Prenatal administration of lipophilic macrocyclic lactone anthelmintics can alter neonatal lamb behavioural development

The treatment of pregnant ewes with persistent macrocyclic lactone (ML) anthelmintic products is common in some countries, with the intent of alleviating the potential negative impacts of gastrointestinal nematode parasites on production, health and welfare. However, some MLs such as moxidectin, are lipophilic compounds that can cross the placenta, and many are secreted into colostrum and milk, yet the effect of MLs on neonatal lamb behaviour remains unknown. The objective of this study was to evaluate the effect of prenatal administration of persistent ML anthelmintics on the behaviour of ewes and their new-born lambs. Sixty mixed-age pregnant twin-bearing ewes were randomly allocated to one of three groups (n = 20/group): 1) administered a subcutaneous injection containing moxidectin 2–3 weeks prior to lambing (MOX), 2) administered one controlled-release capsule containing abamectin and albendazole 2–3 weeks prior to lambing (CRC), and 3) a control group that was not treated or handled other than routine care (CON). The ewes were housed indoors 4 weeks before parturition was expected to commence and remained in the same pens (n = 2 ewes / treatment) until 1 week after lambing. Behaviour of the ewes and lambs was monitored continuously using security cameras positioned above each pen. Ewe and lamb behaviour was evaluated for 3 h after parturition. Ewe grooming of the lamb, and lamb head shaking, standing, udder seeking and suckling behaviour was assessed using Cox Proportional hazard models. There was no evidence that abamectin (and albendazole) affected ewe or lamb behaviour ( P > 0.10). Neonatal lambs from ewes administered moxidectin tended to be slower to shake their heads after parturition (hazard ratio: 0.65; P = 0.08), and were slower to attempt to stand (hazard ratio: 0.61; P = 0.05) compared with lambs from control ewes. These are the first known data to indicate changes in neonatal lamb behaviour associated with anthelmintic treatment of their dam. Further research is required to validate these findings and understand whether the delayed neonatal lamb behavioural development associated with moxidectin has implications for lamb vigour or survivability. • Prenatal administration of ewes with persistent anthelmintics is common on farms. • Abamectin had no effect on ewe or lamb behaviour after parturition. • Lambs from moxidectin-treated ewes showed delayed behavioural development.