In New Zealand, it is a common practice to give a long-acting drench to pregnant ewes. Similarly, ewes (and does) in the US are often dewormed prior to parturition. But is this a good idea? Or does it select for resistant worms through the offspring?
It has long been known that some drugs transfer through the ewe's milk to the lambs. In particular, Macrocylic Lactones (ML), because of their lipophilic nature, are detectable in the milk of treated animals and in the plasma of the suckling offspring. So, a study was conducted to confirm the transfer of ML actives to lambs in the ewe's milk, and to assess whether this could result in selection for ML resistant nematodes in the lamb.
Long-acting anthelmintics were administered to pregnant ewes. Moxidectin and abamectin were detected in ewe's milk for >60 days, meaning lambs were receiving a low dose of anthelmintic every day. Moxidectin and abamectin were detected in plasma of lambs for >60 days. Abamectin treatment had no effect on establishment of larvae in the lambs. Moxidectin treatment reduced establishment of susceptible, but not resistant larvae.
Conclusion: Pre-lamb drenching of ewes can select for resistant worms in the lambs, even though the lambs themselves may never have been drenched.
Read article in International Journal of Veterinary Parasitology, December 2015.