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.