Efficacy against sheep lice (Bovicola ovis) and fleece wetting of six shower dip preparations.
The relative efficacy of 6 shower dip chemicals most frequently used for the treatment of sheep lice (Bovicola ovis) in Western Australia was examined. Groups of 20 sheep infested with lice were treated with products containing either alphamethrin, cyhalothrin, diazinon or diazinon plus piperonyl butoxide and rotenone, formulated as emulsifiable concentrates, and with products containing either coumaphos or magnesium fluorosilicate, formulated as wettable powders. All treatments were applied through a shower dip (Sunbeam model SSD). Inspections for lice were conducted until 9 months after dipping. No lice were found on sheep treated with the 4 emulsifiable concentrate products. In contrast, treatment with the wettable powders, which contained either coumaphos or magnesium fluorosilicate as the active ingredient, did not eradicate the lice infestations. The degree to which the fleece was wetted was assessed 20 minutes after dipping and showed that the wettable powder dips penetrated the fleece less than the emulsifiable concentrate dips. Less fluid was retained by wool staples in an in-vitro test when dip wash was made with the wettable powders. It was concluded that the degree of wetting attained at dipping was an important factor in achieving eradication of sheep lice.
Further evidence that zinc sulphate compromises the efficacy of dipping treatments using diazinon to control sheep lice (Bovicola ovis).
OBJECTIVE
To compare the wettability and efficacy of diazinon dip wash made with and without the addition of zinc sulphate.


DESIGN
Field experiments using a shower and a plunge dip complemented by in-vitro wettability experiments.


PROCEDURE
A flock of infested sheep was divided into groups and treated in a shower dip with clear or cloudy dam water plus up to 1.5% zinc sulphate. Another infested line of sheep was treated using a plunge dip with nil or 1% zinc sulphate. In both experiments, wetting was assessed after dipping and louse counts were conducted for 9 months after treatment. Five in-vitro experiments compared the wettability of dip wash containing diazinon with up to 1.5% zinc sulphate added.


RESULTS
In the shower dipping experiment, live lice were found at 1 month after dipping in the cloudy water groups with 0.75%, 1.0% and 1.5% zinc sulphate and at 2 months in the 0.75% zinc sulphate group. No lice were found at subsequent inspections or at any time in the groups that were plunge dipped. Zinc sulphate decreased the amount of dip wash retained by wool staples in all in-vitro experiments (P < 0.05).


CONCLUSION
Zinc sulphate should be considered as a risk factor that could cause failure to eradicate a lice infestation. The risk can be overcome by ensuring that all sheep are saturated at dipping and that the dip wash, and any holding tanks, are agitated throughout the dipping event.
Resistance to synthetic pyrethroid pour-on insecticides in strains of the sheep body louse Bovicola (Damalinia) ovis.
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.