Monitoring macrocyclic lactone resistance in Cooperia oncophora on a Belgian cattle farm during four consecutive years.

In this study we have used the faecal egg count reduction test (FECRT) to monitor the evolution of ivermectin resistance on a Belgian cattle farm between 2006 and 2009. The presence of ivermectin resistant Cooperia oncophora worms on this farm was first detected in 2006. During the following years, the FECRs on day 21 post-treatment decreased from 73% (95% CI: 8-99) in 2006, over 40% (95% CI: 0-89) in 2007, to 0% (95% CI: 0-41) in 2008. The ivermectin resistant C. oncophora showed side-resistance against moxidectin, indicated by a FECR of 83%, suggesting that the use of any type of ML is discouraged once ivermectin resistance has been detected. Benzimidazoles on the other hand were still fully effective on this farm. The resistant C. oncophora larvae collected in 2007 (CoIVR07) and 2008 (CoIVR08) were also used to evaluate a modified version of the larval migration inhibition assay (LMIA). The results indicated that it is possible with the LMIA to differentiate susceptible from ivermectin resistant C. oncophora isolates. The EC50 values of CoIVR07 (542 nM) and CoIVR08 (698 nM) were, respectively, 4.5- and 5.8-fold higher than the value of a susceptible isolate (120 nM). Furthermore, the results of the LMIA reflected the outcome of the FECRT, with the C. oncophora isolate collected in 2008 being more resistant than the isolate collected in 2007. However, the test should be further optimized, e.g. more isolates with different susceptibility to ivermectin and mixtures of species, in order to use the test in field conditions. Copyright (c) 2010 Elsevier B.V. All rights reserved.