVM) has been used indiscriminately and the selection pressure to which bovine gastrointestinal nematodes have been exposed has been intense, resulting in considerable economic losses due to parasitic resistance. One possibility for the control of resistant parasites is the use of P-glycoprotein (P-gp) modulators, because one of the main biochemical changes in ivermectin-resistant parasites is the increased activity of membrane proteins responsible for the efflux of drugs and xenobiotics. This study aimed to evaluate the in vitro effect of eight P-gp modulating drugs to potentiate IVM efficacy against an IVM-resistant field isolate of Haemonchus placei (Nematoda: Trichostrongylidae). The association of IVM with cyclosporin-A, ceftriaxone, dexamethasone, diminazene aceturate, quercetin, trifluoperazine, verapamil, or vinblastine resulted in increased IVM (10(-4)M) efficacy of 5.1%, 49.06%, 76.42%, 3.31%, 28.85%, 13.74%, 45.64% and 43.61%, respectively, and reduced the IVM half maximal effective concentration (EC50) from 4.381 × 10(-6)M to 9.877 × 10(-8), 2.739 × 10(-7), 1.240 × 10(-6), 1.651 × 10(-6), 2.710 × 10(-7), 1.159 × 10(-7), 1.026 × 10(-6) and 7.136 × 10(-7)M, respectively. Only diminazene aceturate did not significantly reduce the number of migrating larvae when associated with IVM (P > 0.05). The effect of P-gp modulating drugs depended on IVM concentration, with greater potentiating effect at lower IVM concentrations. The in vitro application of trifluoperazine, dexamethasone, quercetin, verapamil, cyclosporin A, vinblastine, and ceftriaxone potentiated IVM efficacy against an IVM-resistant field isolate of H. placei, resulting in higher efficacy and lower IVM EC50.

Ivermectin and moxidectin resistance characterization by larval migration inhibition test in field isolates of Cooperia spp. in beef cattle, Mato Grosso do Sul, Brazil.

Ivermectin (IVM) resistance of Cooperia spp. in cattle has become an increasing and global problem. The early detection of anthelmintic resistance (AR) is important to propose strategies to slow down the development of resistance and requires sensitive, reliable, economic high-throughput and practical tests. The purpose of the present study was to apply a larval migration inhibition test (LMIT) for evaluating IVM and MOX efficacy against well-characterized field isolates of Cooperia spp. infecting cattle in Brazil. Eight isolates were used for IVM and seven for MOX. The following EC50 values of IVM were observed for the isolates: susceptible, 1.16 ηmol; Nova Alvorada do Sul I, 4.09 ηmol (RF=3.52); Campo Grande BNA, 3.57 ηmol (RF=3.07); Campo Grande TBR, 4.09 ηmol (RF=3,52); Nova Alvorada do Sul II, 2.50 ηmol (RF=2.15); Bandeirantes, 11.35 ηmol (RF=9.78); Campo Grande II, 6.03 ηmol (RF=5.20); and Porto Mortinho, 8.63 ηmol (RF=7.44). For MOX, the following EC50 values were observed: susceptible, 0.75 ηmol; Campo Grande BNA, 0.93 ηmol (RF=1.24); Campo Grande TBR, 0.36 ηmol (RF=0.48); Nova Alvorada do Sul II, 2.57 ηmol (RF=3.42); Bandeirantes, 1.43 ηmol (RF=1.90); Campo Grande II, 1.08 ηmol (RF=1.44); and Porto Mortinho, 0.49 ηmol (RF=0.65). The LMIT used in the present study can be a useful tool for in vitro evaluation of IVM, but not of MOX. However, such methodology cannot be used in large-scale studies yet. The isolates of Cooperia spp. showed various degrees of resistance to IVM, though remaining susceptible to MOX.