AbstractIvermectin (IVM) resistance of Cooperia spp. in dairy heifers from Puerto Rico was evaluated using the larval migration inhibition assay (LMIA). In eight of the farms where anthelmintic resistance had previously been evaluated using a fecal egg count reduction test (FECRT), third stage larvae (L3) were recovered from fecal cultures prior to the administration of IVM treatment. The effective concentration at which 50% of larval migration is inhibited (EC50) was determined in susceptible and resistant Cooperia spp. isolates from one farm. The susceptible and resistant isolates had an EC50 of 0.7224 uM and an EC50 of 7.0778 uM, respectively. This indicates that on average, larvae from the resistant isolate are 10X more resistant than those from the susceptible isolates. However, the LMIA was unable to discriminate consistently between susceptible and resistant larvae obtained from dairy farms evaluated. In one case, an EC50 value of 0.7263 uM, which is close to that of the susceptible isolate, did not agree with the FECR result of -136.19% documented on this farm. In two additional instances the assay was unable to discriminate between susceptible and resistant isolates. Indeed LMIA EC50 values obtained from some resistant isolates were lower than the EC50 value from susceptible isolates, whereas their FECRT results indicated that resistance to IVM was present. Possibly, the FECRT could have given a false positive for macrocyclic lactones resistance at these two farms. Therefore, more studies are needed to validate the effectiveness of the LMIA to detect anthelmintic resistance to macrocyclic lactones in Cooperia spp. larvae isolated from bovine fecal field samples.
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