Field efficacy and safety of fluralaner oral solution for sheep for control of naturally acquired biting louse (Bovicola ovis) infestations on long-wool sheep across Australia and New Zealand

The primary control strategy for the sheep biting louse ( Bovicola ovis ) relies on topical chemical-based treatments applied postshearing, with limited use of long-wool interventions. Resistance to commonly used chemical classes, including synthetic pyrethroids (SPs) and insect growth regulators (IGRs), has been reported, particularly in Australia, and may compromise control under some conditions. This series of studies evaluated the field efficacy and safety of a 10 mg/mL fluralaner oral solution (which was later registered as Flexolt ® , MSD Animal Health) against naturally acquired B. ovis infestations on long-wool sheep. The studies were conducted across seven commercial sites (six in Australia and one in New Zealand [NZ]) with naturally infested flocks. At each site, 25 tracer sheep with ≥ 40 lice from 40 × 10 cm wool partings were selected pretreatment and monitored over time. All sheep in each flock were treated once on Day 0 with fluralaner at a nominal dose of 1.5 mg/kg body weight. Post-treatment lice and viable egg counts were conducted on 3 to 7 occasions per site. Efficacy was calculated relative to pretreatment lice counts. Resistance to SPs and IGRs was assessed in selected lice populations using laboratory bioassays. Lice counts were reduced by 100% at all Australian sites and by 99.9–100% at the NZ site. No viable eggs were detected from Day 14–29 at five sites and from the first or second post-treatment assessment at Day 54 and Day 70 at the remaining sites. High efficacy was observed across all sites, including those where laboratory bioassays identified low- to high-level SP resistance and limited IGR resistance. No adverse events were observed other than mild, transient coughing in some sheep immediately after dosing. A single oral administration of fluralaner at a nominal dose of 1.5 mg/kg body weight provided effective control of B. ovis infestations on long-wool sheep under field conditions in Australia and NZ. Efficacy was maintained across diverse field situations, including sites with documented resistance to established lousicide chemical classes. These findings support fluralaner as an effective option for lice control on long-wool sheep.