Veterinary Parasitology 189 (2012) 378–382 Contents lists available at SciVerse ScienceDirect Veterinary Parasitology jou rn al h om epa ge: www.elsevier.com/locate/vetpar Short communication Assessing resistance against macrocyclic lactones in gastro-intestinal nematodes in cattle using the faecal egg count reduction test and the controlled efficacy test J. De Graef a,1 , C. Sarre a,1 , B.J. Mills b , S. Mahabir b , S. Casaert a , N. De Wilde a , M. Van Weyenberg a , P. Geldhof a , A. Marchiondo b , J. Vercruysse a , P. Meeus b , E. Claerebout a,∗ a Laboratory for Parasitology, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, B-9820 Merelbeke, Belgium b Pfizer Animal Health, 333 Portage Road, Kalamazoo, MI 49007, USA a r t i c l e i n f o Article history: Received 28 March 2012 Received in revised form 25 April 2012 Accepted 28 April 2012 Keywords: Ivermectin Moxidectin Resistance Ostertagia ostertagi Cooperia oncophora Cattle a b s t r a c t The main objective of the present study was to evaluate the accuracy of the faecal egg count reduction test (FECRT) to assess the resistance status of ivermectin (IVM)-resistant isolates of the cattle nematodes Ostertagia ostertagi and Cooperia oncophora, using the con- trolled efficacy test (worm counts) as a reference. The second objective was to investigate whether both IVM-resistant isolates showed side-resistance against moxidectin (MOX) under controlled conditions. Thirty male Holstein calves were experimentally infected with 25,000 L3 of an IVM- resistant O. ostertagi isolate and 25,000 L3 of an IVM-resistant C. oncophora isolate. Twenty- eight days later the calves were randomly divided into 2 treatment groups and 1 untreated control group. Animals in groups 1 and 2 received MOX (Cydectin ® 1%, Pfizer) and IVM (Ivomec ® 1%, Merial) respectively, by subcutaneous injection at a dose rate of 0.2 mg/kg bodyweight. Faecal samples were collected 7 and 14 days after treatment and animals were necropsied 14/15 days post-treatment. Both the FECRT and the controlled efficacy test demonstrated that the O. ostertagi and C. oncophora isolates were resistant against IVM, with efficacies below 90%. The IVM-resistant O. ostertagia isolate was still susceptible to MOX treatment, as shown by over 99% reduction in egg counts and worm burden. The FECRT suggested borderline resistance against MOX in the IVM-resistant C. oncophora isolate, with egg count reductions between 97% (95% CI: 76; 100) at day 7 and 86% (95% CI: 49; 96) at day 14. However, the controlled efficacy test clearly showed MOX-resistance, with a decrease of only 31% (95% CI: −12; 57) in C. oncophora worm numbers. After MOX treatment, a significantly lower number of eggs per female C. oncophora worms was counted compared to the control group (43% reduction). Due to this reduced fecundity, the FECRT may fail to detect MOX-resistance. © 2012 Elsevier B.V. All rights reserved. ∗ Corresponding author. Tel.: +32 09 2647393; fax: +32 09 2647496. E-mail address: edwin.claerebout@ugent.be (E. Claerebout). 1 These authors contributed equally. 1. Introduction Anthelmintic-resistance is occurring in parasites of small ruminants, cattle and horses against all major classes of anthelmintics (Kaplan, 2004; Kaplan and Vidyashankar, 2012). In cattle, in the majority of the cases, resis- tance against the macrocyclic lactones (MLs) has been reported. MLs are divided into two groups: avermectins 0304-4017/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vetpar.2012.04.040