Between July 1981 and December 1983, 116 randomly selected sheep farms in the south west of Western Australian were surveyed for resistance to anthelmintics. A faecal worm egg count reduction test was carried out on each farm. Anthelmintics tested were thiabendazole (44 mg/kg) and levamisole (7.5 mg/kg) given by intra-ruminal injection and comparisons were made with an untreated group on each farm. Successful tests were carried out on 84 farms and 68% of these had resistant worms present. The prevalence of thiabendazole resistant populations was for H. contortus 18%; Teladorsagia, 41% and Trichostrongylus, 48% and for levamisole resistant populations H. contortus, 10%; Teladorsagia, 41%; Trichostrongylus, 24%, and Nematodirus, 10%. Multiple resistant populations were found on 17% of farms. Although the distribution of nematode genera varied between the 400 to 750 mm and the greater than 750 mm rainfall zones there was no significant difference in the prevalence of resistance between zones. About one third of resistant populations were severely resistant (less than 60% reduction). It is likely that resistant worms were present on many farms without causing clinical disease and continued anthelmintic selection pressure will result in further development of resistance.
Owners of 116 farms, whose flocks had been tested for anthelmintic resistance, were interviewed to determine their use of various sheep management and parasite control practices and their knowledge and adoption of recommended procedures for the prevention and control of resistance. Farmers knowledge of current recommendations related mainly to changing drenches and drench groups. Other aspects of the recommended program including reduction of drenching frequency and the use of alternative management strategies were not considered as important by farmers. For most questions a high proportion of farmers (greater than 20%) had no opinion. Associations between various strategies for nematode control and resistance of Trichostrongylus and Teladorsagia to thiabendazole and levamisole were examined. These relationships differed between anthelmintics and nematode genera. A number of factors were related to resistance of one or both nematode genera to one or both anthelmintic groups. These factors included flock size, percentage of ewes in the flock, cattle number, main sheep production activity, grazing strategy, frequency of drenching, changes in the frequency of drenching, number of summer drenches and the method of estimating dose rates. It was concluded that the methods employed to control anthelmintic resistance may vary with the nematode, its resistance status and the anthelmintic to which it is exposed. Modifications to the previously recommended program have been proposed which incorporate selection of the anthelmintic to be used following a test for anthelmintic resistance.
The failure to eradicate sheep lice, Damalinia ovis, after the application of a synthetic pyrethroid to the backline of sheep may be due to a number of factors, including the management of the sheep, the application of insufficient chemical and poor application technique. Recently it has been recognised that a lowered susceptibility of the lice to the chemical may also be responsible for the breakdown in eradication, and that some strains of lice have developed a degree of resistance to the synthetic pyrethroids (Boray el al1988; de Chaneet el all989). We report the results of trials in which sheep, infested with one of 4 strains of lice (Table 1) with different susceptibilities to cypermehin and alphamethrin, as assessed by an in-vitro assay (Boray et al1988). were treated with 3 synthetic pyrethroids at increasing dose rates.Four groups of sheep, each infested with one of these strains, were penned separately until shearing. Portions of the fleece of these sheep were then used to experimentally infest 4 flocks of lice-free, adult Merino sheep. Three of the flocks, each comprising approximately 60 sheep, were pastured at Wokalup while the fourth, a flock of 150, was infested with the B strain of lice and pastured at Esperance.The lice populations on the experimentally infested sheep were allowed to increase for 16 weeks until all sheep were infested. The Wokalup sheep were shorn in March and the Esperance sheepinMay.Thesheep were weighedwithinanhourofshearing and 5 to 8 of them were allocated to a treatment so that the average number of lice counted at 40 sites of 10 cm partings in the wool of eachsheep (Wilkinsonetd 1982) in each treatment group was greater than 18.A synthetic pyrethroid was then applied along the backline of each sheep from poll to rump. The doses were adjusted according to the individual body weight of the sheep at the rates shown in Table 2. In the case of the long wool treatments, a standard dose TABLE 1 The LC50 of cypermethrln and alphamethrin when used In an In-vltro asray agalnst 4 strains of lice. Strains LC50 (95% confidence intervals) of lice C ypermethrin Alphamethrin (Wg) W g ) K 4.8 (3.8,6.1) 1.3 (1.2,5.5) B 3.7 (3.3,4.2) 2.9 (2.5,3.3) S 3.1 (2.9,3.4) 0.8 (0.6,l .O) SP 2.0 (142.2) 0.7 (0.6,0.8)was used (Table 2). Four groups of 5 to 8 sheep, each infested with one of the strains of lice, remained untreated. Each group was grazed in a separate small paddock Conventional fences with offset elechified wires separated the paddocks and prevented physical contact betweensheep in adjacent groups. Inspections for the presence of lice were made at 40 sites of 10 cm long partings in the wool every 6 weeks until 180 days after treatment. The detection of live lice in each treatment group was This study found that the S strain of lice survived an off-shears treatment with cypermethrin at the recommended dose and the B strain survived off-shears treatments with cypexmedrin at the recommended dose and both cypexmethrin and a l p h a m e at twice the recommended dose ( Table 2). The lice counts ...
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