Groups of sheep infested with strains of Bovicola (Damalinia) ovis were obtained from flocks either with a history of failure to control lice with synthetic pyrethroid (SP) pour-on insecticides, or from farms where SP compounds were not used. The sheep were treated according to the manufacturer's recommendations with registered "off-shears" SP formulations. All treatments were applied under ideal conditions with doses calculated on an individual body weight basis and applied to the dorsal mid-line from the base of the neck to the butt of the tail. Treated sheep were kept in pens and maintained in separate groups. The pour-on SP treatments significantly reduced the lice population but failed to eliminate the infestation in 7 of 13 experiments in sheep carrying strains of lice with resistance factors of greater than 4 to at least one of the SP compounds. Failures occurred with all three of the SP pour-ons currently registered for lice control in NSW and with both water-based and organic solvent-based formulations.
Under the conditions of this study intradermal injection of cetrimide had no permanent effect on bare area measurements on the breech or the amount of wool-bearing skin on the tail. It also caused signs of discomfort and pain that raise welfare concerns.
Synthetic pyrethroid (SP) resistance has developed in Australian field populations of the sheep body louse, Bovicola (Damalinia) ovis. Laboratory bioassays were used to measure the susceptibility of lice to cypermethrin and the other registered SPs. Results of these bioassays indicated resistance to cypermethrin, deltamethrin, cyhalothrin and alphacypermethrin. So far, high-level resistance has been diagnosed in only a few strains. The toxicological responses of these strains were clearly separated from those of the majority of louse strains tested. Furthermore, these strains had survived immersion in commercial SP dips. The level of resistance described in some strains was sufficient to cause pour-on products to fail despite the fact that the LC50s of these strains fell within the normal range of field responses.
Fine wool Merino sheep with short (4 mm) or long (30-80 mm) wool and moderate to heavy infestations of lice (Bovicola ovis) were treated with cyhalothrin at a concentration of 50 mg/L, applied using Alison, Aussie Dip and Boral Cyclone Superace automatic jetting races. Each machine was set up and operated by representatives of the manufacturer. With all 3 machines, arithmetic mean louse counts before and after treatment showed that in the sheep with short wool, lice numbers were reduced by > 98% at 8-12 weeks after treatment but were not eliminated. In the sheep with long wool, the maximum reductions in mean louse counts were 79, 68 and 98% for Alison, Aussie Dip and Boral Cyclone Superace machines at 7, 5 and 8 weeks after treatment, respectively. The Alison and Aussie Dip machines were each tested in alternative configurations which affected the disposition of the jetting fluid on the fleece. The distribution of live lice on the bodies of the shorn sheep after treatment using the Alison machine and 1 configuration of the Aussie Dip machine, indicated that more insecticide may have been delivered to the left side of the body than to the right. There were no significant differences between sides in sheep treated in the Boral Cyclone Superace machine. Our results suggest that the jetting races tested would control infestations of lice for at least 12 weeks in sheep treated within 2 weeks after shearing, but would be unlikely to eradicate lice from an infested flock. The machines differed markedly in their ability to control moderate to heavy infestations of lice in sheep with long (30-80 mm) wool, but in all cases there was evidence of a resurgence in lice numbers beyond 12 weeks after treatment.
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