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

- Decline in faecal worm egg counts (WEC) for lambs suckling ewes treated with lipophilic anthelmintics, indicating ingestion of sub-therapeutic amounts of anthelmintic active/s via milk.
- The sub-therapeutic nature of the anthelmintic transfer via milk would increase selection within the gastrointestinal nematode (GIN) population (predominantly Haemonchus contortus) for anthelmintic resistance.
- Significant and negative relationships between WEC and packed cell volume (PCV), and female H. contortus burden and WEC and a significant and positive relationship between H. contortus burden and WEC.

The aim for this experiment was to look for evidence of milk transfer of anthelmintic actives from ewes to their suckling lambs by reference to lambs' faecal worm egg count (WEC). The hypothesis was that WEC will decline in lambs suckling ewes treated with anthelmintics known to be lipophilic. One group of lactating Border Leicester × Merino ewes were treated (TX) with a combination of short (2.5 mg/kg monepantel) and long-acting (1 mg/kg moxidectin long-acting injection and a sustained release of 4.62 g albendazole over 100 days) anthelmintics to remove gastrointestinal nematode (GIN) burden on day 0. The other group of lactating ewes (UTX) and all lambs (White Suffolk sires) were not treated. Ewes and lambs grazed as a single group and were exposed to GIN (predominately Haemonchus contortus) infection from pasture. Measurements were taken on days 0 and 7. WEC of lambs suckling UTX ewes increased from 6441 to 10,341 eggs per gram (epg) between days 0 and 7, while there was a 51% reduction in WEC for lambs suckling TX ewes. Packed cell volume (PCV) was significantly higher for lambs suckling TX ewes on day 7 compared to lambs suckling UTX ewes (28.5% vs. 24.9%, p = 0.039). These results suggest that lambs suckling ewes treated with lipophilic anthelmintics received a sub-therapeutic dose via milk which would increase selection within the GIN (H. contortus) population for anthelmintic resistance.