Reduced efficacy of moxidectin and abamectin in young red deer (Cervus elaphus) after 20 years of moxidectin pour-on use on a New Zealand deer farm

A study was undertaken on weaned 4–5 month old farmed red deer to test the efficacy ofmoxidectin and abamectin anthelmintics, given by three different routes of administration,compared with an untreated control. Faecal samples were collected on days 0, 7 and 14 fora faecal egg count reduction test (FECRT), blood samples were collected on days 0, 0.5, 1, 2,3, 5, 7, 10 and 14 for pharmacokinetics, and the deer were killed on days 14 or 15 for totalnematode count.The control group averaged 1264 adult Ostertagia-type nematode parasite species andtreatment efficacy was 77.4% for moxidectin injection, 26% for oral moxidectin and 27.6%for pour-on moxidectin, while the treatment efficacy was 72.4% for abamectin injection,70.1% for oral abamectin (Hi-Mineral) and 34.1% for pour-on abamectin. Both moxidectinand abamectin injections were significantly more efficacious than their equivalent pour-ons. There was a significant difference in efficacy between oral abamectin (Hi-Mineral) andoral moxidectin (P < 0.01).The control group averaged 2956 adult lungworm (Dictyocaulus eckerti) and 50Oesophagostomum venulosum in the large intestine and treatment efficacy against thesenematodes was 100% for all treatments. There were negligible numbers of other gastro-intestinal nematodes.At slaughter, there was a significant correlation (P = 0.02) between FEC and Ostertagia-type nematodes in the untreated controls. Relatively few eggs were found in faeces fromtreated animals at 7 and 14 days post-treatment despite significant worm burdens in allsix treatment groups, suggesting egg-laying suppression in resistant nematodes, and allthree different FECRT calculations tended to overestimate the efficacy of the treatmentscompared with actual nematode counts.Peak plasma concentrations (Cmax) for both actives were measured 12 h after treatmentfor injection and oral and at 5 days for pour-on. Cmax(ng/ml) for moxidectin injection,oral and pour-on were 71.8, 8.3 and 0.4, respectively, and for abamectin injection, oraland pour-on were 62.1, 30.3 and 10.0, respectively. Area under the curve (AUC) estimatesfor moxidectin injection, oral and pour-on were 106.6, 12.9 and 6.1, respectively, and forabamectin injection, oral and pour-on were 162.7, 57.5 and 74.3, respectively.The results demonstrate that significant anthelmintic resistance to moxidectin andabamectin is present on this deer farm. However, the injection was the most effectiveroute of administration in young deer for both anthelmintics, although <80% efficacious.We conclude that the FECRT is unreliable in deer when anthelmintic resistance is present.