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An experiment was carried out to study the possible interaction between dexamethasone (DXM) treatment and the efficacy of ivermectin (IVM) treatment in young cattle. Two groups, each of seven calves, were experimentally inoculated with an equal mixture containing 15,000 third stage larvae of Cooperia oncophora and Ostertagia ostertagi each, and with no history of being resistant to any anthelmintics. However, in this study C. oncophora was unexpectedly classified as IVM-resistant according to the outcome from the faecal egg count reduction test (FECRT). Blood parameters and faecal egg counts (FEC) were monitored from 0 to 35 days post infection (d.p.i.). The calves in one group received intramuscular injections of short and long-term acting DXM at 22 and 24 d.p.i., respectively. The other group remained as a control. Three days post patency (24 d.p.i.) both groups were injected subcutaneously with IVM (Merial) at the recommended dose (0.2mg/kg). A significant difference (p<0.001) in FEC patterns was observed between groups. Although both groups still excreted eggs (100-200 eggs per gram faeces) 11 days post anthelmintic treatment, the control group had a significantly higher reduction between 23 and 35 d.p.i. (p=0.025). After 35 days, four animals per group were euthanized, and worms in the gastrointestinal tract were counted. No O. ostertagi were found in the abomasums, but low to high numbers (800-6200) of C. oncophora remained in the small intestines in both groups. Overall, these findings indicated that there was an interaction between the efficacy of IVM and DXM treatment. As significantly lower plasma levels of IVM were observed in the DXM group, we conclude that the impaired efficacy of ivermectin was most likely due to the altered pharmacokinetics.