Decline in faecal worm egg counts in lambs suckling ewes treated with lipophilic anthelmintics: Implications for hastening development of anthelmintic resistance

Selection for anthelmintic resistant Teladorsagia circumcincta in pre-weaned lambs by treating their dams with long-acting moxidectin injection

“…A large scale replicated field study in New Zealand confirmed the potential of a pre-lambing treatment of ewes with a CRC containing albendazole to accelerate the development of resistance to benzimidazole anthelmintics ( Leathwick et al., 2006 ). Albendazole CRC, such as the ones used in that study, have no withholding periods for meat or milk ( Chartier et al., 1996 ; http://www.merial.co.nz/Sheep/Products/Pages/extender_seco.aspx ), reflecting the fact that albendazole does not transfer into milk of treated lactating animals to the extent that ML actives do ( De Liguoro et al., 1996 , Dever and Kahn, 2015 ), and so the mechanism of resistance selection in that study must have involved only effects against worms in the ewe. Therefore, the process of selection demonstrated here (i.e.…”

“…Therefore, the process of selection demonstrated here (i.e. selection for resistant worms in the lamb via transfer of drug in the ewe's milk) represents a new, and until recently unconsidered ( Dever and Kahn, 2015 ), pathway by which use of moxidectin injection in lactating ewes could select for resistance. The results of this study, therefore, represent yet another reason why administration of long-acting anthelmintic products to pre-lambing ewes must be considered high-risk for selecting anthelmintic resistance and their use approached with caution.…”

“…The administration of anthelmintics to adult ewes in the weeks pre- or post-lambing has been a common practice amongst sheep farmers in New Zealand for many years ( Brunsdon et al., 1983 , Lawrence et al., 2007 ) and the practice is also common in other countries ( Sargison et al., 2012 , Dever and Kahn, 2015 ). The potential of treating ewes at this time to accelerate the development of anthelmintic resistance has been recognized for almost as long ( Dash et al., 1985 , Michel, 1985 ).…”

“…Like all ML compounds, the lipophylic nature of moxidectin and abamectin results in a proportion of the administered dose being excreted in the milk of lactating animals ( Alvinerie et al., 1996 , Oukessou et al., 1999 , Carceles et al., 2001 , Imperiale et al., 2004 , Dupuy et al., 2008 , Barrera et al., 2013 ) and being detectable in the plasma of suckling offspring ( Bogan and McKellar, 1988 , Alvinerie et al., 1996 , Cerkvenik-Flajs et al., 2007 ). This raises the possibility that treatment of ewes pre-lambing with these long-acting products could result in sufficient transfer of active ingredient to the suckling lamb to result in anthelmintic activity, and the potential for subsequent selection for ML-resistant parasites in the lambs ( Dever and Kahn, 2015 ). Here we describe a study which was designed to test firstly for the transfer of moxidectin and abamectin from treated ewes into their lambs, and secondly for the potential of any such transfer to select for ML-resistance in Teladorsagia circumcincta .…”

Effect of Moxidectin Treatment at Peripartum on Gastrointestinal Parasite Infections in Ewes Raised under Tropical Andes High Altitude Conditions

“…Although this temporal excretion of Mox could reduce FEC, it also exposes lambs' nematodes to subtherapeutic doses reducing “ refugia” population of parasites and predisposing to resistance processes [ 47 ]. Recently Dever and Kahn [ 48 ] have demonstrated that anthelmintics extremely lipophilic as Mox administered at the rate of 1 mg kg −1 of body weight to lactating ewes could reduce significantly FEC in suckling offspring and expose lambs to subtherapeutic doses of the drug, a risk factor for the development of anthelmintic resistance. Our study employed only 200 mg mL −1 and due to differences in treatment period application (late pregnancy or immediate peripartum), T 1 offspring received lower Mox by milk than T 2 .…”